[Concepts] Requires Expressions

Implement support for C++2a requires-expressions.

Re-commit after compilation failure on some platforms due to alignment issues with PointerIntPair.

Differential Revision: https://reviews.llvm.org/D50360

clang/include/clang/AST/ASTConcept.h

@@ -22,6 +22,7 @@
 #include <utility>
 namespace clang {
 class ConceptDecl;
+class ConceptSpecializationExpr;
 
 /// \brief The result of a constraint satisfaction check, containing the
 /// necessary information to diagnose an unsatisfied constraint.

clang/include/clang/AST/DeclCXX.h

@@ -1893,6 +1893,37 @@ public:
   static bool classofKind(Kind K) { return K == CXXDeductionGuide; }
 };
 
+/// \brief Represents the body of a requires-expression.
+///
+/// This decl exists merely to serve as the DeclContext for the local
+/// parameters of the requires expression as well as other declarations inside
+/// it.
+///
+/// \code
+/// template<typename T> requires requires (T t) { {t++} -> regular; }
+/// \endcode
+///
+/// In this example, a RequiresExpr object will be generated for the expression,
+/// and a RequiresExprBodyDecl will be created to hold the parameter t and the
+/// template argument list imposed by the compound requirement.
+class RequiresExprBodyDecl : public Decl, public DeclContext {
+  RequiresExprBodyDecl(ASTContext &C, DeclContext *DC, SourceLocation StartLoc)
+      : Decl(RequiresExprBody, DC, StartLoc), DeclContext(RequiresExprBody) {}
+
+public:
+  friend class ASTDeclReader;
+  friend class ASTDeclWriter;
+
+  static RequiresExprBodyDecl *Create(ASTContext &C, DeclContext *DC,
+                                      SourceLocation StartLoc);
+
+  static RequiresExprBodyDecl *CreateDeserialized(ASTContext &C, unsigned ID);
+
+  // Implement isa/cast/dyncast/etc.
+  static bool classof(const Decl *D) { return classofKind(D->getKind()); }
+  static bool classofKind(Kind K) { return K == RequiresExprBody; }
+};
+
 /// Represents a static or instance method of a struct/union/class.
 ///
 /// In the terminology of the C++ Standard, these are the (static and

clang/include/clang/AST/ExprCXX.h

@@ -14,7 +14,6 @@
 #ifndef LLVM_CLANG_AST_EXPRCXX_H
 #define LLVM_CLANG_AST_EXPRCXX_H
 
-#include "clang/AST/ASTConcept.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclBase.h"
 #include "clang/AST/DeclCXX.h"
@@ -4836,99 +4835,6 @@ public:
   }
 };
 
-/// \brief Represents the specialization of a concept - evaluates to a prvalue
-/// of type bool.
-///
-/// According to C++2a [expr.prim.id]p3 an id-expression that denotes the
-/// specialization of a concept results in a prvalue of type bool.
-class ConceptSpecializationExpr final : public Expr, public ConceptReference,
-      private llvm::TrailingObjects<ConceptSpecializationExpr,
-                                    TemplateArgument> {
-  friend class ASTStmtReader;
-  friend TrailingObjects;
-public:
-  using SubstitutionDiagnostic = std::pair<SourceLocation, std::string>;
-
-protected:
-  /// \brief The number of template arguments in the tail-allocated list of
-  /// converted template arguments.
-  unsigned NumTemplateArgs;
-
-  /// \brief Information about the satisfaction of the named concept with the
-  /// given arguments. If this expression is value dependent, this is to be
-  /// ignored.
-  ASTConstraintSatisfaction *Satisfaction;
-
-  ConceptSpecializationExpr(const ASTContext &C, NestedNameSpecifierLoc NNS,
-                            SourceLocation TemplateKWLoc,
-                            DeclarationNameInfo ConceptNameInfo,
-                            NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
-                            const ASTTemplateArgumentListInfo *ArgsAsWritten,
-                            ArrayRef<TemplateArgument> ConvertedArgs,
-                            const ConstraintSatisfaction *Satisfaction);
-
-  ConceptSpecializationExpr(EmptyShell Empty, unsigned NumTemplateArgs);
-
-public:
-
-  static ConceptSpecializationExpr *
-  Create(const ASTContext &C, NestedNameSpecifierLoc NNS,
-         SourceLocation TemplateKWLoc, DeclarationNameInfo ConceptNameInfo,
-         NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
-         const ASTTemplateArgumentListInfo *ArgsAsWritten,
-         ArrayRef<TemplateArgument> ConvertedArgs,
-         const ConstraintSatisfaction *Satisfaction);
-
-  static ConceptSpecializationExpr *
-  Create(ASTContext &C, EmptyShell Empty, unsigned NumTemplateArgs);
-
-  ArrayRef<TemplateArgument> getTemplateArguments() const {
-    return ArrayRef<TemplateArgument>(getTrailingObjects<TemplateArgument>(),
-                                      NumTemplateArgs);
-  }
-
-  /// \brief Set new template arguments for this concept specialization.
-  void setTemplateArguments(ArrayRef<TemplateArgument> Converted);
-
-  /// \brief Whether or not the concept with the given arguments was satisfied
-  /// when the expression was created.
-  /// The expression must not be dependent.
-  bool isSatisfied() const {
-    assert(!isValueDependent()
-           && "isSatisfied called on a dependent ConceptSpecializationExpr");
-    return Satisfaction->IsSatisfied;
-  }
-
-  /// \brief Get elaborated satisfaction info about the template arguments'
-  /// satisfaction of the named concept.
-  /// The expression must not be dependent.
-  const ASTConstraintSatisfaction &getSatisfaction() const {
-    assert(!isValueDependent()
-           && "getSatisfaction called on dependent ConceptSpecializationExpr");
-    return *Satisfaction;
-  }
-
-  static bool classof(const Stmt *T) {
-    return T->getStmtClass() == ConceptSpecializationExprClass;
-  }
-
-  SourceLocation getBeginLoc() const LLVM_READONLY {
-    return ConceptName.getBeginLoc();
-  }
-
-  SourceLocation getEndLoc() const LLVM_READONLY {
-    return ArgsAsWritten->RAngleLoc;
-  }
-
-  // Iterators
-  child_range children() {
-    return child_range(child_iterator(), child_iterator());
-  }
-  const_child_range children() const {
-    return const_child_range(const_child_iterator(), const_child_iterator());
-  }
-};
-
 } // namespace clang
 
 #endif // LLVM_CLANG_AST_EXPRCXX_H

clang/include/clang/AST/ExprConcepts.h

@@ -0,0 +1,540 @@
+//===- ExprConcepts.h - C++2a Concepts expressions --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file
+/// Defines Expressions and AST nodes for C++2a concepts.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_EXPRCONCEPTS_H
+#define LLVM_CLANG_AST_EXPRCONCEPTS_H
+
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/ASTConcept.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/DeclarationName.h"
+#include "clang/AST/DeclTemplate.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/NestedNameSpecifier.h"
+#include "clang/AST/TemplateBase.h"
+#include "clang/AST/Type.h"
+#include "clang/Basic/SourceLocation.h"
+#include "llvm/Support/TrailingObjects.h"
+#include <utility>
+#include <string>
+
+namespace clang {
+class ASTStmtReader;
+class ASTStmtWriter;
+
+/// \brief Represents the specialization of a concept - evaluates to a prvalue
+/// of type bool.
+///
+/// According to C++2a [expr.prim.id]p3 an id-expression that denotes the
+/// specialization of a concept results in a prvalue of type bool.
+class ConceptSpecializationExpr final : public Expr, public ConceptReference,
+      private llvm::TrailingObjects<ConceptSpecializationExpr,
+                                    TemplateArgument> {
+  friend class ASTStmtReader;
+  friend TrailingObjects;
+public:
+  using SubstitutionDiagnostic = std::pair<SourceLocation, std::string>;
+
+protected:
+  /// \brief The number of template arguments in the tail-allocated list of
+  /// converted template arguments.
+  unsigned NumTemplateArgs;
+
+  /// \brief Information about the satisfaction of the named concept with the
+  /// given arguments. If this expression is value dependent, this is to be
+  /// ignored.
+  ASTConstraintSatisfaction *Satisfaction;
+
+  ConceptSpecializationExpr(const ASTContext &C, NestedNameSpecifierLoc NNS,
+                            SourceLocation TemplateKWLoc,
+                            DeclarationNameInfo ConceptNameInfo,
+                            NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
+                            const ASTTemplateArgumentListInfo *ArgsAsWritten,
+                            ArrayRef<TemplateArgument> ConvertedArgs,
+                            const ConstraintSatisfaction *Satisfaction);
+
+  ConceptSpecializationExpr(EmptyShell Empty, unsigned NumTemplateArgs);
+
+public:
+
+  static ConceptSpecializationExpr *
+  Create(const ASTContext &C, NestedNameSpecifierLoc NNS,
+         SourceLocation TemplateKWLoc, DeclarationNameInfo ConceptNameInfo,
+         NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
+         const ASTTemplateArgumentListInfo *ArgsAsWritten,
+         ArrayRef<TemplateArgument> ConvertedArgs,
+         const ConstraintSatisfaction *Satisfaction);
+
+  static ConceptSpecializationExpr *
+  Create(ASTContext &C, EmptyShell Empty, unsigned NumTemplateArgs);
+
+  ArrayRef<TemplateArgument> getTemplateArguments() const {
+    return ArrayRef<TemplateArgument>(getTrailingObjects<TemplateArgument>(),
+                                      NumTemplateArgs);
+  }
+
+  /// \brief Set new template arguments for this concept specialization.
+  void setTemplateArguments(ArrayRef<TemplateArgument> Converted);
+
+  /// \brief Whether or not the concept with the given arguments was satisfied
+  /// when the expression was created.
+  /// The expression must not be dependent.
+  bool isSatisfied() const {
+    assert(!isValueDependent()
+           && "isSatisfied called on a dependent ConceptSpecializationExpr");
+    return Satisfaction->IsSatisfied;
+  }
+
+  /// \brief Get elaborated satisfaction info about the template arguments'
+  /// satisfaction of the named concept.
+  /// The expression must not be dependent.
+  const ASTConstraintSatisfaction &getSatisfaction() const {
+    assert(!isValueDependent()
+           && "getSatisfaction called on dependent ConceptSpecializationExpr");
+    return *Satisfaction;
+  }
+
+  static bool classof(const Stmt *T) {
+    return T->getStmtClass() == ConceptSpecializationExprClass;
+  }
+
+  SourceLocation getBeginLoc() const LLVM_READONLY {
+    return ConceptName.getBeginLoc();
+  }
+
+  SourceLocation getEndLoc() const LLVM_READONLY {
+    return ArgsAsWritten->RAngleLoc;
+  }
+
+  // Iterators
+  child_range children() {
+    return child_range(child_iterator(), child_iterator());
+  }
+  const_child_range children() const {
+    return const_child_range(const_child_iterator(), const_child_iterator());
+  }
+};
+
+namespace concepts {
+
+/// \brief A static requirement that can be used in a requires-expression to
+/// check properties of types and expression.
+class Requirement {
+public:
+  // Note - simple and compound requirements are both represented by the same
+  // class (ExprRequirement).
+  enum RequirementKind { RK_Type, RK_Simple, RK_Compound, RK_Nested };
+private:
+  const RequirementKind Kind;
+  bool Dependent : 1;
+  bool ContainsUnexpandedParameterPack : 1;
+  bool Satisfied : 1;
+public:
+  struct SubstitutionDiagnostic {
+    StringRef SubstitutedEntity;
+    // FIXME: Store diagnostics semantically and not as prerendered strings.
+    //  Fixing this probably requires serialization of PartialDiagnostic
+    //  objects.
+    SourceLocation DiagLoc;
+    StringRef DiagMessage;
+  };
+
+  Requirement(RequirementKind Kind, bool IsDependent,
+              bool ContainsUnexpandedParameterPack, bool IsSatisfied = true) :
+      Kind(Kind), Dependent(IsDependent),
+      ContainsUnexpandedParameterPack(ContainsUnexpandedParameterPack),
+      Satisfied(IsSatisfied) {}
+
+  RequirementKind getKind() const { return Kind; }
+
+  bool isSatisfied() const {
+    assert(!Dependent &&
+           "isSatisfied can only be called on non-dependent requirements.");
+    return Satisfied;
+  }
+
+  void setSatisfied(bool IsSatisfied) {
+    assert(!Dependent &&
+           "setSatisfied can only be called on non-dependent requirements.");
+    Satisfied = IsSatisfied;
+  }
+
+  void setDependent(bool IsDependent) { Dependent = IsDependent; }
+  bool isDependent() const { return Dependent; }
+
+  void setContainsUnexpandedParameterPack(bool Contains) {
+    ContainsUnexpandedParameterPack = Contains;
+  }
+  bool containsUnexpandedParameterPack() const {
+    return ContainsUnexpandedParameterPack;
+  }
+};
+
+/// \brief A requires-expression requirement which queries the existence of a
+/// type name or type template specialization ('type' requirements).
+class TypeRequirement : public Requirement {
+public:
+  enum SatisfactionStatus {
+      SS_Dependent,
+      SS_SubstitutionFailure,
+      SS_Satisfied
+  };
+private:
+  llvm::PointerUnion<SubstitutionDiagnostic *, TypeSourceInfo *> Value;
+  SatisfactionStatus Status;
+public:
+  friend ASTStmtReader;
+  friend ASTStmtWriter;
+
+  /// \brief Construct a type requirement from a type. If the given type is not
+  /// dependent, this indicates that the type exists and the requirement will be
+  /// satisfied. Otherwise, the SubstitutionDiagnostic constructor is to be
+  /// used.
+  TypeRequirement(TypeSourceInfo *T);
+
+  /// \brief Construct a type requirement when the nested name specifier is
+  /// invalid due to a bad substitution. The requirement is unsatisfied.
+  TypeRequirement(SubstitutionDiagnostic *Diagnostic) :
+      Requirement(RK_Type, false, false, false), Value(Diagnostic),
+      Status(SS_SubstitutionFailure) {}
+
+  SatisfactionStatus getSatisfactionStatus() const { return Status; }
+  void setSatisfactionStatus(SatisfactionStatus Status) {
+    this->Status = Status;
+  }
+
+  bool isSubstitutionFailure() const {
+    return Status == SS_SubstitutionFailure;
+  }
+
+  SubstitutionDiagnostic *getSubstitutionDiagnostic() const {
+    assert(Status == SS_SubstitutionFailure &&
+           "Attempted to get substitution diagnostic when there has been no "
+           "substitution failure.");
+    return Value.get<SubstitutionDiagnostic *>();
+  }
+
+  TypeSourceInfo *getType() const {
+    assert(!isSubstitutionFailure() &&
+           "Attempted to get type when there has been a substitution failure.");
+    return Value.get<TypeSourceInfo *>();
+  }
+
+  static bool classof(const Requirement *R) {
+    return R->getKind() == RK_Type;
+  }
+};
+
+/// \brief A requires-expression requirement which queries the validity and
+/// properties of an expression ('simple' and 'compound' requirements).
+class ExprRequirement : public Requirement {
+public:
+  enum SatisfactionStatus {
+      SS_Dependent,
+      SS_ExprSubstitutionFailure,
+      SS_NoexceptNotMet,
+      SS_TypeRequirementSubstitutionFailure,
+      SS_ConstraintsNotSatisfied,
+      SS_Satisfied
+  };
+  class ReturnTypeRequirement {
+      llvm::PointerIntPair<
+          llvm::PointerUnion<TemplateParameterList *, SubstitutionDiagnostic *>,
+          1, bool>
+          TypeConstraintInfo;
+  public:
+      friend ASTStmtReader;
+      friend ASTStmtWriter;
+
+      /// \brief No return type requirement was specified.
+      ReturnTypeRequirement() : TypeConstraintInfo(nullptr, 0) {}
+
+      /// \brief A return type requirement was specified but it was a
+      /// substitution failure.
+      ReturnTypeRequirement(SubstitutionDiagnostic *SubstDiag) :
+          TypeConstraintInfo(SubstDiag, 0) {}
+
+      /// \brief A 'type constraint' style return type requirement.
+      /// \param TPL an invented template parameter list containing a single
+      /// type parameter with a type-constraint.
+      // TODO: Can we maybe not save the whole template parameter list and just
+      //  the type constraint? Saving the whole TPL makes it easier to handle in
+      //  serialization but is less elegant.
+      ReturnTypeRequirement(TemplateParameterList *TPL);
+
+      bool isDependent() const {
+        return TypeConstraintInfo.getInt();
+      }
+
+      bool containsUnexpandedParameterPack() const {
+        if (!isTypeConstraint())
+          return false;
+        return getTypeConstraintTemplateParameterList()
+                ->containsUnexpandedParameterPack();
+      }
+
+      bool isEmpty() const {
+        return TypeConstraintInfo.getPointer().isNull();
+      }
+
+      bool isSubstitutionFailure() const {
+        return !isEmpty() &&
+            TypeConstraintInfo.getPointer().is<SubstitutionDiagnostic *>();
+      }
+
+      bool isTypeConstraint() const {
+        return !isEmpty() &&
+            TypeConstraintInfo.getPointer().is<TemplateParameterList *>();
+      }
+
+      SubstitutionDiagnostic *getSubstitutionDiagnostic() const {
+        assert(isSubstitutionFailure());
+        return TypeConstraintInfo.getPointer().get<SubstitutionDiagnostic *>();
+      }
+
+      const TypeConstraint *getTypeConstraint() const;
+
+      TemplateParameterList *getTypeConstraintTemplateParameterList() const {
+        assert(isTypeConstraint());
+        return TypeConstraintInfo.getPointer().get<TemplateParameterList *>();
+      }
+  };
+private:
+  llvm::PointerUnion<Expr *, SubstitutionDiagnostic *> Value;
+  SourceLocation NoexceptLoc; // May be empty if noexcept wasn't specified.
+  ReturnTypeRequirement TypeReq;
+  ConceptSpecializationExpr *SubstitutedConstraintExpr;
+  SatisfactionStatus Status;
+public:
+  friend ASTStmtReader;
+  friend ASTStmtWriter;
+
+  /// \brief Construct a compound requirement.
+  /// \param E the expression which is checked by this requirement.
+  /// \param IsSimple whether this was a simple requirement in source.
+  /// \param NoexceptLoc the location of the noexcept keyword, if it was
+  /// specified, otherwise an empty location.
+  /// \param Req the requirement for the type of the checked expression.
+  /// \param Status the satisfaction status of this requirement.
+  ExprRequirement(
+      Expr *E, bool IsSimple, SourceLocation NoexceptLoc,
+      ReturnTypeRequirement Req, SatisfactionStatus Status,
+      ConceptSpecializationExpr *SubstitutedConstraintExpr = nullptr);
+
+  /// \brief Construct a compound requirement whose expression was a
+  /// substitution failure. The requirement is not satisfied.
+  /// \param E the diagnostic emitted while instantiating the original
+  /// expression.
+  /// \param IsSimple whether this was a simple requirement in source.
+  /// \param NoexceptLoc the location of the noexcept keyword, if it was
+  /// specified, otherwise an empty location.
+  /// \param Req the requirement for the type of the checked expression (omit
+  /// if no requirement was specified).
+  ExprRequirement(SubstitutionDiagnostic *E, bool IsSimple,
+                  SourceLocation NoexceptLoc, ReturnTypeRequirement Req = {});
+
+  bool isSimple() const { return getKind() == RK_Simple; }
+  bool isCompound() const { return getKind() == RK_Compound; }
+
+  bool hasNoexceptRequirement() const { return NoexceptLoc.isValid(); }
+  SourceLocation getNoexceptLoc() const { return NoexceptLoc; }
+
+  SatisfactionStatus getSatisfactionStatus() const { return Status; }
+
+  bool isExprSubstitutionFailure() const {
+    return Status == SS_ExprSubstitutionFailure;
+  }
+
+  const ReturnTypeRequirement &getReturnTypeRequirement() const {
+    return TypeReq;
+  }
+
+  ConceptSpecializationExpr *
+  getReturnTypeRequirementSubstitutedConstraintExpr() const {
+    assert(Status >= SS_TypeRequirementSubstitutionFailure);
+    return SubstitutedConstraintExpr;
+  }
+
+  SubstitutionDiagnostic *getExprSubstitutionDiagnostic() const {
+    assert(isExprSubstitutionFailure() &&
+           "Attempted to get expression substitution diagnostic when there has "
+           "been no expression substitution failure");
+    return Value.get<SubstitutionDiagnostic *>();
+  }
+
+  Expr *getExpr() const {
+    assert(!isExprSubstitutionFailure() &&
+           "ExprRequirement has no expression because there has been a "
+           "substitution failure.");
+    return Value.get<Expr *>();
+  }
+
+  static bool classof(const Requirement *R) {
+    return R->getKind() == RK_Compound || R->getKind() == RK_Simple;
+  }
+};
+
+/// \brief A requires-expression requirement which is satisfied when a general
+/// constraint expression is satisfied ('nested' requirements).
+class NestedRequirement : public Requirement {
+  llvm::PointerUnion<Expr *, SubstitutionDiagnostic *> Value;
+  const ASTConstraintSatisfaction *Satisfaction = nullptr;
+
+public:
+  friend ASTStmtReader;
+  friend ASTStmtWriter;
+
+  NestedRequirement(SubstitutionDiagnostic *SubstDiag) :
+      Requirement(RK_Nested, /*Dependent=*/false,
+                  /*ContainsUnexpandedParameterPack*/false,
+                  /*Satisfied=*/false), Value(SubstDiag) {}
+
+  NestedRequirement(Expr *Constraint) :
+      Requirement(RK_Nested, /*Dependent=*/true,
+                  Constraint->containsUnexpandedParameterPack()),
+      Value(Constraint) {
+    assert(Constraint->isInstantiationDependent() &&
+           "Nested requirement with non-dependent constraint must be "
+           "constructed with a ConstraintSatisfaction object");
+  }
+
+  NestedRequirement(ASTContext &C, Expr *Constraint,
+                    const ConstraintSatisfaction &Satisfaction) :
+      Requirement(RK_Nested, Constraint->isInstantiationDependent(),
+                  Constraint->containsUnexpandedParameterPack(),
+                  Satisfaction.IsSatisfied),
+      Value(Constraint),
+      Satisfaction(ASTConstraintSatisfaction::Create(C, Satisfaction)) {}
+
+  bool isSubstitutionFailure() const {
+    return Value.is<SubstitutionDiagnostic *>();
+  }
+
+  SubstitutionDiagnostic *getSubstitutionDiagnostic() const {
+    assert(isSubstitutionFailure() &&
+           "getSubstitutionDiagnostic() may not be called when there was no "
+           "substitution failure.");
+    return Value.get<SubstitutionDiagnostic *>();
+  }
+
+  Expr *getConstraintExpr() const {
+    assert(!isSubstitutionFailure() && "getConstraintExpr() may not be called "
+                                       "on nested requirements with "
+                                       "substitution failures.");
+    return Value.get<Expr *>();
+  }
+
+  const ASTConstraintSatisfaction &getConstraintSatisfaction() const {
+    assert(!isSubstitutionFailure() && "getConstraintSatisfaction() may not be "
+                                       "called on nested requirements with "
+                                       "substitution failures.");
+    return *Satisfaction;
+  }
+
+  static bool classof(const Requirement *R) {
+    return R->getKind() == RK_Nested;
+  }
+};
+
+} // namespace concepts
+
+/// C++2a [expr.prim.req]:
+///     A requires-expression provides a concise way to express requirements on
+///     template arguments. A requirement is one that can be checked by name
+///     lookup (6.4) or by checking properties of types and expressions.
+///     [...]
+///     A requires-expression is a prvalue of type bool [...]
+class RequiresExpr final : public Expr,
+    llvm::TrailingObjects<RequiresExpr, ParmVarDecl *,
+                          concepts::Requirement *> {
+  friend TrailingObjects;
+  friend class ASTStmtReader;
+
+  unsigned NumLocalParameters;
+  unsigned NumRequirements;
+  RequiresExprBodyDecl *Body;
+  SourceLocation RBraceLoc;
+
+  unsigned numTrailingObjects(OverloadToken<ParmVarDecl *>) const {
+    return NumLocalParameters;
+  }
+
+  unsigned numTrailingObjects(OverloadToken<concepts::Requirement *>) const {
+    return NumRequirements;
+  }
+
+  RequiresExpr(ASTContext &C, SourceLocation RequiresKWLoc,
+               RequiresExprBodyDecl *Body,
+               ArrayRef<ParmVarDecl *> LocalParameters,
+               ArrayRef<concepts::Requirement *> Requirements,
+               SourceLocation RBraceLoc);
+  RequiresExpr(ASTContext &C, EmptyShell Empty, unsigned NumLocalParameters,
+               unsigned NumRequirements);
+
+public:
+  static RequiresExpr *
+  Create(ASTContext &C, SourceLocation RequiresKWLoc,
+         RequiresExprBodyDecl *Body, ArrayRef<ParmVarDecl *> LocalParameters,
+         ArrayRef<concepts::Requirement *> Requirements,
+         SourceLocation RBraceLoc);
+  static RequiresExpr *
+  Create(ASTContext &C, EmptyShell Empty, unsigned NumLocalParameters,
+         unsigned NumRequirements);
+
+  ArrayRef<ParmVarDecl *> getLocalParameters() const {
+    return {getTrailingObjects<ParmVarDecl *>(), NumLocalParameters};
+  }
+
+  RequiresExprBodyDecl *getBody() const { return Body; }
+
+  ArrayRef<concepts::Requirement *> getRequirements() const {
+    return {getTrailingObjects<concepts::Requirement *>(), NumRequirements};
+  }
+
+  /// \brief Whether or not the requires clause is satisfied.
+  /// The expression must not be dependent.
+  bool isSatisfied() const {
+    assert(!isValueDependent()
+           && "isSatisfied called on a dependent RequiresExpr");
+    return RequiresExprBits.IsSatisfied;
+  }
+
+  SourceLocation getRequiresKWLoc() const {
+    return RequiresExprBits.RequiresKWLoc;
+  }
+
+  SourceLocation getRBraceLoc() const { return RBraceLoc; }
+
+  static bool classof(const Stmt *T) {
+    return T->getStmtClass() == RequiresExprClass;
+  }
+
+  SourceLocation getBeginLoc() const LLVM_READONLY {
+    return RequiresExprBits.RequiresKWLoc;
+  }
+  SourceLocation getEndLoc() const LLVM_READONLY {
+    return RBraceLoc;
+  }
+
+  // Iterators
+  child_range children() {
+    return child_range(child_iterator(), child_iterator());
+  }
+  const_child_range children() const {
+    return const_child_range(const_child_iterator(), const_child_iterator());
+  }
+};
+
+} // namespace clang
+
+#endif // LLVM_CLANG_AST_EXPRCONCEPTS_H

clang/include/clang/AST/RecursiveASTVisitor.h

@@ -23,6 +23,7 @@
 #include "clang/AST/DeclOpenMP.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/ExprOpenMP.h"
@@ -2138,6 +2139,8 @@ DEF_TRAVERSE_DECL(ParmVarDecl, {
     TRY_TO(TraverseStmt(D->getDefaultArg()));
 })
 
+DEF_TRAVERSE_DECL(RequiresExprBodyDecl, {})
+
 #undef DEF_TRAVERSE_DECL
 
 // ----------------- Stmt traversal -----------------
@@ -2709,6 +2712,28 @@ DEF_TRAVERSE_STMT(ConceptSpecializationExpr, {
   TRY_TO(TraverseConceptReference(*S));
 })
 
+DEF_TRAVERSE_STMT(RequiresExpr, {
+  TRY_TO(TraverseDecl(S->getBody()));
+  for (ParmVarDecl *Parm : S->getLocalParameters())
+    TRY_TO(TraverseDecl(Parm));
+  for (concepts::Requirement *Req : S->getRequirements())
+    if (auto *TypeReq = dyn_cast<concepts::TypeRequirement>(Req)) {
+      if (!TypeReq->isSubstitutionFailure())
+        TRY_TO(TraverseTypeLoc(TypeReq->getType()->getTypeLoc()));
+    } else if (auto *ExprReq = dyn_cast<concepts::ExprRequirement>(Req)) {
+      if (!ExprReq->isExprSubstitutionFailure())
+        TRY_TO(TraverseStmt(ExprReq->getExpr()));
+      auto &RetReq = ExprReq->getReturnTypeRequirement();
+      if (RetReq.isTypeConstraint())
+        TRY_TO(TraverseTemplateParameterListHelper(
+                   RetReq.getTypeConstraintTemplateParameterList()));
+    } else {
+      auto *NestedReq = cast<concepts::NestedRequirement>(Req);
+      if (!NestedReq->isSubstitutionFailure())
+        TRY_TO(TraverseStmt(NestedReq->getConstraintExpr()));
+    }
+})
+
 // These literals (all of them) do not need any action.
 DEF_TRAVERSE_STMT(IntegerLiteral, {})
 DEF_TRAVERSE_STMT(FixedPointLiteral, {})

clang/include/clang/AST/Stmt.h

@@ -910,6 +910,17 @@ protected:
     SourceLocation NameLoc;
   };
 
+  class RequiresExprBitfields {
+    friend class ASTStmtReader;
+    friend class ASTStmtWriter;
+    friend class RequiresExpr;
+
+    unsigned : NumExprBits;
+
+    unsigned IsSatisfied : 1;
+    SourceLocation RequiresKWLoc;
+  };
+
   //===--- C++ Coroutines TS bitfields classes ---===//
 
   class CoawaitExprBitfields {
@@ -1008,6 +1019,7 @@ protected:
     UnresolvedMemberExprBitfields UnresolvedMemberExprBits;
     CXXNoexceptExprBitfields CXXNoexceptExprBits;
     SubstNonTypeTemplateParmExprBitfields SubstNonTypeTemplateParmExprBits;
+    RequiresExprBitfields RequiresExprBits;
 
     // C++ Coroutines TS expressions
     CoawaitExprBitfields CoawaitBits;

clang/include/clang/AST/StmtVisitor.h

@@ -13,6 +13,7 @@
 #ifndef LLVM_CLANG_AST_STMTVISITOR_H
 #define LLVM_CLANG_AST_STMTVISITOR_H
 
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/ExprOpenMP.h"

clang/include/clang/Basic/DeclNodes.td

@@ -100,5 +100,6 @@ def OMPThreadPrivate : DeclNode<Decl>;
 def OMPAllocate : DeclNode<Decl>;
 def OMPRequires : DeclNode<Decl>;
 def Empty : DeclNode<Decl>;
+def RequiresExprBody : DeclNode<Decl>, DeclContext;
 def LifetimeExtendedTemporary : DeclNode<Decl>;
 

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -744,6 +744,33 @@ def err_friend_explicit_instantiation : Error<
 def err_explicit_instantiation_enum : Error<
   "enumerations cannot be explicitly instantiated">;
 def err_expected_template_parameter : Error<"expected template parameter">;
+def note_ill_formed_requires_expression_outside_template : Note<
+  "requires expression outside a template declaration may not contain invalid "
+  "types or expressions">;
+def err_empty_requires_expr : Error<
+  "a requires expression must contain at least one requirement">;
+def err_requires_expr_parameter_list_ellipsis : Error<
+  "varargs not allowed in requires expression">;
+def err_requires_expr_type_req_illegal_identifier : Error<
+  "expected identifier or template-id in type requirement">;
+def err_requires_expr_type_req_template_args_on_non_template : Error<
+  "template arguments provided for non-template '%0'">;
+def err_expected_semi_requirement : Error<
+  "expected ';' at end of requirement">;
+def err_requires_expr_missing_arrow : Error<
+  "expected '->' before expression type requirement">;
+def err_requires_expr_expected_type_constraint : Error<
+  "expected concept name with optional arguments">;
+def err_requires_expr_simple_requirement_noexcept : Error<
+  "'noexcept' can only be used in a compound requirement (with '{' '}' around "
+  "the expression)">;
+def err_requires_expr_simple_requirement_unexpected_tok : Error<
+  "unexpected %0 after expression; did you intend to use a compound "
+  "requirement (with '{' '}' around the expression)?">;
+def warn_requires_expr_in_simple_requirement : Warning<
+  "this requires expression will only be checked for syntactic validity; did "
+  "you intend to place it in a nested requirement? (add another 'requires' "
+  "before the expression)">, InGroup<DiagGroup<"requires-expression">>;
 
 def err_missing_dependent_template_keyword : Error<
   "use 'template' keyword to treat '%0' as a dependent template name">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2102,12 +2102,21 @@ def err_auto_not_allowed : Error<
   "|in template argument|in typedef|in type alias|in function return type"
   "|in conversion function type|here|in lambda parameter"
   "|in type allocated by 'new'|in K&R-style function parameter"
-  "|in template parameter|in friend declaration}1">;
+  "|in template parameter|in friend declaration"
+  "|in requires expression parameter}1">;
+def err_auto_not_allowed_in_return_type_requirement : Error<
+  "%select{'auto'|'decltype(auto)'|'__auto_type'}0 not allowed in expression "
+  "type requirement">;
 def err_dependent_deduced_tst : Error<
   "typename specifier refers to "
   "%select{class template|function template|variable template|alias template|"
   "template template parameter|template}0 member in %1; "
   "argument deduction not allowed here">;
+def err_deduced_tst : Error<
+  "typename specifier refers to "
+  "%select{class template|function template|variable template|alias template|"
+  "template template parameter|template}0; argument deduction not allowed "
+  "here">;
 def err_auto_not_allowed_var_inst : Error<
   "'auto' variable template instantiation is not allowed">;
 def err_auto_var_requires_init : Error<
@@ -2590,19 +2599,56 @@ def note_constraints_not_satisfied : Note<
 def note_substituted_constraint_expr_is_ill_formed : Note<
   "because substituted constraint expression is ill-formed%0">;
 def note_atomic_constraint_evaluated_to_false : Note<
-  "%select{and |because }0'%1' evaluated to false">;
+  "%select{and|because}0 '%1' evaluated to false">;
 def note_concept_specialization_constraint_evaluated_to_false : Note<
-  "%select{and |because }0'%1' evaluated to false">;
+  "%select{and|because}0 '%1' evaluated to false">;
 def note_single_arg_concept_specialization_constraint_evaluated_to_false : Note<
-  "%select{and |because }0%1 does not satisfy %2">;
+  "%select{and|because}0 %1 does not satisfy %2">;
 def note_atomic_constraint_evaluated_to_false_elaborated : Note<
-  "%select{and |because }0'%1' (%2 %3 %4) evaluated to false">;
+  "%select{and|because}0 '%1' (%2 %3 %4) evaluated to false">;
 def err_constrained_virtual_method : Error<
   "virtual function cannot have a requires clause">;
 def err_trailing_requires_clause_on_deduction_guide : Error<
   "deduction guide cannot have a requires clause">;
 def err_reference_to_function_with_unsatisfied_constraints : Error<
   "invalid reference to function %0: constraints not satisfied">;
+def note_requires_expr_ill_formed_expr : Note<
+  "expression is invalid: %0">;
+def note_requires_expr_no_implicit_conversion : Note<
+  "no implicit conversion exists between expression type %0 and expected type "
+  "%1">;
+def err_requires_expr_local_parameter_default_argument : Error<
+  "default arguments not allowed for parameters of a requires expression">;
+def err_requires_expr_parameter_referenced_in_evaluated_context : Error<
+  "constraint variable %0 cannot be used in an evaluated context">;
+def note_expr_requirement_expr_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid: %2">;
+def note_expr_requirement_expr_unknown_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid">;
+def note_expr_requirement_noexcept_not_met : Note<
+  "%select{and|because}0 '%1' may throw an exception">;
+def note_expr_requirement_type_requirement_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid: %2">;
+def note_expr_requirement_type_requirement_unknown_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid">;
+def note_expr_requirement_constraints_not_satisfied : Note<
+  "%select{and|because}0 type constraint '%1' was not satisfied:">;
+def note_expr_requirement_constraints_not_satisfied_simple : Note<
+  "%select{and|because}0 %1 does not satisfy %2:">;
+def note_type_requirement_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid: %2">;
+def note_type_requirement_unknown_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid">;
+def err_type_requirement_non_type_template : Error<
+  "'%0' refers to a %select{class template|function template|"
+  "variable template|alias template|template template parameter|template}1, "
+  "not a type template">;
+def err_type_requirement_no_such_type : Error<
+  "'%0' does not name a type">;
+def note_nested_requirement_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid: %2">;
+def note_nested_requirement_unknown_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid">;
 def note_ambiguous_atomic_constraints : Note<
   "similar constraint expressions not considered equivalent; constraint "
   "expressions cannot be considered equivalent unless they originate from the "
@@ -4588,6 +4634,8 @@ def note_template_type_alias_instantiation_here : Note<
   "in instantiation of template type alias %0 requested here">;
 def note_template_exception_spec_instantiation_here : Note<
   "in instantiation of exception specification for %0 requested here">;
+def note_template_requirement_instantiation_here : Note<
+  "in instantiation of requirement here">;
 def warn_var_template_missing : Warning<"instantiation of variable %q0 "
   "required here, but no definition is available">,
   InGroup<UndefinedVarTemplate>;
@@ -4623,6 +4671,8 @@ def note_template_default_arg_checking : Note<
   "while checking a default template argument used here">;
 def note_concept_specialization_here : Note<
   "while checking the satisfaction of concept '%0' requested here">;
+def note_nested_requirement_here : Note<
+  "while checking the satisfaction of nested requirement requested here">;
 def note_checking_constraints_for_template_id_here : Note<
   "while checking constraint satisfaction for template '%0' required here">;
 def note_checking_constraints_for_var_spec_id_here : Note<
@@ -4756,8 +4806,12 @@ def err_typename_nested_not_found_requirement : Error<
   "declaration">;
 def err_typename_nested_not_type : Error<
     "typename specifier refers to non-type member %0 in %1">;
-def note_typename_refers_here : Note<
+def err_typename_not_type : Error<
+    "typename specifier refers to non-type %0">;
+def note_typename_member_refers_here : Note<
     "referenced member %0 is declared here">;
+def note_typename_refers_here : Note<
+    "referenced %0 is declared here">;
 def err_typename_missing : Error<
   "missing 'typename' prior to dependent type name '%0%1'">;
 def err_typename_missing_template : Error<

clang/include/clang/Basic/StmtNodes.td

@@ -164,6 +164,7 @@ def CoyieldExpr : StmtNode<CoroutineSuspendExpr>;
 
 // C++2a Concepts expressions
 def ConceptSpecializationExpr : StmtNode<Expr>;
+def RequiresExpr : StmtNode<Expr>;
 
 // Obj-C Expressions.
 def ObjCStringLiteral : StmtNode<Expr>;

clang/include/clang/Parse/Parser.h

@@ -1933,6 +1933,7 @@ private:
   //===--------------------------------------------------------------------===//
   // C++ Concepts
 
+  ExprResult ParseRequiresExpression();
   void ParseTrailingRequiresClause(Declarator &D);
 
   //===--------------------------------------------------------------------===//
@@ -2771,7 +2772,7 @@ private:
          Declarator &D,
          SmallVectorImpl<DeclaratorChunk::ParamInfo> &ParamInfo);
   void ParseParameterDeclarationClause(
-         Declarator &D,
+         DeclaratorContext DeclaratorContext,
          ParsedAttributes &attrs,
          SmallVectorImpl<DeclaratorChunk::ParamInfo> &ParamInfo,
          SourceLocation &EllipsisLoc);

clang/include/clang/Sema/DeclSpec.h

@@ -1757,7 +1757,8 @@ enum class DeclaratorContext {
     TemplateArgContext,  // Any template argument (in template argument list).
     TemplateTypeArgContext, // Template type argument (in default argument).
     AliasDeclContext,    // C++11 alias-declaration.
-    AliasTemplateContext // C++11 alias-declaration template.
+    AliasTemplateContext, // C++11 alias-declaration template.
+    RequiresExprContext   // C++2a requires-expression.
 };
 
 
@@ -1981,6 +1982,7 @@ public:
     case DeclaratorContext::TemplateTypeArgContext:
     case DeclaratorContext::TrailingReturnContext:
     case DeclaratorContext::TrailingReturnVarContext:
+    case DeclaratorContext::RequiresExprContext:
       return true;
     }
     llvm_unreachable("unknown context kind!");
@@ -2003,6 +2005,7 @@ public:
     case DeclaratorContext::TemplateParamContext:
     case DeclaratorContext::CXXCatchContext:
     case DeclaratorContext::ObjCCatchContext:
+    case DeclaratorContext::RequiresExprContext:
       return true;
 
     case DeclaratorContext::TypeNameContext:
@@ -2039,6 +2042,7 @@ public:
     case DeclaratorContext::MemberContext:
     case DeclaratorContext::PrototypeContext:
     case DeclaratorContext::TemplateParamContext:
+    case DeclaratorContext::RequiresExprContext:
       // Maybe one day...
       return false;
 
@@ -2116,6 +2120,7 @@ public:
     case DeclaratorContext::TemplateArgContext:
     case DeclaratorContext::TemplateTypeArgContext:
     case DeclaratorContext::TrailingReturnContext:
+    case DeclaratorContext::RequiresExprContext:
       return false;
     }
     llvm_unreachable("unknown context kind!");
@@ -2337,6 +2342,7 @@ public:
     case DeclaratorContext::TemplateTypeArgContext:
     case DeclaratorContext::TrailingReturnContext:
     case DeclaratorContext::TrailingReturnVarContext:
+    case DeclaratorContext::RequiresExprContext:
       return false;
     }
     llvm_unreachable("unknown context kind!");
@@ -2370,6 +2376,7 @@ public:
     case DeclaratorContext::TrailingReturnContext:
     case DeclaratorContext::TrailingReturnVarContext:
     case DeclaratorContext::TemplateTypeArgContext:
+    case DeclaratorContext::RequiresExprContext:
       return false;
 
     case DeclaratorContext::BlockContext:

clang/include/clang/Sema/Sema.h

@@ -21,6 +21,7 @@
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/DeclarationName.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/ExternalASTSource.h"
@@ -6282,13 +6283,17 @@ public:
 
   /// \brief Emit diagnostics explaining why a constraint expression was deemed
   /// unsatisfied.
+  /// \param First whether this is the first time an unsatisfied constraint is
+  /// diagnosed for this error.
   void
-  DiagnoseUnsatisfiedConstraint(const ConstraintSatisfaction& Satisfaction);
+  DiagnoseUnsatisfiedConstraint(const ConstraintSatisfaction &Satisfaction,
+                                bool First = true);
 
   /// \brief Emit diagnostics explaining why a constraint expression was deemed
   /// unsatisfied.
   void
-  DiagnoseUnsatisfiedConstraint(const ASTConstraintSatisfaction& Satisfaction);
+  DiagnoseUnsatisfiedConstraint(const ASTConstraintSatisfaction &Satisfaction,
+                                bool First = true);
 
   /// \brief Emit diagnostics explaining why a constraint expression was deemed
   /// unsatisfied because it was ill-formed.
@@ -7262,7 +7267,17 @@ public:
                              SourceLocation KeywordLoc,
                              NestedNameSpecifierLoc QualifierLoc,
                              const IdentifierInfo &II,
-                             SourceLocation IILoc);
+                             SourceLocation IILoc,
+                             TypeSourceInfo **TSI,
+                             bool DeducedTSTContext);
+
+  QualType CheckTypenameType(ElaboratedTypeKeyword Keyword,
+                             SourceLocation KeywordLoc,
+                             NestedNameSpecifierLoc QualifierLoc,
+                             const IdentifierInfo &II,
+                             SourceLocation IILoc,
+                             bool DeducedTSTContext = true);
+
 
   TypeSourceInfo *RebuildTypeInCurrentInstantiation(TypeSourceInfo *T,
                                                     SourceLocation Loc,
@@ -7282,11 +7297,52 @@ public:
                                   const TemplateArgument *Args,
                                   unsigned NumArgs);
 
-  // Concepts
+  //===--------------------------------------------------------------------===//
+  // C++ Concepts
+  //===--------------------------------------------------------------------===//
   Decl *ActOnConceptDefinition(
       Scope *S, MultiTemplateParamsArg TemplateParameterLists,
       IdentifierInfo *Name, SourceLocation NameLoc, Expr *ConstraintExpr);
 
+  RequiresExprBodyDecl *
+  ActOnStartRequiresExpr(SourceLocation RequiresKWLoc,
+                         ArrayRef<ParmVarDecl *> LocalParameters,
+                         Scope *BodyScope);
+  void ActOnFinishRequiresExpr();
+  concepts::Requirement *ActOnSimpleRequirement(Expr *E);
+  concepts::Requirement *ActOnTypeRequirement(
+      SourceLocation TypenameKWLoc, CXXScopeSpec &SS, SourceLocation NameLoc,
+      IdentifierInfo *TypeName, TemplateIdAnnotation *TemplateId);
+  concepts::Requirement *ActOnCompoundRequirement(Expr *E,
+                                                  SourceLocation NoexceptLoc);
+  concepts::Requirement *
+  ActOnCompoundRequirement(
+      Expr *E, SourceLocation NoexceptLoc, CXXScopeSpec &SS,
+      TemplateIdAnnotation *TypeConstraint, unsigned Depth);
+  concepts::Requirement *ActOnNestedRequirement(Expr *Constraint);
+  concepts::ExprRequirement *
+  BuildExprRequirement(
+      Expr *E, bool IsSatisfied, SourceLocation NoexceptLoc,
+      concepts::ExprRequirement::ReturnTypeRequirement ReturnTypeRequirement);
+  concepts::ExprRequirement *
+  BuildExprRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *ExprSubstDiag,
+      bool IsSatisfied, SourceLocation NoexceptLoc,
+      concepts::ExprRequirement::ReturnTypeRequirement ReturnTypeRequirement);
+  concepts::TypeRequirement *BuildTypeRequirement(TypeSourceInfo *Type);
+  concepts::TypeRequirement *
+  BuildTypeRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *SubstDiag);
+  concepts::NestedRequirement *BuildNestedRequirement(Expr *E);
+  concepts::NestedRequirement *
+  BuildNestedRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *SubstDiag);
+  ExprResult ActOnRequiresExpr(SourceLocation RequiresKWLoc,
+                               RequiresExprBodyDecl *Body,
+                               ArrayRef<ParmVarDecl *> LocalParameters,
+                               ArrayRef<concepts::Requirement *> Requirements,
+                               SourceLocation ClosingBraceLoc);
+
   //===--------------------------------------------------------------------===//
   // C++ Variadic Templates (C++0x [temp.variadic])
   //===--------------------------------------------------------------------===//
@@ -7933,6 +7989,13 @@ public:
       /// template which was deferred until it was needed.
       ExceptionSpecInstantiation,
 
+      /// We are instantiating a requirement of a requires expression.
+      RequirementInstantiation,
+
+      /// We are checking the satisfaction of a nested requirement of a requires
+      /// expression.
+      NestedRequirementConstraintsCheck,
+
       /// We are declaring an implicit special member function.
       DeclaringSpecialMember,
 
@@ -8254,6 +8317,19 @@ public:
                           ParameterMappingSubstitution, NamedDecl *Template,
                           SourceRange InstantiationRange);
 
+    /// \brief Note that we are substituting template arguments into a part of
+    /// a requirement of a requires expression.
+    InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
+                          concepts::Requirement *Req,
+                          sema::TemplateDeductionInfo &DeductionInfo,
+                          SourceRange InstantiationRange = SourceRange());
+
+    /// \brief Note that we are checking the satisfaction of the constraint
+    /// expression inside of a nested requirement.
+    InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
+                          concepts::NestedRequirement *Req, ConstraintsCheck,
+                          SourceRange InstantiationRange = SourceRange());
+
     /// Note that we have finished instantiating this template.
     void Clear();
 

clang/include/clang/Sema/SemaConcept.h

@@ -13,10 +13,17 @@
 
 #ifndef LLVM_CLANG_SEMA_SEMACONCEPT_H
 #define LLVM_CLANG_SEMA_SEMACONCEPT_H
+#include "clang/AST/ASTConcept.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/DeclTemplate.h"
+#include "clang/Basic/SourceLocation.h"
+#include "llvm/ADT/PointerUnion.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallVector.h"
+#include <string>
+#include <utility>
+
 namespace clang {
 class Sema;
 

clang/include/clang/Serialization/ASTBitCodes.h

@@ -1403,6 +1403,9 @@ namespace serialization {
       /// An LifetimeExtendedTemporaryDecl record.
       DECL_LIFETIME_EXTENDED_TEMPORARY,
 
+      /// A RequiresExprBodyDecl record.
+      DECL_REQUIRES_EXPR_BODY,
+
       /// An ObjCTypeParamDecl record.
       DECL_OBJC_TYPE_PARAM,
 
@@ -1785,6 +1788,7 @@ namespace serialization {
       EXPR_MATERIALIZE_TEMPORARY, // MaterializeTemporaryExpr
       EXPR_CXX_FOLD,              // CXXFoldExpr
       EXPR_CONCEPT_SPECIALIZATION,// ConceptSpecializationExpr
+      EXPR_REQUIRES,              // RequiresExpr
 
       // CUDA
       EXPR_CUDA_KERNEL_CALL,       // CUDAKernelCallExpr

clang/lib/AST/CMakeLists.txt

@@ -46,6 +46,7 @@ add_clang_library(clangAST
   DeclTemplate.cpp
   Expr.cpp
   ExprClassification.cpp
+  ExprConcepts.cpp
   ExprConstant.cpp
   ExprCXX.cpp
   ExprObjC.cpp

clang/lib/AST/DeclBase.cpp

@@ -804,6 +804,7 @@ unsigned Decl::getIdentifierNamespaceForKind(Kind DeclKind) {
     case OMPCapturedExpr:
     case Empty:
     case LifetimeExtendedTemporary:
+    case RequiresExprBody:
       // Never looked up by name.
       return 0;
   }
@@ -1177,6 +1178,7 @@ DeclContext *DeclContext::getPrimaryContext() {
   case Decl::Captured:
   case Decl::OMPDeclareReduction:
   case Decl::OMPDeclareMapper:
+  case Decl::RequiresExprBody:
     // There is only one DeclContext for these entities.
     return this;
 

clang/lib/AST/DeclCXX.cpp

@@ -1968,6 +1968,16 @@ CXXDeductionGuideDecl *CXXDeductionGuideDecl::CreateDeserialized(ASTContext &C,
       QualType(), nullptr, SourceLocation());
 }
 
+RequiresExprBodyDecl *RequiresExprBodyDecl::Create(
+    ASTContext &C, DeclContext *DC, SourceLocation StartLoc) {
+  return new (C, DC) RequiresExprBodyDecl(C, DC, StartLoc);
+}
+
+RequiresExprBodyDecl *RequiresExprBodyDecl::CreateDeserialized(ASTContext &C,
+                                                               unsigned ID) {
+  return new (C, ID) RequiresExprBodyDecl(C, nullptr, SourceLocation());
+}
+
 void CXXMethodDecl::anchor() {}
 
 bool CXXMethodDecl::isStatic() const {

clang/lib/AST/Expr.cpp

@@ -3457,6 +3457,7 @@ bool Expr::HasSideEffects(const ASTContext &Ctx,
   case OpaqueValueExprClass:
   case SourceLocExprClass:
   case ConceptSpecializationExprClass:
+  case RequiresExprClass:
     // These never have a side-effect.
     return false;
 

clang/lib/AST/ExprCXX.cpp

@@ -17,6 +17,7 @@
 #include "clang/AST/DeclAccessPair.h"
 #include "clang/AST/DeclBase.h"
 #include "clang/AST/DeclCXX.h"
+#include "clang/AST/DeclTemplate.h"
 #include "clang/AST/DeclarationName.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/LambdaCapture.h"
@@ -1764,81 +1765,3 @@ CUDAKernelCallExpr *CUDAKernelCallExpr::CreateEmpty(const ASTContext &Ctx,
                            alignof(CUDAKernelCallExpr));
   return new (Mem) CUDAKernelCallExpr(NumArgs, Empty);
 }
-
-ConceptSpecializationExpr::ConceptSpecializationExpr(const ASTContext &C,
-    NestedNameSpecifierLoc NNS, SourceLocation TemplateKWLoc,
-    DeclarationNameInfo ConceptNameInfo, NamedDecl *FoundDecl,
-    ConceptDecl *NamedConcept, const ASTTemplateArgumentListInfo *ArgsAsWritten,
-    ArrayRef<TemplateArgument> ConvertedArgs,
-    const ConstraintSatisfaction *Satisfaction)
-    : Expr(ConceptSpecializationExprClass, C.BoolTy, VK_RValue, OK_Ordinary,
-           /*TypeDependent=*/false,
-           // All the flags below are set in setTemplateArguments.
-           /*ValueDependent=*/!Satisfaction, /*InstantiationDependent=*/false,
-           /*ContainsUnexpandedParameterPacks=*/false),
-      ConceptReference(NNS, TemplateKWLoc, ConceptNameInfo, FoundDecl,
-                       NamedConcept, ArgsAsWritten),
-      NumTemplateArgs(ConvertedArgs.size()),
-      Satisfaction(Satisfaction ?
-                   ASTConstraintSatisfaction::Create(C, *Satisfaction) :
-                   nullptr) {
-  setTemplateArguments(ConvertedArgs);
-}
-
-ConceptSpecializationExpr::ConceptSpecializationExpr(EmptyShell Empty,
-    unsigned NumTemplateArgs)
-    : Expr(ConceptSpecializationExprClass, Empty), ConceptReference(),
-      NumTemplateArgs(NumTemplateArgs) { }
-
-void ConceptSpecializationExpr::setTemplateArguments(
-    ArrayRef<TemplateArgument> Converted) {
-  assert(Converted.size() == NumTemplateArgs);
-  std::uninitialized_copy(Converted.begin(), Converted.end(),
-                          getTrailingObjects<TemplateArgument>());
-  bool IsInstantiationDependent = false;
-  bool ContainsUnexpandedParameterPack = false;
-  for (const TemplateArgument& Arg : Converted) {
-    if (Arg.isInstantiationDependent())
-      IsInstantiationDependent = true;
-    if (Arg.containsUnexpandedParameterPack())
-      ContainsUnexpandedParameterPack = true;
-    if (ContainsUnexpandedParameterPack && IsInstantiationDependent)
-      break;
-  }
-
-  // Currently guaranteed by the fact concepts can only be at namespace-scope.
-  assert(!NestedNameSpec ||
-         (!NestedNameSpec.getNestedNameSpecifier()->isInstantiationDependent() &&
-          !NestedNameSpec.getNestedNameSpecifier()
-              ->containsUnexpandedParameterPack()));
-  setInstantiationDependent(IsInstantiationDependent);
-  setContainsUnexpandedParameterPack(ContainsUnexpandedParameterPack);
-  assert((!isValueDependent() || isInstantiationDependent()) &&
-         "should not be value-dependent");
-}
-
-ConceptSpecializationExpr *
-ConceptSpecializationExpr::Create(const ASTContext &C,
-                                  NestedNameSpecifierLoc NNS,
-                                  SourceLocation TemplateKWLoc,
-                                  DeclarationNameInfo ConceptNameInfo,
-                                  NamedDecl *FoundDecl,
-                                  ConceptDecl *NamedConcept,
-                               const ASTTemplateArgumentListInfo *ArgsAsWritten,
-                                  ArrayRef<TemplateArgument> ConvertedArgs,
-                                  const ConstraintSatisfaction *Satisfaction) {
-  void *Buffer = C.Allocate(totalSizeToAlloc<TemplateArgument>(
-                                ConvertedArgs.size()));
-  return new (Buffer) ConceptSpecializationExpr(C, NNS, TemplateKWLoc,
-                                                ConceptNameInfo, FoundDecl,
-                                                NamedConcept, ArgsAsWritten,
-                                                ConvertedArgs, Satisfaction);
-}
-
-ConceptSpecializationExpr *
-ConceptSpecializationExpr::Create(ASTContext &C, EmptyShell Empty,
-                                  unsigned NumTemplateArgs) {
-  void *Buffer = C.Allocate(totalSizeToAlloc<TemplateArgument>(
-                                NumTemplateArgs));
-  return new (Buffer) ConceptSpecializationExpr(Empty, NumTemplateArgs);
-}

clang/lib/AST/ExprClassification.cpp

@@ -193,6 +193,7 @@ static Cl::Kinds ClassifyInternal(ASTContext &Ctx, const Expr *E) {
   case Expr::DesignatedInitUpdateExprClass:
   case Expr::SourceLocExprClass:
   case Expr::ConceptSpecializationExprClass:
+  case Expr::RequiresExprClass:
     return Cl::CL_PRValue;
 
   case Expr::ConstantExprClass:

clang/lib/AST/ExprConcepts.cpp

@@ -0,0 +1,185 @@
+//===- ExprCXX.cpp - (C++) Expression AST Node Implementation -------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the subclesses of Expr class declared in ExprCXX.h
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/AST/ExprConcepts.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/ASTConcept.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/DeclarationName.h"
+#include "clang/AST/DeclTemplate.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/NestedNameSpecifier.h"
+#include "clang/AST/TemplateBase.h"
+#include "clang/AST/Type.h"
+#include "clang/Basic/SourceLocation.h"
+#include "llvm/Support/TrailingObjects.h"
+#include <algorithm>
+#include <utility>
+#include <string>
+
+using namespace clang;
+
+ConceptSpecializationExpr::ConceptSpecializationExpr(const ASTContext &C,
+    NestedNameSpecifierLoc NNS, SourceLocation TemplateKWLoc,
+    DeclarationNameInfo ConceptNameInfo, NamedDecl *FoundDecl,
+    ConceptDecl *NamedConcept, const ASTTemplateArgumentListInfo *ArgsAsWritten,
+    ArrayRef<TemplateArgument> ConvertedArgs,
+    const ConstraintSatisfaction *Satisfaction)
+    : Expr(ConceptSpecializationExprClass, C.BoolTy, VK_RValue, OK_Ordinary,
+           /*TypeDependent=*/false,
+           // All the flags below are set in setTemplateArguments.
+           /*ValueDependent=*/!Satisfaction, /*InstantiationDependent=*/false,
+           /*ContainsUnexpandedParameterPacks=*/false),
+      ConceptReference(NNS, TemplateKWLoc, ConceptNameInfo, FoundDecl,
+                       NamedConcept, ArgsAsWritten),
+      NumTemplateArgs(ConvertedArgs.size()),
+      Satisfaction(Satisfaction ?
+                   ASTConstraintSatisfaction::Create(C, *Satisfaction) :
+                   nullptr) {
+  setTemplateArguments(ConvertedArgs);
+}
+
+ConceptSpecializationExpr::ConceptSpecializationExpr(EmptyShell Empty,
+    unsigned NumTemplateArgs)
+    : Expr(ConceptSpecializationExprClass, Empty), ConceptReference(),
+      NumTemplateArgs(NumTemplateArgs) { }
+
+void ConceptSpecializationExpr::setTemplateArguments(
+    ArrayRef<TemplateArgument> Converted) {
+  assert(Converted.size() == NumTemplateArgs);
+  std::uninitialized_copy(Converted.begin(), Converted.end(),
+                          getTrailingObjects<TemplateArgument>());
+  bool IsInstantiationDependent = false;
+  bool ContainsUnexpandedParameterPack = false;
+  for (const TemplateArgument& Arg : Converted) {
+    if (Arg.isInstantiationDependent())
+      IsInstantiationDependent = true;
+    if (Arg.containsUnexpandedParameterPack())
+      ContainsUnexpandedParameterPack = true;
+    if (ContainsUnexpandedParameterPack && IsInstantiationDependent)
+      break;
+  }
+
+  // Currently guaranteed by the fact concepts can only be at namespace-scope.
+  assert(!NestedNameSpec ||
+         (!NestedNameSpec.getNestedNameSpecifier()->isInstantiationDependent() &&
+          !NestedNameSpec.getNestedNameSpecifier()
+              ->containsUnexpandedParameterPack()));
+  setInstantiationDependent(IsInstantiationDependent);
+  setContainsUnexpandedParameterPack(ContainsUnexpandedParameterPack);
+  assert((!isValueDependent() || isInstantiationDependent()) &&
+         "should not be value-dependent");
+}
+
+ConceptSpecializationExpr *
+ConceptSpecializationExpr::Create(const ASTContext &C,
+                                  NestedNameSpecifierLoc NNS,
+                                  SourceLocation TemplateKWLoc,
+                                  DeclarationNameInfo ConceptNameInfo,
+                                  NamedDecl *FoundDecl,
+                                  ConceptDecl *NamedConcept,
+                               const ASTTemplateArgumentListInfo *ArgsAsWritten,
+                                  ArrayRef<TemplateArgument> ConvertedArgs,
+                                  const ConstraintSatisfaction *Satisfaction) {
+  void *Buffer = C.Allocate(totalSizeToAlloc<TemplateArgument>(
+                                ConvertedArgs.size()));
+  return new (Buffer) ConceptSpecializationExpr(C, NNS, TemplateKWLoc,
+                                                ConceptNameInfo, FoundDecl,
+                                                NamedConcept, ArgsAsWritten,
+                                                ConvertedArgs, Satisfaction);
+}
+
+ConceptSpecializationExpr *
+ConceptSpecializationExpr::Create(ASTContext &C, EmptyShell Empty,
+                                  unsigned NumTemplateArgs) {
+  void *Buffer = C.Allocate(totalSizeToAlloc<TemplateArgument>(
+                                NumTemplateArgs));
+  return new (Buffer) ConceptSpecializationExpr(Empty, NumTemplateArgs);
+}
+
+const TypeConstraint *
+concepts::ExprRequirement::ReturnTypeRequirement::getTypeConstraint() const {
+  assert(isTypeConstraint());
+  auto TPL =
+      TypeConstraintInfo.getPointer().get<TemplateParameterList *>();
+  return cast<TemplateTypeParmDecl>(TPL->getParam(0))
+      ->getTypeConstraint();
+}
+
+RequiresExpr::RequiresExpr(ASTContext &C, SourceLocation RequiresKWLoc,
+                           RequiresExprBodyDecl *Body,
+                           ArrayRef<ParmVarDecl *> LocalParameters,
+                           ArrayRef<concepts::Requirement *> Requirements,
+                           SourceLocation RBraceLoc)
+  : Expr(RequiresExprClass, C.BoolTy, VK_RValue, OK_Ordinary,
+         /*TD=*/false, /*VD=*/false, /*ID=*/false,
+         /*ContainsUnexpandedParameterPack=*/false),
+    NumLocalParameters(LocalParameters.size()),
+    NumRequirements(Requirements.size()), Body(Body), RBraceLoc(RBraceLoc) {
+  RequiresExprBits.IsSatisfied = false;
+  RequiresExprBits.RequiresKWLoc = RequiresKWLoc;
+  bool Dependent = false;
+  bool ContainsUnexpandedParameterPack = false;
+  for (ParmVarDecl *P : LocalParameters) {
+    Dependent |= P->getType()->isInstantiationDependentType();
+    ContainsUnexpandedParameterPack |=
+        P->getType()->containsUnexpandedParameterPack();
+  }
+  RequiresExprBits.IsSatisfied = true;
+  for (concepts::Requirement *R : Requirements) {
+    Dependent |= R->isDependent();
+    ContainsUnexpandedParameterPack |= R->containsUnexpandedParameterPack();
+    if (!Dependent) {
+      RequiresExprBits.IsSatisfied = R->isSatisfied();
+      if (!RequiresExprBits.IsSatisfied)
+        break;
+    }
+  }
+  std::copy(LocalParameters.begin(), LocalParameters.end(),
+            getTrailingObjects<ParmVarDecl *>());
+  std::copy(Requirements.begin(), Requirements.end(),
+            getTrailingObjects<concepts::Requirement *>());
+  RequiresExprBits.IsSatisfied |= Dependent;
+  setValueDependent(Dependent);
+  setInstantiationDependent(Dependent);
+  setContainsUnexpandedParameterPack(ContainsUnexpandedParameterPack);
+}
+
+RequiresExpr::RequiresExpr(ASTContext &C, EmptyShell Empty,
+                           unsigned NumLocalParameters,
+                           unsigned NumRequirements)
+  : Expr(RequiresExprClass, Empty), NumLocalParameters(NumLocalParameters),
+    NumRequirements(NumRequirements) { }
+
+RequiresExpr *
+RequiresExpr::Create(ASTContext &C, SourceLocation RequiresKWLoc,
+                     RequiresExprBodyDecl *Body,
+                     ArrayRef<ParmVarDecl *> LocalParameters,
+                     ArrayRef<concepts::Requirement *> Requirements,
+                     SourceLocation RBraceLoc) {
+  void *Mem =
+      C.Allocate(totalSizeToAlloc<ParmVarDecl *, concepts::Requirement *>(
+                     LocalParameters.size(), Requirements.size()),
+                 alignof(RequiresExpr));
+  return new (Mem) RequiresExpr(C, RequiresKWLoc, Body, LocalParameters,
+                                Requirements, RBraceLoc);
+}
+
+RequiresExpr *
+RequiresExpr::Create(ASTContext &C, EmptyShell Empty,
+                     unsigned NumLocalParameters, unsigned NumRequirements) {
+  void *Mem =
+      C.Allocate(totalSizeToAlloc<ParmVarDecl *, concepts::Requirement *>(
+                     NumLocalParameters, NumRequirements),
+                 alignof(RequiresExpr));
+  return new (Mem) RequiresExpr(C, Empty, NumLocalParameters, NumRequirements);
+}

clang/lib/AST/ExprConstant.cpp

@@ -9912,6 +9912,7 @@ public:
   bool VisitSizeOfPackExpr(const SizeOfPackExpr *E);
   bool VisitSourceLocExpr(const SourceLocExpr *E);
   bool VisitConceptSpecializationExpr(const ConceptSpecializationExpr *E);
+  bool VisitRequiresExpr(const RequiresExpr *E);
   // FIXME: Missing: array subscript of vector, member of vector
 };
 
@@ -12524,6 +12525,9 @@ bool IntExprEvaluator::VisitConceptSpecializationExpr(
   return Success(E->isSatisfied(), E);
 }
 
+bool IntExprEvaluator::VisitRequiresExpr(const RequiresExpr *E) {
+  return Success(E->isSatisfied(), E);
+}
 
 bool FixedPointExprEvaluator::VisitUnaryOperator(const UnaryOperator *E) {
   switch (E->getOpcode()) {
@@ -14182,6 +14186,7 @@ static ICEDiag CheckICE(const Expr* E, const ASTContext &Ctx) {
   case Expr::CXXScalarValueInitExprClass:
   case Expr::TypeTraitExprClass:
   case Expr::ConceptSpecializationExprClass:
+  case Expr::RequiresExprClass:
   case Expr::ArrayTypeTraitExprClass:
   case Expr::ExpressionTraitExprClass:
   case Expr::CXXNoexceptExprClass:

clang/lib/AST/ItaniumMangle.cpp

@@ -22,6 +22,7 @@
 #include "clang/AST/DeclOpenMP.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/TypeLoc.h"
@@ -3668,6 +3669,7 @@ recurse:
   case Expr::ConvertVectorExprClass:
   case Expr::StmtExprClass:
   case Expr::TypeTraitExprClass:
+  case Expr::RequiresExprClass:
   case Expr::ArrayTypeTraitExprClass:
   case Expr::ExpressionTraitExprClass:
   case Expr::VAArgExprClass:

clang/lib/AST/Stmt.cpp

@@ -16,6 +16,7 @@
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclGroup.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/ExprOpenMP.h"

clang/lib/AST/StmtPrinter.cpp

@@ -2269,6 +2269,60 @@ void StmtPrinter::VisitConceptSpecializationExpr(ConceptSpecializationExpr *E) {
                             Policy);
 }
 
+void StmtPrinter::VisitRequiresExpr(RequiresExpr *E) {
+  OS << "requires ";
+  auto LocalParameters = E->getLocalParameters();
+  if (!LocalParameters.empty()) {
+    OS << "(";
+    for (ParmVarDecl *LocalParam : LocalParameters) {
+      PrintRawDecl(LocalParam);
+      if (LocalParam != LocalParameters.back())
+        OS << ", ";
+    }
+
+    OS << ") ";
+  }
+  OS << "{ ";
+  auto Requirements = E->getRequirements();
+  for (concepts::Requirement *Req : Requirements) {
+    if (auto *TypeReq = dyn_cast<concepts::TypeRequirement>(Req)) {
+      if (TypeReq->isSubstitutionFailure())
+        OS << "<<error-type>>";
+      else
+        TypeReq->getType()->getType().print(OS, Policy);
+    } else if (auto *ExprReq = dyn_cast<concepts::ExprRequirement>(Req)) {
+      if (ExprReq->isCompound())
+        OS << "{ ";
+      if (ExprReq->isExprSubstitutionFailure())
+        OS << "<<error-expression>>";
+      else
+        PrintExpr(ExprReq->getExpr());
+      if (ExprReq->isCompound()) {
+        OS << " }";
+        if (ExprReq->getNoexceptLoc().isValid())
+          OS << " noexcept";
+        const auto &RetReq = ExprReq->getReturnTypeRequirement();
+        if (!RetReq.isEmpty()) {
+          OS << " -> ";
+          if (RetReq.isSubstitutionFailure())
+            OS << "<<error-type>>";
+          else if (RetReq.isTypeConstraint())
+            RetReq.getTypeConstraint()->print(OS, Policy);
+        }
+      }
+    } else {
+      auto *NestedReq = cast<concepts::NestedRequirement>(Req);
+      OS << "requires ";
+      if (NestedReq->isSubstitutionFailure())
+        OS << "<<error-expression>>";
+      else
+        PrintExpr(NestedReq->getConstraintExpr());
+    }
+    OS << "; ";
+  }
+  OS << "}";
+}
+
 // C++ Coroutines TS
 
 void StmtPrinter::VisitCoroutineBodyStmt(CoroutineBodyStmt *S) {

clang/lib/AST/StmtProfile.cpp

@@ -1340,6 +1340,49 @@ void StmtProfiler::VisitConceptSpecializationExpr(
     VisitTemplateArgument(Arg);
 }
 
+void StmtProfiler::VisitRequiresExpr(const RequiresExpr *S) {
+  VisitExpr(S);
+  ID.AddInteger(S->getLocalParameters().size());
+  for (ParmVarDecl *LocalParam : S->getLocalParameters())
+    VisitDecl(LocalParam);
+  ID.AddInteger(S->getRequirements().size());
+  for (concepts::Requirement *Req : S->getRequirements()) {
+    if (auto *TypeReq = dyn_cast<concepts::TypeRequirement>(Req)) {
+      ID.AddInteger(concepts::Requirement::RK_Type);
+      ID.AddBoolean(TypeReq->isSubstitutionFailure());
+      if (!TypeReq->isSubstitutionFailure())
+        VisitType(TypeReq->getType()->getType());
+    } else if (auto *ExprReq = dyn_cast<concepts::ExprRequirement>(Req)) {
+      ID.AddInteger(concepts::Requirement::RK_Compound);
+      ID.AddBoolean(ExprReq->isExprSubstitutionFailure());
+      if (!ExprReq->isExprSubstitutionFailure())
+        Visit(ExprReq->getExpr());
+      // C++2a [expr.prim.req.compound]p1 Example:
+      //    [...] The compound-requirement in C1 requires that x++ is a valid
+      //    expression. It is equivalent to the simple-requirement x++; [...]
+      // We therefore do not profile isSimple() here.
+      ID.AddBoolean(ExprReq->getNoexceptLoc().isValid());
+      const concepts::ExprRequirement::ReturnTypeRequirement &RetReq =
+          ExprReq->getReturnTypeRequirement();
+      if (RetReq.isEmpty()) {
+        ID.AddInteger(0);
+      } else if (RetReq.isTypeConstraint()) {
+        ID.AddInteger(1);
+        Visit(RetReq.getTypeConstraint()->getImmediatelyDeclaredConstraint());
+      } else {
+        assert(RetReq.isSubstitutionFailure());
+        ID.AddInteger(2);
+      }
+    } else {
+      ID.AddInteger(concepts::Requirement::RK_Nested);
+      auto *NestedReq = cast<concepts::NestedRequirement>(Req);
+      ID.AddBoolean(NestedReq->isSubstitutionFailure());
+      if (!NestedReq->isSubstitutionFailure())  
+        Visit(NestedReq->getConstraintExpr());
+    }
+  }
+}
+
 static Stmt::StmtClass DecodeOperatorCall(const CXXOperatorCallExpr *S,
                                           UnaryOperatorKind &UnaryOp,
                                           BinaryOperatorKind &BinaryOp) {

clang/lib/CodeGen/CGDecl.cpp

@@ -111,6 +111,7 @@ void CodeGenFunction::EmitDecl(const Decl &D) {
   case Decl::Empty:
   case Decl::Concept:
   case Decl::LifetimeExtendedTemporary:
+  case Decl::RequiresExprBody:
     // None of these decls require codegen support.
     return;
 

clang/lib/CodeGen/CGExprScalar.cpp

@@ -680,6 +680,10 @@ public:
     return Builder.getInt1(E->isSatisfied());
   }
 
+  Value *VisitRequiresExpr(const RequiresExpr *E) {
+    return Builder.getInt1(E->isSatisfied());
+  }
+
   Value *VisitArrayTypeTraitExpr(const ArrayTypeTraitExpr *E) {
     return llvm::ConstantInt::get(Builder.getInt32Ty(), E->getValue());
   }

clang/lib/Frontend/FrontendActions.cpp

@@ -429,6 +429,10 @@ private:
       return "ConstraintNormalization";
     case CodeSynthesisContext::ParameterMappingSubstitution:
       return "ParameterMappingSubstitution";
+    case CodeSynthesisContext::RequirementInstantiation:
+      return "RequirementInstantiation";
+    case CodeSynthesisContext::NestedRequirementConstraintsCheck:
+      return "NestedRequirementConstraintsCheck";
     }
     return "";
   }

clang/lib/Frontend/InitPreprocessor.cpp

@@ -385,6 +385,9 @@ static void InitializeStandardPredefinedMacros(const TargetInfo &TI,
     else
       Builder.defineMacro("__cplusplus", "199711L");
 
+    if (LangOpts.ConceptsTS)
+      Builder.defineMacro("__cpp_concepts", "201707L");
+
     // C++1z [cpp.predefined]p1:
     //   An integer literal of type std::size_t whose value is the alignment
     //   guaranteed by a call to operator new(std::size_t)

clang/lib/Parse/ParseDecl.cpp

@@ -6366,7 +6366,7 @@ void Parser::ParseFunctionDeclarator(Declarator &D,
     ProhibitAttributes(FnAttrs);
   } else {
     if (Tok.isNot(tok::r_paren))
-      ParseParameterDeclarationClause(D, FirstArgAttrs, ParamInfo,
+      ParseParameterDeclarationClause(D.getContext(), FirstArgAttrs, ParamInfo,
                                       EllipsisLoc);
     else if (RequiresArg)
       Diag(Tok, diag::err_argument_required_after_attribute);
@@ -6584,9 +6584,9 @@ void Parser::ParseFunctionDeclaratorIdentifierList(
 /// after the opening parenthesis. This function will not parse a K&R-style
 /// identifier list.
 ///
-/// D is the declarator being parsed.  If FirstArgAttrs is non-null, then the
-/// caller parsed those arguments immediately after the open paren - they should
-/// be considered to be part of the first parameter.
+/// DeclContext is the context of the declarator being parsed.  If FirstArgAttrs
+/// is non-null, then the caller parsed those attributes immediately after the
+/// open paren - they should be considered to be part of the first parameter.
 ///
 /// After returning, ParamInfo will hold the parsed parameters. EllipsisLoc will
 /// be the location of the ellipsis, if any was parsed.
@@ -6612,7 +6612,7 @@ void Parser::ParseFunctionDeclaratorIdentifierList(
 /// [C++11] attribute-specifier-seq parameter-declaration
 ///
 void Parser::ParseParameterDeclarationClause(
-       Declarator &D,
+       DeclaratorContext DeclaratorContext,
        ParsedAttributes &FirstArgAttrs,
        SmallVectorImpl<DeclaratorChunk::ParamInfo> &ParamInfo,
        SourceLocation &EllipsisLoc) {
@@ -6661,9 +6661,11 @@ void Parser::ParseParameterDeclarationClause(
     // "LambdaExprParameterContext", because we must accept either
     // 'declarator' or 'abstract-declarator' here.
     Declarator ParmDeclarator(
-        DS, D.getContext() == DeclaratorContext::LambdaExprContext
-                ? DeclaratorContext::LambdaExprParameterContext
-                : DeclaratorContext::PrototypeContext);
+        DS, DeclaratorContext == DeclaratorContext::RequiresExprContext
+                ? DeclaratorContext::RequiresExprContext
+                : DeclaratorContext == DeclaratorContext::LambdaExprContext
+                      ? DeclaratorContext::LambdaExprParameterContext
+                      : DeclaratorContext::PrototypeContext);
     ParseDeclarator(ParmDeclarator);
 
     // Parse GNU attributes, if present.
@@ -6717,7 +6719,7 @@ void Parser::ParseParameterDeclarationClause(
         SourceLocation EqualLoc = Tok.getLocation();
 
         // Parse the default argument
-        if (D.getContext() == DeclaratorContext::MemberContext) {
+        if (DeclaratorContext == DeclaratorContext::MemberContext) {
           // If we're inside a class definition, cache the tokens
           // corresponding to the default argument. We'll actually parse
           // them when we see the end of the class definition.

clang/lib/Parse/ParseExpr.cpp

@@ -756,6 +756,7 @@ class CastExpressionIdValidator final : public CorrectionCandidateCallback {
 /// [C++11] user-defined-literal
 ///         '(' expression ')'
 /// [C11]   generic-selection
+/// [C++2a] requires-expression
 ///         '__func__'        [C99 6.4.2.2]
 /// [GNU]   '__FUNCTION__'
 /// [MS]    '__FUNCDNAME__'
@@ -1601,6 +1602,9 @@ ExprResult Parser::ParseCastExpression(CastParseKind ParseKind,
       *NotPrimaryExpression = true;
     return ParseCXXDeleteExpression(false, Tok.getLocation());
 
+  case tok::kw_requires: // [C++2a] requires-expression
+    return ParseRequiresExpression();
+
   case tok::kw_noexcept: { // [C++0x] 'noexcept' '(' expression ')'
     if (NotPrimaryExpression)
       *NotPrimaryExpression = true;

clang/lib/Parse/ParseExprCXX.cpp

@@ -11,7 +11,9 @@
 //===----------------------------------------------------------------------===//
 #include "clang/Parse/Parser.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/AST/Decl.h"
 #include "clang/AST/DeclTemplate.h"
+#include "clang/AST/ExprCXX.h"
 #include "clang/Basic/PrettyStackTrace.h"
 #include "clang/Lex/LiteralSupport.h"
 #include "clang/Parse/ParseDiagnostic.h"
@@ -1299,9 +1301,9 @@ ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
       Actions.RecordParsingTemplateParameterDepth(
           CurTemplateDepthTracker.getOriginalDepth());
 
-      ParseParameterDeclarationClause(D, Attr, ParamInfo, EllipsisLoc);
-
-      // For a generic lambda, each 'auto' within the parameter declaration
+      ParseParameterDeclarationClause(D.getContext(), Attr, ParamInfo,
+                                      EllipsisLoc);
+      // For a generic lambda, each 'auto' within the parameter declaration 
       // clause creates a template type parameter, so increment the depth.
       // If we've parsed any explicit template parameters, then the depth will
       // have already been incremented. So we make sure that at most a single
@@ -3255,6 +3257,324 @@ Parser::ParseCXXDeleteExpression(bool UseGlobal, SourceLocation Start) {
   return Actions.ActOnCXXDelete(Start, UseGlobal, ArrayDelete, Operand.get());
 }
 
+/// ParseRequiresExpression - Parse a C++2a requires-expression.
+/// C++2a [expr.prim.req]p1
+///     A requires-expression provides a concise way to express requirements on
+///     template arguments. A requirement is one that can be checked by name
+///     lookup (6.4) or by checking properties of types and expressions.
+///
+///     requires-expression:
+///         'requires' requirement-parameter-list[opt] requirement-body
+///
+///     requirement-parameter-list:
+///         '(' parameter-declaration-clause[opt] ')'
+///
+///     requirement-body:
+///         '{' requirement-seq '}'
+///
+///     requirement-seq:
+///         requirement
+///         requirement-seq requirement
+///
+///     requirement:
+///         simple-requirement
+///         type-requirement
+///         compound-requirement
+///         nested-requirement
+ExprResult Parser::ParseRequiresExpression() {
+  assert(Tok.is(tok::kw_requires) && "Expected 'requires' keyword");
+  SourceLocation RequiresKWLoc = ConsumeToken(); // Consume 'requires'
+
+  llvm::SmallVector<ParmVarDecl *, 2> LocalParameterDecls;
+  if (Tok.is(tok::l_paren)) {
+    // requirement parameter list is present.
+    ParseScope LocalParametersScope(this, Scope::FunctionPrototypeScope |
+                                    Scope::DeclScope);
+    BalancedDelimiterTracker Parens(*this, tok::l_paren);
+    Parens.consumeOpen();
+    if (!Tok.is(tok::r_paren)) {
+      ParsedAttributes FirstArgAttrs(getAttrFactory());
+      SourceLocation EllipsisLoc;
+      llvm::SmallVector<DeclaratorChunk::ParamInfo, 2> LocalParameters;
+      DiagnosticErrorTrap Trap(Diags);
+      ParseParameterDeclarationClause(DeclaratorContext::RequiresExprContext,
+                                      FirstArgAttrs, LocalParameters,
+                                      EllipsisLoc);
+      if (EllipsisLoc.isValid())
+        Diag(EllipsisLoc, diag::err_requires_expr_parameter_list_ellipsis);
+      for (auto &ParamInfo : LocalParameters)
+        LocalParameterDecls.push_back(cast<ParmVarDecl>(ParamInfo.Param));
+      if (Trap.hasErrorOccurred())
+        SkipUntil(tok::r_paren, StopBeforeMatch);
+    }
+    Parens.consumeClose();
+  }
+
+  BalancedDelimiterTracker Braces(*this, tok::l_brace);
+  if (Braces.expectAndConsume())
+    return ExprError();
+
+  // Start of requirement list
+  llvm::SmallVector<concepts::Requirement *, 2> Requirements;
+
+  // C++2a [expr.prim.req]p2
+  //   Expressions appearing within a requirement-body are unevaluated operands.
+  EnterExpressionEvaluationContext Ctx(
+      Actions, Sema::ExpressionEvaluationContext::Unevaluated);
+
+  ParseScope BodyScope(this, Scope::DeclScope);
+  RequiresExprBodyDecl *Body = Actions.ActOnStartRequiresExpr(
+      RequiresKWLoc, LocalParameterDecls, getCurScope());
+
+  if (Tok.is(tok::r_brace)) {
+    // Grammar does not allow an empty body.
+    // requirement-body:
+    //   { requirement-seq }
+    // requirement-seq:
+    //   requirement
+    //   requirement-seq requirement
+    Diag(Tok, diag::err_empty_requires_expr);
+    // Continue anyway and produce a requires expr with no requirements.
+  } else {
+    while (!Tok.is(tok::r_brace)) {
+      switch (Tok.getKind()) {
+      case tok::l_brace: {
+        // Compound requirement
+        // C++ [expr.prim.req.compound]
+        //     compound-requirement:
+        //         '{' expression '}' 'noexcept'[opt]
+        //             return-type-requirement[opt] ';'
+        //     return-type-requirement:
+        //         trailing-return-type
+        //         '->' cv-qualifier-seq[opt] constrained-parameter
+        //             cv-qualifier-seq[opt] abstract-declarator[opt]
+        BalancedDelimiterTracker ExprBraces(*this, tok::l_brace);
+        ExprBraces.consumeOpen();
+        ExprResult Expression =
+            Actions.CorrectDelayedTyposInExpr(ParseExpression());
+        if (!Expression.isUsable()) {
+          ExprBraces.skipToEnd();
+          SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+          break;
+        }
+        if (ExprBraces.consumeClose())
+          ExprBraces.skipToEnd();
+
+        concepts::Requirement *Req = nullptr;
+        SourceLocation NoexceptLoc;
+        TryConsumeToken(tok::kw_noexcept, NoexceptLoc);
+        if (Tok.is(tok::semi)) {
+          Req = Actions.ActOnCompoundRequirement(Expression.get(), NoexceptLoc);
+          if (Req)
+            Requirements.push_back(Req);
+          break;
+        }
+        if (!TryConsumeToken(tok::arrow))
+          // User probably forgot the arrow, remind them and try to continue.
+          Diag(Tok, diag::err_requires_expr_missing_arrow)
+              << FixItHint::CreateInsertion(Tok.getLocation(), "->");
+        // Try to parse a 'type-constraint'
+        CXXScopeSpec SS;
+        if (ParseOptionalCXXScopeSpecifier(SS, ParsedType(),
+                                           /*EnteringContext=*/false,
+                                           /*MayBePseudoDestructor=*/nullptr,
+                                           // If this is not a type-constraint,
+                                           // then this scope-spec is part of
+                                           // the typename of a non-type
+                                           // template parameter
+                                           /*IsTypename=*/true,
+                                           /*LastII=*/nullptr,
+                                           // We won't find concepts in
+                                           // non-namespaces anyway, so might as
+                                           // well parse this correctly for
+                                           // possible type names.
+                                           /*OnlyNamespace=*/false,
+                                           /*SuppressDiagnostic=*/true)) {
+          SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+          break;
+        }
+        if (TryAnnotateTypeConstraint()) {
+          SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+          break;
+        }
+        if (!isTypeConstraintAnnotation()) {
+          Diag(Tok, diag::err_requires_expr_expected_type_constraint);
+          SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+          break;
+        }
+        if (Tok.is(tok::annot_cxxscope))
+          ConsumeAnnotationToken();
+
+        Req = Actions.ActOnCompoundRequirement(
+            Expression.get(), NoexceptLoc, SS, takeTemplateIdAnnotation(Tok),
+            TemplateParameterDepth);
+        ConsumeAnnotationToken();
+        if (Req)
+          Requirements.push_back(Req);
+        break;
+      }
+      default: {
+        bool PossibleRequiresExprInSimpleRequirement = false;
+        if (Tok.is(tok::kw_requires)) {
+          auto IsNestedRequirement = [&] {
+            RevertingTentativeParsingAction TPA(*this);
+            ConsumeToken(); // 'requires'
+            if (Tok.is(tok::l_brace))
+              // This is a requires expression
+              // requires (T t) {
+              //   requires { t++; };
+              //   ...      ^
+              // }
+              return false;
+            if (Tok.is(tok::l_paren)) {
+              // This might be the parameter list of a requires expression
+              ConsumeParen();
+              auto Res = TryParseParameterDeclarationClause();
+              if (Res != TPResult::False) {
+                // Skip to the closing parenthesis
+                // FIXME: Don't traverse these tokens twice (here and in
+                //  TryParseParameterDeclarationClause).
+                unsigned Depth = 1;
+                while (Depth != 0) {
+                  if (Tok.is(tok::l_paren))
+                    Depth++;
+                  else if (Tok.is(tok::r_paren))
+                    Depth--;
+                  ConsumeAnyToken();
+                }
+                // requires (T t) {
+                //   requires () ?
+                //   ...         ^
+                //   - OR -
+                //   requires (int x) ?
+                //   ...              ^
+                // }
+                if (Tok.is(tok::l_brace))
+                  // requires (...) {
+                  //                ^ - a requires expression as a
+                  //                    simple-requirement.
+                  return false;
+              }
+            }
+            return true;
+          };
+          if (IsNestedRequirement()) {
+            ConsumeToken();
+            // Nested requirement
+            // C++ [expr.prim.req.nested]
+            //     nested-requirement:
+            //         'requires' constraint-expression ';'
+            ExprResult ConstraintExpr =
+                Actions.CorrectDelayedTyposInExpr(ParseConstraintExpression());
+            if (ConstraintExpr.isInvalid() || !ConstraintExpr.isUsable()) {
+              SkipUntil(tok::semi, tok::r_brace,
+                        SkipUntilFlags::StopBeforeMatch);
+              break;
+            }
+            if (auto *Req =
+                    Actions.ActOnNestedRequirement(ConstraintExpr.get()))
+              Requirements.push_back(Req);
+            else {
+              SkipUntil(tok::semi, tok::r_brace,
+                        SkipUntilFlags::StopBeforeMatch);
+              break;
+            }
+            break;
+          } else
+            PossibleRequiresExprInSimpleRequirement = true;
+        } else if (Tok.is(tok::kw_typename)) {
+          // This might be 'typename T::value_type;' (a type requirement) or
+          // 'typename T::value_type{};' (a simple requirement).
+          TentativeParsingAction TPA(*this);
+
+          // We need to consume the typename to allow 'requires { typename a; }'
+          SourceLocation TypenameKWLoc = ConsumeToken();
+          if (TryAnnotateCXXScopeToken()) {
+            SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+            break;
+          }
+          CXXScopeSpec SS;
+          if (Tok.is(tok::annot_cxxscope)) {
+            Actions.RestoreNestedNameSpecifierAnnotation(
+                Tok.getAnnotationValue(), Tok.getAnnotationRange(), SS);
+            ConsumeAnnotationToken();
+          }
+
+          if (Tok.isOneOf(tok::identifier, tok::annot_template_id) &&
+              !NextToken().isOneOf(tok::l_brace, tok::l_paren)) {
+            TPA.Commit();
+            SourceLocation NameLoc = Tok.getLocation();
+            IdentifierInfo *II = nullptr;
+            TemplateIdAnnotation *TemplateId = nullptr;
+            if (Tok.is(tok::identifier)) {
+              II = Tok.getIdentifierInfo();
+              ConsumeToken();
+            } else {
+              TemplateId = takeTemplateIdAnnotation(Tok);
+              ConsumeAnnotationToken();
+            }
+
+            if (auto *Req = Actions.ActOnTypeRequirement(TypenameKWLoc, SS,
+                                                         NameLoc, II,
+                                                         TemplateId)) {
+              Requirements.push_back(Req);
+            }
+            break;
+          }
+          TPA.Revert();
+        }
+        // Simple requirement
+        // C++ [expr.prim.req.simple]
+        //     simple-requirement:
+        //         expression ';'
+        SourceLocation StartLoc = Tok.getLocation();
+        ExprResult Expression =
+            Actions.CorrectDelayedTyposInExpr(ParseExpression());
+        if (!Expression.isUsable()) {
+          SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+          break;
+        }
+        if (!Expression.isInvalid() && PossibleRequiresExprInSimpleRequirement)
+          Diag(StartLoc, diag::warn_requires_expr_in_simple_requirement)
+              << FixItHint::CreateInsertion(StartLoc, "requires");
+        if (auto *Req = Actions.ActOnSimpleRequirement(Expression.get()))
+          Requirements.push_back(Req);
+        else {
+          SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+          break;
+        }
+        // User may have tried to put some compound requirement stuff here
+        if (Tok.is(tok::kw_noexcept)) {
+          Diag(Tok, diag::err_requires_expr_simple_requirement_noexcept)
+              << FixItHint::CreateInsertion(StartLoc, "{")
+              << FixItHint::CreateInsertion(Tok.getLocation(), "}");
+          SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+          break;
+        }
+        break;
+      }
+      }
+      if (ExpectAndConsumeSemi(diag::err_expected_semi_requirement)) {
+        SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+        TryConsumeToken(tok::semi);
+        break;
+      }
+    }
+    if (Requirements.empty()) {
+      // Don't emit an empty requires expr here to avoid confusing the user with
+      // other diagnostics quoting an empty requires expression they never
+      // wrote.
+      Braces.consumeClose();
+      Actions.ActOnFinishRequiresExpr();
+      return ExprError();
+    }
+  }
+  Braces.consumeClose();
+  Actions.ActOnFinishRequiresExpr();
+  return Actions.ActOnRequiresExpr(RequiresKWLoc, Body, LocalParameterDecls,
+                                   Requirements, Braces.getCloseLocation());
+}
+
 static TypeTrait TypeTraitFromTokKind(tok::TokenKind kind) {
   switch (kind) {
   default: llvm_unreachable("Not a known type trait");

clang/lib/Sema/Sema.cpp

@@ -1261,7 +1261,8 @@ DeclContext *Sema::getFunctionLevelDeclContext() {
   DeclContext *DC = CurContext;
 
   while (true) {
-    if (isa<BlockDecl>(DC) || isa<EnumDecl>(DC) || isa<CapturedDecl>(DC)) {
+    if (isa<BlockDecl>(DC) || isa<EnumDecl>(DC) || isa<CapturedDecl>(DC) ||
+        isa<RequiresExprBodyDecl>(DC)) {
       DC = DC->getParent();
     } else if (isa<CXXMethodDecl>(DC) &&
                cast<CXXMethodDecl>(DC)->getOverloadedOperator() == OO_Call &&

clang/lib/Sema/SemaConcept.cpp

@@ -17,7 +17,10 @@
 #include "clang/Sema/SemaDiagnostic.h"
 #include "clang/Sema/TemplateDeduction.h"
 #include "clang/Sema/Template.h"
-#include "clang/AST/ExprCXX.h"
+#include "clang/Sema/Overload.h"
+#include "clang/Sema/Initialization.h"
+#include "clang/Sema/SemaInternal.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/Basic/OperatorPrecedence.h"
 #include "llvm/ADT/DenseMap.h"
@@ -336,6 +339,118 @@ bool Sema::EnsureTemplateArgumentListConstraints(
   return false;
 }
 
+static void diagnoseUnsatisfiedRequirement(Sema &S,
+                                           concepts::ExprRequirement *Req,
+                                           bool First) {
+  assert(!Req->isSatisfied()
+         && "Diagnose() can only be used on an unsatisfied requirement");
+  switch (Req->getSatisfactionStatus()) {
+    case concepts::ExprRequirement::SS_Dependent:
+      llvm_unreachable("Diagnosing a dependent requirement");
+      break;
+    case concepts::ExprRequirement::SS_ExprSubstitutionFailure: {
+      auto *SubstDiag = Req->getExprSubstitutionDiagnostic();
+      if (!SubstDiag->DiagMessage.empty())
+        S.Diag(SubstDiag->DiagLoc,
+               diag::note_expr_requirement_expr_substitution_error)
+               << (int)First << SubstDiag->SubstitutedEntity
+               << SubstDiag->DiagMessage;
+      else
+        S.Diag(SubstDiag->DiagLoc,
+               diag::note_expr_requirement_expr_unknown_substitution_error)
+            << (int)First << SubstDiag->SubstitutedEntity;
+      break;
+    }
+    case concepts::ExprRequirement::SS_NoexceptNotMet:
+      S.Diag(Req->getNoexceptLoc(),
+             diag::note_expr_requirement_noexcept_not_met)
+          << (int)First << Req->getExpr();
+      break;
+    case concepts::ExprRequirement::SS_TypeRequirementSubstitutionFailure: {
+      auto *SubstDiag =
+          Req->getReturnTypeRequirement().getSubstitutionDiagnostic();
+      if (!SubstDiag->DiagMessage.empty())
+        S.Diag(SubstDiag->DiagLoc,
+               diag::note_expr_requirement_type_requirement_substitution_error)
+            << (int)First << SubstDiag->SubstitutedEntity
+            << SubstDiag->DiagMessage;
+      else
+        S.Diag(SubstDiag->DiagLoc,
+               diag::note_expr_requirement_type_requirement_unknown_substitution_error)
+            << (int)First << SubstDiag->SubstitutedEntity;
+      break;
+    }
+    case concepts::ExprRequirement::SS_ConstraintsNotSatisfied: {
+      ConceptSpecializationExpr *ConstraintExpr =
+          Req->getReturnTypeRequirementSubstitutedConstraintExpr();
+      if (ConstraintExpr->getTemplateArgsAsWritten()->NumTemplateArgs == 1)
+        // A simple case - expr type is the type being constrained and the concept
+        // was not provided arguments.
+        S.Diag(ConstraintExpr->getBeginLoc(),
+               diag::note_expr_requirement_constraints_not_satisfied_simple)
+            << (int)First << S.BuildDecltypeType(Req->getExpr(),
+                                                 Req->getExpr()->getBeginLoc())
+            << ConstraintExpr->getNamedConcept();
+      else
+        S.Diag(ConstraintExpr->getBeginLoc(),
+               diag::note_expr_requirement_constraints_not_satisfied)
+            << (int)First << ConstraintExpr;
+      S.DiagnoseUnsatisfiedConstraint(ConstraintExpr->getSatisfaction());
+      break;
+    }
+    case concepts::ExprRequirement::SS_Satisfied:
+      llvm_unreachable("We checked this above");
+  }
+}
+
+static void diagnoseUnsatisfiedRequirement(Sema &S,
+                                           concepts::TypeRequirement *Req,
+                                           bool First) {
+  assert(!Req->isSatisfied()
+         && "Diagnose() can only be used on an unsatisfied requirement");
+  switch (Req->getSatisfactionStatus()) {
+  case concepts::TypeRequirement::SS_Dependent:
+    llvm_unreachable("Diagnosing a dependent requirement");
+    return;
+  case concepts::TypeRequirement::SS_SubstitutionFailure: {
+    auto *SubstDiag = Req->getSubstitutionDiagnostic();
+    if (!SubstDiag->DiagMessage.empty())
+      S.Diag(SubstDiag->DiagLoc,
+             diag::note_type_requirement_substitution_error) << (int)First
+          << SubstDiag->SubstitutedEntity << SubstDiag->DiagMessage;
+    else
+      S.Diag(SubstDiag->DiagLoc,
+             diag::note_type_requirement_unknown_substitution_error)
+          << (int)First << SubstDiag->SubstitutedEntity;
+    return;
+  }
+  default:
+    llvm_unreachable("Unknown satisfaction status");
+    return;
+  }
+}
+
+static void diagnoseUnsatisfiedRequirement(Sema &S,
+                                           concepts::NestedRequirement *Req,
+                                           bool First) {
+  if (Req->isSubstitutionFailure()) {
+    concepts::Requirement::SubstitutionDiagnostic *SubstDiag =
+        Req->getSubstitutionDiagnostic();
+    if (!SubstDiag->DiagMessage.empty())
+      S.Diag(SubstDiag->DiagLoc,
+             diag::note_nested_requirement_substitution_error)
+             << (int)First << SubstDiag->SubstitutedEntity
+             << SubstDiag->DiagMessage;
+    else
+      S.Diag(SubstDiag->DiagLoc,
+             diag::note_nested_requirement_unknown_substitution_error)
+          << (int)First << SubstDiag->SubstitutedEntity;
+    return;
+  }
+  S.DiagnoseUnsatisfiedConstraint(Req->getConstraintSatisfaction(), First);
+}
+
+
 static void diagnoseWellFormedUnsatisfiedConstraintExpr(Sema &S,
                                                         Expr *SubstExpr,
                                                         bool First = true) {
@@ -412,6 +527,19 @@ static void diagnoseWellFormedUnsatisfiedConstraintExpr(Sema &S,
     }
     S.DiagnoseUnsatisfiedConstraint(CSE->getSatisfaction());
     return;
+  } else if (auto *RE = dyn_cast<RequiresExpr>(SubstExpr)) {
+    for (concepts::Requirement *Req : RE->getRequirements())
+      if (!Req->isDependent() && !Req->isSatisfied()) {
+        if (auto *E = dyn_cast<concepts::ExprRequirement>(Req))
+          diagnoseUnsatisfiedRequirement(S, E, First);
+        else if (auto *T = dyn_cast<concepts::TypeRequirement>(Req))
+          diagnoseUnsatisfiedRequirement(S, T, First);
+        else
+          diagnoseUnsatisfiedRequirement(
+              S, cast<concepts::NestedRequirement>(Req), First);
+        break;
+      }
+    return;
   }
 
   S.Diag(SubstExpr->getSourceRange().getBegin(),
@@ -434,11 +562,11 @@ static void diagnoseUnsatisfiedConstraintExpr(
       Record.template get<Expr *>(), First);
 }
 
-void Sema::DiagnoseUnsatisfiedConstraint(
-    const ConstraintSatisfaction& Satisfaction) {
+void
+Sema::DiagnoseUnsatisfiedConstraint(const ConstraintSatisfaction& Satisfaction,
+                                    bool First) {
   assert(!Satisfaction.IsSatisfied &&
          "Attempted to diagnose a satisfied constraint");
-  bool First = true;
   for (auto &Pair : Satisfaction.Details) {
     diagnoseUnsatisfiedConstraintExpr(*this, Pair.first, Pair.second, First);
     First = false;
@@ -446,10 +574,10 @@ void Sema::DiagnoseUnsatisfiedConstraint(
 }
 
 void Sema::DiagnoseUnsatisfiedConstraint(
-    const ASTConstraintSatisfaction &Satisfaction) {
+    const ASTConstraintSatisfaction &Satisfaction,
+    bool First) {
   assert(!Satisfaction.IsSatisfied &&
          "Attempted to diagnose a satisfied constraint");
-  bool First = true;
   for (auto &Pair : Satisfaction) {
     diagnoseUnsatisfiedConstraintExpr(*this, Pair.first, Pair.second, First);
     First = false;
@@ -826,3 +954,67 @@ bool Sema::MaybeEmitAmbiguousAtomicConstraintsDiagnostic(NamedDecl *D1,
       << AmbiguousAtomic2->getSourceRange();
   return true;
 }
+
+concepts::ExprRequirement::ExprRequirement(
+    Expr *E, bool IsSimple, SourceLocation NoexceptLoc,
+    ReturnTypeRequirement Req, SatisfactionStatus Status,
+    ConceptSpecializationExpr *SubstitutedConstraintExpr) :
+    Requirement(IsSimple ? RK_Simple : RK_Compound, Status == SS_Dependent,
+                Status == SS_Dependent &&
+                (E->containsUnexpandedParameterPack() ||
+                 Req.containsUnexpandedParameterPack()),
+                Status == SS_Satisfied), Value(E), NoexceptLoc(NoexceptLoc),
+    TypeReq(Req), SubstitutedConstraintExpr(SubstitutedConstraintExpr),
+    Status(Status) {
+  assert((!IsSimple || (Req.isEmpty() && NoexceptLoc.isInvalid())) &&
+         "Simple requirement must not have a return type requirement or a "
+         "noexcept specification");
+  assert((Status > SS_TypeRequirementSubstitutionFailure && Req.isTypeConstraint()) ==
+         (SubstitutedConstraintExpr != nullptr));
+}
+
+concepts::ExprRequirement::ExprRequirement(
+    SubstitutionDiagnostic *ExprSubstDiag, bool IsSimple,
+    SourceLocation NoexceptLoc, ReturnTypeRequirement Req) :
+    Requirement(IsSimple ? RK_Simple : RK_Compound, Req.isDependent(),
+                Req.containsUnexpandedParameterPack(), /*IsSatisfied=*/false),
+    Value(ExprSubstDiag), NoexceptLoc(NoexceptLoc), TypeReq(Req),
+    Status(SS_ExprSubstitutionFailure) {
+  assert((!IsSimple || (Req.isEmpty() && NoexceptLoc.isInvalid())) &&
+         "Simple requirement must not have a return type requirement or a "
+         "noexcept specification");
+}
+
+concepts::ExprRequirement::ReturnTypeRequirement::
+ReturnTypeRequirement(TemplateParameterList *TPL) :
+    TypeConstraintInfo(TPL, 0) {
+  assert(TPL->size() == 1);
+  const TypeConstraint *TC =
+      cast<TemplateTypeParmDecl>(TPL->getParam(0))->getTypeConstraint();
+  assert(TC &&
+         "TPL must have a template type parameter with a type constraint");
+  auto *Constraint =
+      cast_or_null<ConceptSpecializationExpr>(
+          TC->getImmediatelyDeclaredConstraint());
+  bool Dependent = false;
+  if (Constraint->getTemplateArgsAsWritten()) {
+    for (auto &ArgLoc :
+         Constraint->getTemplateArgsAsWritten()->arguments().drop_front(1)) {
+      if (ArgLoc.getArgument().isDependent()) {
+        Dependent = true;
+        break;
+      }
+    }
+  }
+  TypeConstraintInfo.setInt(Dependent ? 1 : 0);
+}
+
+concepts::TypeRequirement::TypeRequirement(TypeSourceInfo *T) :
+    Requirement(RK_Type, T->getType()->isDependentType(),
+                T->getType()->containsUnexpandedParameterPack(),
+                // We reach this ctor with either dependent types (in which
+                // IsSatisfied doesn't matter) or with non-dependent type in
+                // which the existence of the type indicates satisfaction.
+                /*IsSatisfied=*/true
+                ), Value(T),
+    Status(T->getType()->isDependentType() ? SS_Dependent : SS_Satisfied) {}

clang/lib/Sema/SemaDecl.cpp

@@ -6468,6 +6468,8 @@ static bool shouldConsiderLinkage(const VarDecl *VD) {
     return true;
   if (DC->isRecord())
     return false;
+  if (isa<RequiresExprBodyDecl>(DC))
+    return false;
   llvm_unreachable("Unexpected context");
 }
 

clang/lib/Sema/SemaExceptionSpec.cpp

@@ -1386,6 +1386,7 @@ CanThrowResult Sema::canThrow(const Stmt *S) {
   case Expr::StringLiteralClass:
   case Expr::SourceLocExprClass:
   case Expr::ConceptSpecializationExprClass:
+  case Expr::RequiresExprClass:
     // These expressions can never throw.
     return CT_Cannot;
 

clang/lib/Sema/SemaExpr.cpp

@@ -350,6 +350,17 @@ bool Sema::DiagnoseUseOfDecl(NamedDecl *D, ArrayRef<SourceLocation> Locs,
     }
   }
 
+  if (isa<ParmVarDecl>(D) && isa<RequiresExprBodyDecl>(D->getDeclContext()) &&
+      !isUnevaluatedContext()) {
+    // C++ [expr.prim.req.nested] p3
+    //   A local parameter shall only appear as an unevaluated operand
+    //   (Clause 8) within the constraint-expression.
+    Diag(Loc, diag::err_requires_expr_parameter_referenced_in_evaluated_context)
+        << D;
+    Diag(D->getLocation(), diag::note_entity_declared_at) << D;
+    return true;
+  }
+
   return false;
 }
 
@@ -1904,7 +1915,7 @@ Sema::BuildDeclRefExpr(ValueDecl *D, QualType Ty, ExprValueKind VK,
   bool RefersToCapturedVariable =
       isa<VarDecl>(D) &&
       NeedToCaptureVariable(cast<VarDecl>(D), NameInfo.getLoc());
-
+  
   DeclRefExpr *E = DeclRefExpr::Create(
       Context, NNS, TemplateKWLoc, D, RefersToCapturedVariable, NameInfo, Ty,
       VK, FoundD, TemplateArgs, getNonOdrUseReasonInCurrentContext(D));

clang/lib/Sema/SemaExprCXX.cpp

@@ -11,6 +11,7 @@
 ///
 //===----------------------------------------------------------------------===//
 
+#include "clang/Sema/Template.h"
 #include "clang/Sema/SemaInternal.h"
 #include "TreeTransform.h"
 #include "TypeLocBuilder.h"
@@ -8331,3 +8332,215 @@ Sema::CheckMicrosoftIfExistsSymbol(Scope *S, SourceLocation KeywordLoc,
 
   return CheckMicrosoftIfExistsSymbol(S, SS, TargetNameInfo);
 }
+
+concepts::Requirement *Sema::ActOnSimpleRequirement(Expr *E) {
+  return BuildExprRequirement(E, /*IsSimple=*/true,
+                              /*NoexceptLoc=*/SourceLocation(),
+                              /*ReturnTypeRequirement=*/{});
+}
+
+concepts::Requirement *
+Sema::ActOnTypeRequirement(SourceLocation TypenameKWLoc, CXXScopeSpec &SS,
+                           SourceLocation NameLoc, IdentifierInfo *TypeName,
+                           TemplateIdAnnotation *TemplateId) {
+  assert(((!TypeName && TemplateId) || (TypeName && !TemplateId)) &&
+         "Exactly one of TypeName and TemplateId must be specified.");
+  TypeSourceInfo *TSI = nullptr;
+  if (TypeName) {
+    QualType T = CheckTypenameType(ETK_Typename, TypenameKWLoc,
+                                   SS.getWithLocInContext(Context), *TypeName,
+                                   NameLoc, &TSI, /*DeducedTypeContext=*/false);
+    if (T.isNull())
+      return nullptr;
+  } else {
+    ASTTemplateArgsPtr ArgsPtr(TemplateId->getTemplateArgs(),
+                               TemplateId->NumArgs);
+    TypeResult T = ActOnTypenameType(CurScope, TypenameKWLoc, SS,
+                                     TemplateId->TemplateKWLoc,
+                                     TemplateId->Template, TemplateId->Name,
+                                     TemplateId->TemplateNameLoc,
+                                     TemplateId->LAngleLoc, ArgsPtr,
+                                     TemplateId->RAngleLoc);
+    if (T.isInvalid())
+      return nullptr;
+    if (GetTypeFromParser(T.get(), &TSI).isNull())
+      return nullptr;
+  }
+  return BuildTypeRequirement(TSI);
+}
+
+concepts::Requirement *
+Sema::ActOnCompoundRequirement(Expr *E, SourceLocation NoexceptLoc) {
+  return BuildExprRequirement(E, /*IsSimple=*/false, NoexceptLoc,
+                              /*ReturnTypeRequirement=*/{});
+}
+
+concepts::Requirement *
+Sema::ActOnCompoundRequirement(
+    Expr *E, SourceLocation NoexceptLoc, CXXScopeSpec &SS,
+    TemplateIdAnnotation *TypeConstraint, unsigned Depth) {
+  // C++2a [expr.prim.req.compound] p1.3.3
+  //   [..] the expression is deduced against an invented function template
+  //   F [...] F is a void function template with a single type template
+  //   parameter T declared with the constrained-parameter. Form a new
+  //   cv-qualifier-seq cv by taking the union of const and volatile specifiers
+  //   around the constrained-parameter. F has a single parameter whose
+  //   type-specifier is cv T followed by the abstract-declarator. [...]
+  //
+  // The cv part is done in the calling function - we get the concept with
+  // arguments and the abstract declarator with the correct CV qualification and
+  // have to synthesize T and the single parameter of F.
+  auto &II = Context.Idents.get("expr-type");
+  auto *TParam = TemplateTypeParmDecl::Create(Context, CurContext,
+                                              SourceLocation(),
+                                              SourceLocation(), Depth,
+                                              /*Index=*/0, &II,
+                                              /*Typename=*/true,
+                                              /*ParameterPack=*/false,
+                                              /*HasTypeConstraint=*/true);
+
+  if (ActOnTypeConstraint(SS, TypeConstraint, TParam,
+                          /*EllpsisLoc=*/SourceLocation()))
+    // Just produce a requirement with no type requirements.
+    return BuildExprRequirement(E, /*IsSimple=*/false, NoexceptLoc, {});
+
+  auto *TPL = TemplateParameterList::Create(Context, SourceLocation(),
+                                            SourceLocation(),
+                                            ArrayRef<NamedDecl *>(TParam),
+                                            SourceLocation(),
+                                            /*RequiresClause=*/nullptr);
+  return BuildExprRequirement(
+      E, /*IsSimple=*/false, NoexceptLoc,
+      concepts::ExprRequirement::ReturnTypeRequirement(TPL));
+}
+
+concepts::ExprRequirement *
+Sema::BuildExprRequirement(
+    Expr *E, bool IsSimple, SourceLocation NoexceptLoc,
+    concepts::ExprRequirement::ReturnTypeRequirement ReturnTypeRequirement) {
+  auto Status = concepts::ExprRequirement::SS_Satisfied;
+  ConceptSpecializationExpr *SubstitutedConstraintExpr = nullptr;
+  if (E->isInstantiationDependent() || ReturnTypeRequirement.isDependent())
+    Status = concepts::ExprRequirement::SS_Dependent;
+  else if (NoexceptLoc.isValid() && canThrow(E) == CanThrowResult::CT_Can)
+    Status = concepts::ExprRequirement::SS_NoexceptNotMet;
+  else if (ReturnTypeRequirement.isSubstitutionFailure())
+    Status = concepts::ExprRequirement::SS_TypeRequirementSubstitutionFailure;
+  else if (ReturnTypeRequirement.isTypeConstraint()) {
+    // C++2a [expr.prim.req]p1.3.3
+    //     The immediately-declared constraint ([temp]) of decltype((E)) shall
+    //     be satisfied.
+    TemplateParameterList *TPL =
+        ReturnTypeRequirement.getTypeConstraintTemplateParameterList();
+    QualType MatchedType =
+        BuildDecltypeType(E, E->getBeginLoc()).getCanonicalType();
+    llvm::SmallVector<TemplateArgument, 1> Args;
+    Args.push_back(TemplateArgument(MatchedType));
+    TemplateArgumentList TAL(TemplateArgumentList::OnStack, Args);
+    MultiLevelTemplateArgumentList MLTAL(TAL);
+    for (unsigned I = 0; I < TPL->getDepth(); ++I)
+      MLTAL.addOuterRetainedLevel();
+    Expr *IDC =
+        cast<TemplateTypeParmDecl>(TPL->getParam(0))->getTypeConstraint()
+            ->getImmediatelyDeclaredConstraint();
+    ExprResult Constraint = SubstExpr(IDC, MLTAL);
+    assert(!Constraint.isInvalid() &&
+           "Substitution cannot fail as it is simply putting a type template "
+           "argument into a concept specialization expression's parameter.");
+
+    SubstitutedConstraintExpr =
+        cast<ConceptSpecializationExpr>(Constraint.get());
+    if (!SubstitutedConstraintExpr->isSatisfied())
+      Status = concepts::ExprRequirement::SS_ConstraintsNotSatisfied;
+  }
+  return new (Context) concepts::ExprRequirement(E, IsSimple, NoexceptLoc,
+                                                 ReturnTypeRequirement, Status,
+                                                 SubstitutedConstraintExpr);
+}
+
+concepts::ExprRequirement *
+Sema::BuildExprRequirement(
+    concepts::Requirement::SubstitutionDiagnostic *ExprSubstitutionDiagnostic,
+    bool IsSimple, SourceLocation NoexceptLoc,
+    concepts::ExprRequirement::ReturnTypeRequirement ReturnTypeRequirement) {
+  return new (Context) concepts::ExprRequirement(ExprSubstitutionDiagnostic,
+                                                 IsSimple, NoexceptLoc,
+                                                 ReturnTypeRequirement);
+}
+
+concepts::TypeRequirement *
+Sema::BuildTypeRequirement(TypeSourceInfo *Type) {
+  return new (Context) concepts::TypeRequirement(Type);
+}
+
+concepts::TypeRequirement *
+Sema::BuildTypeRequirement(
+    concepts::Requirement::SubstitutionDiagnostic *SubstDiag) {
+  return new (Context) concepts::TypeRequirement(SubstDiag);
+}
+
+concepts::Requirement *Sema::ActOnNestedRequirement(Expr *Constraint) {
+  return BuildNestedRequirement(Constraint);
+}
+
+concepts::NestedRequirement *
+Sema::BuildNestedRequirement(Expr *Constraint) {
+  ConstraintSatisfaction Satisfaction;
+  if (!Constraint->isInstantiationDependent() &&
+      CheckConstraintSatisfaction(Constraint, Satisfaction))
+    return nullptr;
+  return new (Context) concepts::NestedRequirement(Context, Constraint,
+                                                   Satisfaction);
+}
+
+concepts::NestedRequirement *
+Sema::BuildNestedRequirement(
+    concepts::Requirement::SubstitutionDiagnostic *SubstDiag) {
+  return new (Context) concepts::NestedRequirement(SubstDiag);
+}
+
+RequiresExprBodyDecl *
+Sema::ActOnStartRequiresExpr(SourceLocation RequiresKWLoc,
+                             ArrayRef<ParmVarDecl *> LocalParameters,
+                             Scope *BodyScope) {
+  assert(BodyScope);
+
+  RequiresExprBodyDecl *Body = RequiresExprBodyDecl::Create(Context, CurContext,
+                                                            RequiresKWLoc);
+
+  PushDeclContext(BodyScope, Body);
+
+  for (ParmVarDecl *Param : LocalParameters) {
+    if (Param->hasDefaultArg())
+      // C++2a [expr.prim.req] p4
+      //     [...] A local parameter of a requires-expression shall not have a
+      //     default argument. [...]
+      Diag(Param->getDefaultArgRange().getBegin(),
+           diag::err_requires_expr_local_parameter_default_argument);
+    // Ignore default argument and move on
+
+    Param->setDeclContext(Body);
+    // If this has an identifier, add it to the scope stack.
+    if (Param->getIdentifier()) {
+      CheckShadow(BodyScope, Param);
+      PushOnScopeChains(Param, BodyScope);
+    }
+  }
+  return Body;
+}
+
+void Sema::ActOnFinishRequiresExpr() {
+  assert(CurContext && "DeclContext imbalance!");
+  CurContext = CurContext->getLexicalParent();
+  assert(CurContext && "Popped translation unit!");
+}
+
+ExprResult
+Sema::ActOnRequiresExpr(SourceLocation RequiresKWLoc,
+                        RequiresExprBodyDecl *Body,
+                        ArrayRef<ParmVarDecl *> LocalParameters,
+                        ArrayRef<concepts::Requirement *> Requirements,
+                        SourceLocation ClosingBraceLoc) {
+  return RequiresExpr::Create(Context, RequiresKWLoc, Body, LocalParameters,
+                              Requirements, ClosingBraceLoc);
+}

clang/lib/Sema/SemaLookup.cpp

@@ -1575,7 +1575,9 @@ llvm::DenseSet<Module*> &Sema::getLookupModules() {
   unsigned N = CodeSynthesisContexts.size();
   for (unsigned I = CodeSynthesisContextLookupModules.size();
        I != N; ++I) {
-    Module *M = getDefiningModule(*this, CodeSynthesisContexts[I].Entity);
+    Module *M = CodeSynthesisContexts[I].Entity ?
+                getDefiningModule(*this, CodeSynthesisContexts[I].Entity) :
+                nullptr;
     if (M && !LookupModulesCache.insert(M).second)
       M = nullptr;
     CodeSynthesisContextLookupModules.push_back(M);

clang/lib/Sema/SemaTemplate.cpp

@@ -10010,24 +10010,12 @@ Sema::ActOnTypenameType(Scope *S, SourceLocation TypenameLoc,
       << FixItHint::CreateRemoval(TypenameLoc);
 
   NestedNameSpecifierLoc QualifierLoc = SS.getWithLocInContext(Context);
+  TypeSourceInfo *TSI = nullptr;
   QualType T = CheckTypenameType(TypenameLoc.isValid()? ETK_Typename : ETK_None,
-                                 TypenameLoc, QualifierLoc, II, IdLoc);
+                                 TypenameLoc, QualifierLoc, II, IdLoc, &TSI,
+                                 /*DeducedTSTContext=*/true);
   if (T.isNull())
     return true;
-
-  TypeSourceInfo *TSI = Context.CreateTypeSourceInfo(T);
-  if (isa<DependentNameType>(T)) {
-    DependentNameTypeLoc TL = TSI->getTypeLoc().castAs<DependentNameTypeLoc>();
-    TL.setElaboratedKeywordLoc(TypenameLoc);
-    TL.setQualifierLoc(QualifierLoc);
-    TL.setNameLoc(IdLoc);
-  } else {
-    ElaboratedTypeLoc TL = TSI->getTypeLoc().castAs<ElaboratedTypeLoc>();
-    TL.setElaboratedKeywordLoc(TypenameLoc);
-    TL.setQualifierLoc(QualifierLoc);
-    TL.getNamedTypeLoc().castAs<TypeSpecTypeLoc>().setNameLoc(IdLoc);
-  }
-
   return CreateParsedType(T, TSI);
 }
 
@@ -10164,6 +10152,35 @@ static bool isEnableIf(NestedNameSpecifierLoc NNS, const IdentifierInfo &II,
   return true;
 }
 
+QualType
+Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
+                        SourceLocation KeywordLoc,
+                        NestedNameSpecifierLoc QualifierLoc,
+                        const IdentifierInfo &II,
+                        SourceLocation IILoc,
+                        TypeSourceInfo **TSI,
+                        bool DeducedTSTContext) {
+  QualType T = CheckTypenameType(Keyword, KeywordLoc, QualifierLoc, II, IILoc,
+                                 DeducedTSTContext);
+  if (T.isNull())
+    return QualType();
+
+  *TSI = Context.CreateTypeSourceInfo(T);
+  if (isa<DependentNameType>(T)) {
+    DependentNameTypeLoc TL =
+        (*TSI)->getTypeLoc().castAs<DependentNameTypeLoc>();
+    TL.setElaboratedKeywordLoc(KeywordLoc);
+    TL.setQualifierLoc(QualifierLoc);
+    TL.setNameLoc(IILoc);
+  } else {
+    ElaboratedTypeLoc TL = (*TSI)->getTypeLoc().castAs<ElaboratedTypeLoc>();
+    TL.setElaboratedKeywordLoc(KeywordLoc);
+    TL.setQualifierLoc(QualifierLoc);
+    TL.getNamedTypeLoc().castAs<TypeSpecTypeLoc>().setNameLoc(IILoc);
+  }
+  return T;
+}
+
 /// Build the type that describes a C++ typename specifier,
 /// e.g., "typename T::type".
 QualType
@@ -10171,32 +10188,38 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
                         SourceLocation KeywordLoc,
                         NestedNameSpecifierLoc QualifierLoc,
                         const IdentifierInfo &II,
-                        SourceLocation IILoc) {
+                        SourceLocation IILoc, bool DeducedTSTContext) {
   CXXScopeSpec SS;
   SS.Adopt(QualifierLoc);
 
-  DeclContext *Ctx = computeDeclContext(SS);
-  if (!Ctx) {
-    // If the nested-name-specifier is dependent and couldn't be
-    // resolved to a type, build a typename type.
-    assert(QualifierLoc.getNestedNameSpecifier()->isDependent());
-    return Context.getDependentNameType(Keyword,
-                                        QualifierLoc.getNestedNameSpecifier(),
-                                        &II);
+  DeclContext *Ctx = nullptr;
+  if (QualifierLoc) {
+    Ctx = computeDeclContext(SS);
+    if (!Ctx) {
+      // If the nested-name-specifier is dependent and couldn't be
+      // resolved to a type, build a typename type.
+      assert(QualifierLoc.getNestedNameSpecifier()->isDependent());
+      return Context.getDependentNameType(Keyword,
+                                          QualifierLoc.getNestedNameSpecifier(),
+                                          &II);
+    }
+
+    // If the nested-name-specifier refers to the current instantiation,
+    // the "typename" keyword itself is superfluous. In C++03, the
+    // program is actually ill-formed. However, DR 382 (in C++0x CD1)
+    // allows such extraneous "typename" keywords, and we retroactively
+    // apply this DR to C++03 code with only a warning. In any case we continue.
+
+    if (RequireCompleteDeclContext(SS, Ctx))
+      return QualType();
   }
 
-  // If the nested-name-specifier refers to the current instantiation,
-  // the "typename" keyword itself is superfluous. In C++03, the
-  // program is actually ill-formed. However, DR 382 (in C++0x CD1)
-  // allows such extraneous "typename" keywords, and we retroactively
-  // apply this DR to C++03 code with only a warning. In any case we continue.
-
-  if (RequireCompleteDeclContext(SS, Ctx))
-    return QualType();
-
   DeclarationName Name(&II);
   LookupResult Result(*this, Name, IILoc, LookupOrdinaryName);
-  LookupQualifiedName(Result, Ctx, SS);
+  if (Ctx)
+    LookupQualifiedName(Result, Ctx, SS);
+  else
+    LookupName(Result, CurScope);
   unsigned DiagID = 0;
   Decl *Referenced = nullptr;
   switch (Result.getResultKind()) {
@@ -10205,7 +10228,7 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
     // a more specific diagnostic.
     SourceRange CondRange;
     Expr *Cond = nullptr;
-    if (isEnableIf(QualifierLoc, II, CondRange, Cond)) {
+    if (Ctx && isEnableIf(QualifierLoc, II, CondRange, Cond)) {
       // If we have a condition, narrow it down to the specific failed
       // condition.
       if (Cond) {
@@ -10221,12 +10244,14 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
         return QualType();
       }
 
-      Diag(CondRange.getBegin(), diag::err_typename_nested_not_found_enable_if)
+      Diag(CondRange.getBegin(),
+           diag::err_typename_nested_not_found_enable_if)
           << Ctx << CondRange;
       return QualType();
     }
 
-    DiagID = diag::err_typename_nested_not_found;
+    DiagID = Ctx ? diag::err_typename_nested_not_found
+                 : diag::err_unknown_typename;
     break;
   }
 
@@ -10292,6 +10317,19 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
     //   is a placeholder for a deduced class type [...].
     if (getLangOpts().CPlusPlus17) {
       if (auto *TD = getAsTypeTemplateDecl(Result.getFoundDecl())) {
+        if (!DeducedTSTContext) {
+          QualType T(QualifierLoc
+                         ? QualifierLoc.getNestedNameSpecifier()->getAsType()
+                         : nullptr, 0);
+          if (!T.isNull())
+            Diag(IILoc, diag::err_dependent_deduced_tst)
+              << (int)getTemplateNameKindForDiagnostics(TemplateName(TD)) << T;
+          else
+            Diag(IILoc, diag::err_deduced_tst)
+              << (int)getTemplateNameKindForDiagnostics(TemplateName(TD));
+          Diag(TD->getLocation(), diag::note_template_decl_here);
+          return QualType();
+        }
         return Context.getElaboratedType(
             Keyword, QualifierLoc.getNestedNameSpecifier(),
             Context.getDeducedTemplateSpecializationType(TemplateName(TD),
@@ -10299,12 +10337,14 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
       }
     }
 
-    DiagID = diag::err_typename_nested_not_type;
+    DiagID = Ctx ? diag::err_typename_nested_not_type
+                 : diag::err_typename_not_type;
     Referenced = Result.getFoundDecl();
     break;
 
   case LookupResult::FoundOverloaded:
-    DiagID = diag::err_typename_nested_not_type;
+    DiagID = Ctx ? diag::err_typename_nested_not_type
+                 : diag::err_typename_not_type;
     Referenced = *Result.begin();
     break;
 
@@ -10316,9 +10356,14 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
   // type. Emit an appropriate diagnostic and return an error.
   SourceRange FullRange(KeywordLoc.isValid() ? KeywordLoc : SS.getBeginLoc(),
                         IILoc);
-  Diag(IILoc, DiagID) << FullRange << Name << Ctx;
+  if (Ctx)
+    Diag(IILoc, DiagID) << FullRange << Name << Ctx;
+  else
+    Diag(IILoc, DiagID) << FullRange << Name;
   if (Referenced)
-    Diag(Referenced->getLocation(), diag::note_typename_refers_here)
+    Diag(Referenced->getLocation(),
+         Ctx ? diag::note_typename_member_refers_here
+             : diag::note_typename_refers_here)
       << Name;
   return QualType();
 }

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -26,6 +26,7 @@
 #include "clang/Sema/Template.h"
 #include "clang/Sema/TemplateDeduction.h"
 #include "clang/Sema/TemplateInstCallback.h"
+#include "clang/Sema/SemaConcept.h"
 #include "llvm/Support/TimeProfiler.h"
 
 using namespace clang;
@@ -199,8 +200,10 @@ bool Sema::CodeSynthesisContext::isInstantiationRecord() const {
   case DeducedTemplateArgumentSubstitution:
   case PriorTemplateArgumentSubstitution:
   case ConstraintsCheck:
+  case NestedRequirementConstraintsCheck:
     return true;
 
+  case RequirementInstantiation:
   case DefaultTemplateArgumentChecking:
   case DeclaringSpecialMember:
   case DeclaringImplicitEqualityComparison:
@@ -247,7 +250,7 @@ Sema::InstantiatingTemplate::InstantiatingTemplate(
     Inst.InstantiationRange = InstantiationRange;
     SemaRef.pushCodeSynthesisContext(Inst);
 
-    AlreadyInstantiating =
+    AlreadyInstantiating = !Inst.Entity ? false :
         !SemaRef.InstantiatingSpecializations
              .insert(std::make_pair(Inst.Entity->getCanonicalDecl(), Inst.Kind))
              .second;
@@ -364,6 +367,26 @@ Sema::InstantiatingTemplate::InstantiatingTemplate(
           PointOfInstantiation, InstantiationRange, Param, Template,
           TemplateArgs) {}
 
+Sema::InstantiatingTemplate::InstantiatingTemplate(
+    Sema &SemaRef, SourceLocation PointOfInstantiation,
+    concepts::Requirement *Req, sema::TemplateDeductionInfo &DeductionInfo,
+    SourceRange InstantiationRange)
+    : InstantiatingTemplate(
+          SemaRef, CodeSynthesisContext::RequirementInstantiation,
+          PointOfInstantiation, InstantiationRange, /*Entity=*/nullptr,
+          /*Template=*/nullptr, /*TemplateArgs=*/None, &DeductionInfo) {}
+
+
+Sema::InstantiatingTemplate::InstantiatingTemplate(
+    Sema &SemaRef, SourceLocation PointOfInstantiation,
+    concepts::NestedRequirement *Req, ConstraintsCheck,
+    SourceRange InstantiationRange)
+    : InstantiatingTemplate(
+          SemaRef, CodeSynthesisContext::NestedRequirementConstraintsCheck,
+          PointOfInstantiation, InstantiationRange, /*Entity=*/nullptr,
+          /*Template=*/nullptr, /*TemplateArgs=*/None) {}
+
+
 Sema::InstantiatingTemplate::InstantiatingTemplate(
     Sema &SemaRef, SourceLocation PointOfInstantiation,
     ConstraintsCheck, NamedDecl *Template,
@@ -446,8 +469,9 @@ void Sema::InstantiatingTemplate::Clear() {
   if (!Invalid) {
     if (!AlreadyInstantiating) {
       auto &Active = SemaRef.CodeSynthesisContexts.back();
-      SemaRef.InstantiatingSpecializations.erase(
-          std::make_pair(Active.Entity, Active.Kind));
+      if (Active.Entity)
+        SemaRef.InstantiatingSpecializations.erase(
+            std::make_pair(Active.Entity, Active.Kind));
     }
 
     atTemplateEnd(SemaRef.TemplateInstCallbacks, SemaRef,
@@ -684,6 +708,18 @@ void Sema::PrintInstantiationStack() {
         << Active->InstantiationRange;
       break;
 
+    case CodeSynthesisContext::RequirementInstantiation:
+      Diags.Report(Active->PointOfInstantiation,
+                   diag::note_template_requirement_instantiation_here)
+        << Active->InstantiationRange;
+      break;
+
+    case CodeSynthesisContext::NestedRequirementConstraintsCheck:
+      Diags.Report(Active->PointOfInstantiation,
+                   diag::note_nested_requirement_here)
+        << Active->InstantiationRange;
+      break;
+
     case CodeSynthesisContext::DeclaringSpecialMember:
       Diags.Report(Active->PointOfInstantiation,
                    diag::note_in_declaration_of_implicit_special_member)
@@ -788,6 +824,7 @@ Optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
     case CodeSynthesisContext::ConstraintsCheck:
     case CodeSynthesisContext::ParameterMappingSubstitution:
     case CodeSynthesisContext::ConstraintNormalization:
+    case CodeSynthesisContext::NestedRequirementConstraintsCheck:
       // This is a template instantiation, so there is no SFINAE.
       return None;
 
@@ -802,9 +839,10 @@ Optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
     case CodeSynthesisContext::ExplicitTemplateArgumentSubstitution:
     case CodeSynthesisContext::DeducedTemplateArgumentSubstitution:
     case CodeSynthesisContext::ConstraintSubstitution:
-      // We're either substituting explicitly-specified template arguments
-      // or deduced template arguments or a constraint expression, so SFINAE
-      // applies.
+    case CodeSynthesisContext::RequirementInstantiation:
+      // We're either substituting explicitly-specified template arguments,
+      // deduced template arguments, a constraint expression or a requirement
+      // in a requires expression, so SFINAE applies.
       assert(Active->DeductionInfo && "Missing deduction info pointer");
       return Active->DeductionInfo;
 
@@ -1056,6 +1094,41 @@ namespace {
       return TreeTransform<TemplateInstantiator>::TransformLambdaExpr(E);
     }
 
+    ExprResult TransformRequiresExpr(RequiresExpr *E) {
+      LocalInstantiationScope Scope(SemaRef, /*CombineWithOuterScope=*/true);
+      return TreeTransform<TemplateInstantiator>::TransformRequiresExpr(E);
+    }
+
+    bool TransformRequiresExprRequirements(
+        ArrayRef<concepts::Requirement *> Reqs,
+        SmallVectorImpl<concepts::Requirement *> &Transformed) {
+      bool SatisfactionDetermined = false;
+      for (concepts::Requirement *Req : Reqs) {
+        concepts::Requirement *TransReq = nullptr;
+        if (!SatisfactionDetermined) {
+          if (auto *TypeReq = dyn_cast<concepts::TypeRequirement>(Req))
+            TransReq = TransformTypeRequirement(TypeReq);
+          else if (auto *ExprReq = dyn_cast<concepts::ExprRequirement>(Req))
+            TransReq = TransformExprRequirement(ExprReq);
+          else
+            TransReq = TransformNestedRequirement(
+                cast<concepts::NestedRequirement>(Req));
+          if (!TransReq)
+            return true;
+          if (!TransReq->isDependent() && !TransReq->isSatisfied())
+            // [expr.prim.req]p6
+            //   [...]  The substitution and semantic constraint checking
+            //   proceeds in lexical order and stops when a condition that
+            //   determines the result of the requires-expression is
+            //   encountered. [..]
+            SatisfactionDetermined = true;
+        } else
+          TransReq = Req;
+        Transformed.push_back(TransReq);
+      }
+      return false;
+    }
+
     TemplateParameterList *TransformTemplateParameterList(
                               TemplateParameterList *OrigTPL)  {
       if (!OrigTPL || !OrigTPL->size()) return OrigTPL;
@@ -1065,6 +1138,14 @@ namespace {
                         /* DeclContext *Owner */ Owner, TemplateArgs);
       return DeclInstantiator.SubstTemplateParams(OrigTPL);
     }
+
+    concepts::TypeRequirement *
+    TransformTypeRequirement(concepts::TypeRequirement *Req);
+    concepts::ExprRequirement *
+    TransformExprRequirement(concepts::ExprRequirement *Req);
+    concepts::NestedRequirement *
+    TransformNestedRequirement(concepts::NestedRequirement *Req);
+
   private:
     ExprResult transformNonTypeTemplateParmRef(NonTypeTemplateParmDecl *parm,
                                                SourceLocation loc,
@@ -1669,6 +1750,163 @@ TemplateInstantiator::TransformSubstTemplateTypeParmPackType(
   return Result;
 }
 
+template<typename EntityPrinter>
+static concepts::Requirement::SubstitutionDiagnostic *
+createSubstDiag(Sema &S, TemplateDeductionInfo &Info, EntityPrinter Printer) {
+  SmallString<128> Message;
+  SourceLocation ErrorLoc;
+  if (Info.hasSFINAEDiagnostic()) {
+    PartialDiagnosticAt PDA(SourceLocation(),
+                            PartialDiagnostic::NullDiagnostic{});
+    Info.takeSFINAEDiagnostic(PDA);
+    PDA.second.EmitToString(S.getDiagnostics(), Message);
+    ErrorLoc = PDA.first;
+  } else {
+    ErrorLoc = Info.getLocation();
+  }
+  char *MessageBuf = new (S.Context) char[Message.size()];
+  std::copy(Message.begin(), Message.end(), MessageBuf);
+  SmallString<128> Entity;
+  llvm::raw_svector_ostream OS(Entity);
+  Printer(OS);
+  char *EntityBuf = new (S.Context) char[Entity.size()];
+  std::copy(Entity.begin(), Entity.end(), EntityBuf);
+  return new (S.Context) concepts::Requirement::SubstitutionDiagnostic{
+      StringRef(EntityBuf, Entity.size()), ErrorLoc,
+      StringRef(MessageBuf, Message.size())};
+}
+
+concepts::TypeRequirement *
+TemplateInstantiator::TransformTypeRequirement(concepts::TypeRequirement *Req) {
+  if (!Req->isDependent() && !AlwaysRebuild())
+    return Req;
+  if (Req->isSubstitutionFailure()) {
+    if (AlwaysRebuild())
+      return RebuildTypeRequirement(
+              Req->getSubstitutionDiagnostic());
+    return Req;
+  }
+
+  Sema::SFINAETrap Trap(SemaRef);
+  TemplateDeductionInfo Info(Req->getType()->getTypeLoc().getBeginLoc());
+  Sema::InstantiatingTemplate TypeInst(SemaRef,
+      Req->getType()->getTypeLoc().getBeginLoc(), Req, Info,
+      Req->getType()->getTypeLoc().getSourceRange());
+  if (TypeInst.isInvalid())
+    return nullptr;
+  TypeSourceInfo *TransType = TransformType(Req->getType());
+  if (!TransType || Trap.hasErrorOccurred())
+    return RebuildTypeRequirement(createSubstDiag(SemaRef, Info,
+        [&] (llvm::raw_ostream& OS) {
+            Req->getType()->getType().print(OS, SemaRef.getPrintingPolicy());
+        }));
+  return RebuildTypeRequirement(TransType);
+}
+
+concepts::ExprRequirement *
+TemplateInstantiator::TransformExprRequirement(concepts::ExprRequirement *Req) {
+  if (!Req->isDependent() && !AlwaysRebuild())
+    return Req;
+
+  Sema::SFINAETrap Trap(SemaRef);
+  TemplateDeductionInfo Info(Req->getExpr()->getBeginLoc());
+
+  llvm::PointerUnion<Expr *, concepts::Requirement::SubstitutionDiagnostic *>
+      TransExpr;
+  if (Req->isExprSubstitutionFailure())
+    TransExpr = Req->getExprSubstitutionDiagnostic();
+  else {
+    Sema::InstantiatingTemplate ExprInst(SemaRef, Req->getExpr()->getBeginLoc(),
+                                         Req, Info,
+                                         Req->getExpr()->getSourceRange());
+    if (ExprInst.isInvalid())
+      return nullptr;
+    ExprResult TransExprRes = TransformExpr(Req->getExpr());
+    if (TransExprRes.isInvalid() || Trap.hasErrorOccurred())
+      TransExpr = createSubstDiag(SemaRef, Info,
+          [&] (llvm::raw_ostream& OS) {
+              Req->getExpr()->printPretty(OS, nullptr,
+                                          SemaRef.getPrintingPolicy());
+          });
+    else
+      TransExpr = TransExprRes.get();
+  }
+
+  llvm::Optional<concepts::ExprRequirement::ReturnTypeRequirement> TransRetReq;
+  const auto &RetReq = Req->getReturnTypeRequirement();
+  if (RetReq.isEmpty())
+    TransRetReq.emplace();
+  else if (RetReq.isSubstitutionFailure())
+    TransRetReq.emplace(RetReq.getSubstitutionDiagnostic());
+  else if (RetReq.isTypeConstraint()) {
+    TemplateParameterList *OrigTPL =
+        RetReq.getTypeConstraintTemplateParameterList();
+    Sema::InstantiatingTemplate TPLInst(SemaRef, OrigTPL->getTemplateLoc(),
+                                        Req, Info, OrigTPL->getSourceRange());
+    if (TPLInst.isInvalid())
+      return nullptr;
+    TemplateParameterList *TPL =
+        TransformTemplateParameterList(OrigTPL);
+    if (!TPL)
+      TransRetReq.emplace(createSubstDiag(SemaRef, Info,
+          [&] (llvm::raw_ostream& OS) {
+              RetReq.getTypeConstraint()->getImmediatelyDeclaredConstraint()
+                  ->printPretty(OS, nullptr, SemaRef.getPrintingPolicy());
+          }));
+    else {
+      TPLInst.Clear();
+      TransRetReq.emplace(TPL);
+    }
+  }
+  assert(TransRetReq.hasValue() &&
+         "All code paths leading here must set TransRetReq");
+  if (Expr *E = TransExpr.dyn_cast<Expr *>())
+    return RebuildExprRequirement(E, Req->isSimple(), Req->getNoexceptLoc(),
+                                  std::move(*TransRetReq));
+  return RebuildExprRequirement(
+      TransExpr.get<concepts::Requirement::SubstitutionDiagnostic *>(),
+      Req->isSimple(), Req->getNoexceptLoc(), std::move(*TransRetReq));
+}
+
+concepts::NestedRequirement *
+TemplateInstantiator::TransformNestedRequirement(
+    concepts::NestedRequirement *Req) {
+  if (!Req->isDependent() && !AlwaysRebuild())
+    return Req;
+  if (Req->isSubstitutionFailure()) {
+    if (AlwaysRebuild())
+      return RebuildNestedRequirement(
+          Req->getSubstitutionDiagnostic());
+    return Req;
+  }
+  Sema::InstantiatingTemplate ReqInst(SemaRef,
+      Req->getConstraintExpr()->getBeginLoc(), Req,
+      Sema::InstantiatingTemplate::ConstraintsCheck{},
+      Req->getConstraintExpr()->getSourceRange());
+
+  ExprResult TransConstraint;
+  TemplateDeductionInfo Info(Req->getConstraintExpr()->getBeginLoc());
+  {
+    EnterExpressionEvaluationContext ContextRAII(
+        SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+    Sema::SFINAETrap Trap(SemaRef);
+    Sema::InstantiatingTemplate ConstrInst(SemaRef,
+        Req->getConstraintExpr()->getBeginLoc(), Req, Info,
+        Req->getConstraintExpr()->getSourceRange());
+    if (ConstrInst.isInvalid())
+      return nullptr;
+    TransConstraint = TransformExpr(Req->getConstraintExpr());
+    if (TransConstraint.isInvalid() || Trap.hasErrorOccurred())
+      return RebuildNestedRequirement(createSubstDiag(SemaRef, Info,
+          [&] (llvm::raw_ostream& OS) {
+              Req->getConstraintExpr()->printPretty(OS, nullptr,
+                                                    SemaRef.getPrintingPolicy());
+          }));
+  }
+  return RebuildNestedRequirement(TransConstraint.get());
+}
+
+
 /// Perform substitution on the type T with a given set of template
 /// arguments.
 ///

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -3600,6 +3600,12 @@ Decl *TemplateDeclInstantiator::VisitConceptDecl(ConceptDecl *D) {
   llvm_unreachable("Concept definitions cannot reside inside a template");
 }
 
+Decl *
+TemplateDeclInstantiator::VisitRequiresExprBodyDecl(RequiresExprBodyDecl *D) {
+  return RequiresExprBodyDecl::Create(SemaRef.Context, D->getDeclContext(),
+                                      D->getBeginLoc());
+}
+
 Decl *TemplateDeclInstantiator::VisitDecl(Decl *D) {
   llvm_unreachable("Unexpected decl");
 }

clang/lib/Sema/SemaType.cpp

@@ -2994,6 +2994,9 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
     case DeclaratorContext::PrototypeContext:
       Error = 0;
       break;
+    case DeclaratorContext::RequiresExprContext:
+      Error = 21;
+      break;
     case DeclaratorContext::LambdaExprParameterContext:
       // In C++14, generic lambdas allow 'auto' in their parameters.
       if (!SemaRef.getLangOpts().CPlusPlus14 ||
@@ -3221,6 +3224,7 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
     case DeclaratorContext::ObjCParameterContext:
     case DeclaratorContext::ObjCResultContext:
     case DeclaratorContext::KNRTypeListContext:
+    case DeclaratorContext::RequiresExprContext:
       // C++ [dcl.fct]p6:
       //   Types shall not be defined in return or parameter types.
       DiagID = diag::err_type_defined_in_param_type;
@@ -4279,6 +4283,7 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
     case DeclaratorContext::TemplateTypeArgContext:
     case DeclaratorContext::TypeNameContext:
     case DeclaratorContext::FunctionalCastContext:
+    case DeclaratorContext::RequiresExprContext:
       // Don't infer in these contexts.
       break;
     }
@@ -5227,6 +5232,7 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
     switch (D.getContext()) {
     case DeclaratorContext::PrototypeContext:
     case DeclaratorContext::LambdaExprParameterContext:
+    case DeclaratorContext::RequiresExprContext:
       // C++0x [dcl.fct]p13:
       //   [...] When it is part of a parameter-declaration-clause, the
       //   parameter pack is a function parameter pack (14.5.3). The type T

clang/lib/Sema/TreeTransform.h

@@ -19,6 +19,7 @@
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/ExprOpenMP.h"
@@ -509,6 +510,15 @@ public:
   DeclarationNameInfo
   TransformDeclarationNameInfo(const DeclarationNameInfo &NameInfo);
 
+  bool TransformRequiresExprRequirements(ArrayRef<concepts::Requirement *> Reqs,
+      llvm::SmallVectorImpl<concepts::Requirement *> &Transformed);
+  concepts::TypeRequirement *
+  TransformTypeRequirement(concepts::TypeRequirement *Req);
+  concepts::ExprRequirement *
+  TransformExprRequirement(concepts::ExprRequirement *Req);
+  concepts::NestedRequirement *
+  TransformNestedRequirement(concepts::NestedRequirement *Req);
+
   /// Transform the given template name.
   ///
   /// \param SS The nested-name-specifier that qualifies the template
@@ -1056,23 +1066,8 @@ public:
     }
 
     if (Keyword == ETK_None || Keyword == ETK_Typename) {
-      QualType T = SemaRef.CheckTypenameType(Keyword, KeywordLoc, QualifierLoc,
-                                             *Id, IdLoc);
-      // If a dependent name resolves to a deduced template specialization type,
-      // check that we're in one of the syntactic contexts permitting it.
-      if (!DeducedTSTContext) {
-        if (auto *Deduced = dyn_cast_or_null<DeducedTemplateSpecializationType>(
-                T.isNull() ? nullptr : T->getContainedDeducedType())) {
-          SemaRef.Diag(IdLoc, diag::err_dependent_deduced_tst)
-            << (int)SemaRef.getTemplateNameKindForDiagnostics(
-                   Deduced->getTemplateName())
-            << QualType(QualifierLoc.getNestedNameSpecifier()->getAsType(), 0);
-          if (auto *TD = Deduced->getTemplateName().getAsTemplateDecl())
-            SemaRef.Diag(TD->getLocation(), diag::note_template_decl_here);
-          return QualType();
-        }
-      }
-      return T;
+      return SemaRef.CheckTypenameType(Keyword, KeywordLoc, QualifierLoc,
+                                       *Id, IdLoc, DeducedTSTContext);
     }
 
     TagTypeKind Kind = TypeWithKeyword::getTagTypeKindForKeyword(Keyword);
@@ -3078,7 +3073,56 @@ public:
     return Result;
   }
 
-    /// \brief Build a new Objective-C boxed expression.
+  /// \brief Build a new requires expression.
+  ///
+  /// By default, performs semantic analysis to build the new expression.
+  /// Subclasses may override this routine to provide different behavior.
+  ExprResult RebuildRequiresExpr(SourceLocation RequiresKWLoc,
+                                 RequiresExprBodyDecl *Body,
+                                 ArrayRef<ParmVarDecl *> LocalParameters,
+                                 ArrayRef<concepts::Requirement *> Requirements,
+                                 SourceLocation ClosingBraceLoc) {
+    return RequiresExpr::Create(SemaRef.Context, RequiresKWLoc, Body,
+                                LocalParameters, Requirements, ClosingBraceLoc);
+  }
+
+  concepts::TypeRequirement *
+  RebuildTypeRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *SubstDiag) {
+    return SemaRef.BuildTypeRequirement(SubstDiag);
+  }
+
+  concepts::TypeRequirement *RebuildTypeRequirement(TypeSourceInfo *T) {
+    return SemaRef.BuildTypeRequirement(T);
+  }
+
+  concepts::ExprRequirement *
+  RebuildExprRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *SubstDiag, bool IsSimple,
+      SourceLocation NoexceptLoc,
+      concepts::ExprRequirement::ReturnTypeRequirement Ret) {
+    return SemaRef.BuildExprRequirement(SubstDiag, IsSimple, NoexceptLoc,
+                                        std::move(Ret));
+  }
+
+  concepts::ExprRequirement *
+  RebuildExprRequirement(Expr *E, bool IsSimple, SourceLocation NoexceptLoc,
+                         concepts::ExprRequirement::ReturnTypeRequirement Ret) {
+    return SemaRef.BuildExprRequirement(E, IsSimple, NoexceptLoc,
+                                        std::move(Ret));
+  }
+
+  concepts::NestedRequirement *
+  RebuildNestedRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *SubstDiag) {
+    return SemaRef.BuildNestedRequirement(SubstDiag);
+  }
+
+  concepts::NestedRequirement *RebuildNestedRequirement(Expr *Constraint) {
+    return SemaRef.BuildNestedRequirement(Constraint);
+  }
+
+  /// \brief Build a new Objective-C boxed expression.
   ///
   /// By default, performs semantic analysis to build the new expression.
   /// Subclasses may override this routine to provide different behavior.
@@ -11179,6 +11223,146 @@ TreeTransform<Derived>::TransformConceptSpecializationExpr(
       &TransArgs);
 }
 
+template<typename Derived>
+ExprResult
+TreeTransform<Derived>::TransformRequiresExpr(RequiresExpr *E) {
+  SmallVector<ParmVarDecl*, 4> TransParams;
+  SmallVector<QualType, 4> TransParamTypes;
+  Sema::ExtParameterInfoBuilder ExtParamInfos;
+
+  // C++2a [expr.prim.req]p2
+  // Expressions appearing within a requirement-body are unevaluated operands.
+  EnterExpressionEvaluationContext Ctx(
+      SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
+
+  RequiresExprBodyDecl *Body = RequiresExprBodyDecl::Create(
+      getSema().Context, E->getBody()->getDeclContext(),
+      E->getBody()->getBeginLoc());
+
+  Sema::ContextRAII SavedContext(getSema(), Body, /*NewThisContext*/false);
+
+  if (getDerived().TransformFunctionTypeParams(E->getRequiresKWLoc(),
+                                               E->getLocalParameters(),
+                                               /*ParamTypes=*/nullptr,
+                                               /*ParamInfos=*/nullptr,
+                                               TransParamTypes, &TransParams,
+                                               ExtParamInfos))
+    return ExprError();
+
+  for (ParmVarDecl *Param : TransParams)
+    Param->setDeclContext(Body);
+
+  SmallVector<concepts::Requirement *, 4> TransReqs;
+  if (getDerived().TransformRequiresExprRequirements(E->getRequirements(),
+                                                     TransReqs))
+    return ExprError();
+
+  for (concepts::Requirement *Req : TransReqs) {
+    if (auto *ER = dyn_cast<concepts::ExprRequirement>(Req)) {
+      if (ER->getReturnTypeRequirement().isTypeConstraint()) {
+        ER->getReturnTypeRequirement()
+                .getTypeConstraintTemplateParameterList()->getParam(0)
+                ->setDeclContext(Body);
+      }
+    }
+  }
+
+  return getDerived().RebuildRequiresExpr(E->getRequiresKWLoc(), Body,
+                                          TransParams, TransReqs,
+                                          E->getRBraceLoc());
+}
+
+template<typename Derived>
+bool TreeTransform<Derived>::TransformRequiresExprRequirements(
+    ArrayRef<concepts::Requirement *> Reqs,
+    SmallVectorImpl<concepts::Requirement *> &Transformed) {
+  for (concepts::Requirement *Req : Reqs) {
+    concepts::Requirement *TransReq = nullptr;
+    if (auto *TypeReq = dyn_cast<concepts::TypeRequirement>(Req))
+      TransReq = getDerived().TransformTypeRequirement(TypeReq);
+    else if (auto *ExprReq = dyn_cast<concepts::ExprRequirement>(Req))
+      TransReq = getDerived().TransformExprRequirement(ExprReq);
+    else
+      TransReq = getDerived().TransformNestedRequirement(
+                     cast<concepts::NestedRequirement>(Req));
+    if (!TransReq)
+      return true;
+    Transformed.push_back(TransReq);
+  }
+  return false;
+}
+
+template<typename Derived>
+concepts::TypeRequirement *
+TreeTransform<Derived>::TransformTypeRequirement(
+    concepts::TypeRequirement *Req) {
+  if (Req->isSubstitutionFailure()) {
+    if (getDerived().AlwaysRebuild())
+      return getDerived().RebuildTypeRequirement(
+              Req->getSubstitutionDiagnostic());
+    return Req;
+  }
+  TypeSourceInfo *TransType = getDerived().TransformType(Req->getType());
+  if (!TransType)
+    return nullptr;
+  return getDerived().RebuildTypeRequirement(TransType);
+}
+
+template<typename Derived>
+concepts::ExprRequirement *
+TreeTransform<Derived>::TransformExprRequirement(concepts::ExprRequirement *Req) {
+  llvm::PointerUnion<Expr *, concepts::Requirement::SubstitutionDiagnostic *> TransExpr;
+  if (Req->isExprSubstitutionFailure())
+    TransExpr = Req->getExprSubstitutionDiagnostic();
+  else {
+    ExprResult TransExprRes = getDerived().TransformExpr(Req->getExpr());
+    if (TransExprRes.isInvalid())
+      return nullptr;
+    TransExpr = TransExprRes.get();
+  }
+
+  llvm::Optional<concepts::ExprRequirement::ReturnTypeRequirement> TransRetReq;
+  const auto &RetReq = Req->getReturnTypeRequirement();
+  if (RetReq.isEmpty())
+    TransRetReq.emplace();
+  else if (RetReq.isSubstitutionFailure())
+    TransRetReq.emplace(RetReq.getSubstitutionDiagnostic());
+  else if (RetReq.isTypeConstraint()) {
+    TemplateParameterList *OrigTPL =
+        RetReq.getTypeConstraintTemplateParameterList();
+    TemplateParameterList *TPL =
+        getDerived().TransformTemplateParameterList(OrigTPL);
+    if (!TPL)
+      return nullptr;
+    TransRetReq.emplace(TPL);
+  }
+  assert(TransRetReq.hasValue() &&
+         "All code paths leading here must set TransRetReq");
+  if (Expr *E = TransExpr.dyn_cast<Expr *>())
+    return getDerived().RebuildExprRequirement(E, Req->isSimple(),
+                                               Req->getNoexceptLoc(),
+                                               std::move(*TransRetReq));
+  return getDerived().RebuildExprRequirement(
+      TransExpr.get<concepts::Requirement::SubstitutionDiagnostic *>(),
+      Req->isSimple(), Req->getNoexceptLoc(), std::move(*TransRetReq));
+}
+
+template<typename Derived>
+concepts::NestedRequirement *
+TreeTransform<Derived>::TransformNestedRequirement(
+    concepts::NestedRequirement *Req) {
+  if (Req->isSubstitutionFailure()) {
+    if (getDerived().AlwaysRebuild())
+      return getDerived().RebuildNestedRequirement(
+          Req->getSubstitutionDiagnostic());
+    return Req;
+  }
+  ExprResult TransConstraint =
+      getDerived().TransformExpr(Req->getConstraintExpr());
+  if (TransConstraint.isInvalid())
+    return nullptr;
+  return getDerived().RebuildNestedRequirement(TransConstraint.get());
+}
 
 template<typename Derived>
 ExprResult

clang/lib/Serialization/ASTCommon.cpp

@@ -402,6 +402,7 @@ bool serialization::isRedeclarableDeclKind(unsigned Kind) {
   case Decl::Binding:
   case Decl::Concept:
   case Decl::LifetimeExtendedTemporary:
+  case Decl::RequiresExprBody:
     return false;
 
   // These indirectly derive from Redeclarable<T> but are not actually

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -375,6 +375,7 @@ namespace clang {
     void VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D);
     DeclID VisitTemplateDecl(TemplateDecl *D);
     void VisitConceptDecl(ConceptDecl *D);
+    void VisitRequiresExprBodyDecl(RequiresExprBodyDecl *D);
     RedeclarableResult VisitRedeclarableTemplateDecl(RedeclarableTemplateDecl *D);
     void VisitClassTemplateDecl(ClassTemplateDecl *D);
     void VisitBuiltinTemplateDecl(BuiltinTemplateDecl *D);
@@ -2037,6 +2038,9 @@ void ASTDeclReader::VisitConceptDecl(ConceptDecl *D) {
   mergeMergeable(D);
 }
 
+void ASTDeclReader::VisitRequiresExprBodyDecl(RequiresExprBodyDecl *D) {
+}
+
 ASTDeclReader::RedeclarableResult
 ASTDeclReader::VisitRedeclarableTemplateDecl(RedeclarableTemplateDecl *D) {
   RedeclarableResult Redecl = VisitRedeclarable(D);
@@ -3839,6 +3843,9 @@ Decl *ASTReader::ReadDeclRecord(DeclID ID) {
   case DECL_CONCEPT:
     D = ConceptDecl::CreateDeserialized(Context, ID);
     break;
+  case DECL_REQUIRES_EXPR_BODY:
+    D = RequiresExprBodyDecl::CreateDeserialized(Context, ID);
+    break;
   case DECL_STATIC_ASSERT:
     D = StaticAssertDecl::CreateDeserialized(Context, ID);
     break;

clang/lib/Serialization/ASTReaderStmt.cpp

@@ -724,6 +724,28 @@ void ASTStmtReader::VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *E) {
   E->setRParenLoc(readSourceLocation());
 }
 
+static ConstraintSatisfaction
+readConstraintSatisfaction(ASTRecordReader &Record) {
+  ConstraintSatisfaction Satisfaction;
+  Satisfaction.IsSatisfied = Record.readInt();
+  if (!Satisfaction.IsSatisfied) {
+    unsigned NumDetailRecords = Record.readInt();
+    for (unsigned i = 0; i != NumDetailRecords; ++i) {
+      Expr *ConstraintExpr = Record.readExpr();
+      if (bool IsDiagnostic = Record.readInt()) {
+        SourceLocation DiagLocation = Record.readSourceLocation();
+        std::string DiagMessage = Record.readString();
+        Satisfaction.Details.emplace_back(
+            ConstraintExpr, new (Record.getContext())
+                                ConstraintSatisfaction::SubstitutionDiagnostic{
+                                    DiagLocation, DiagMessage});
+      } else
+        Satisfaction.Details.emplace_back(ConstraintExpr, Record.readExpr());
+    }
+  }
+  return Satisfaction;
+}
+
 void ASTStmtReader::VisitConceptSpecializationExpr(
         ConceptSpecializationExpr *E) {
   VisitExpr(E);
@@ -737,26 +759,121 @@ void ASTStmtReader::VisitConceptSpecializationExpr(
   for (unsigned I = 0; I < NumTemplateArgs; ++I)
     Args.push_back(Record.readTemplateArgument());
   E->setTemplateArguments(Args);
-  ConstraintSatisfaction Satisfaction;
-  Satisfaction.IsSatisfied = Record.readInt();
-  if (!Satisfaction.IsSatisfied) {
-    unsigned NumDetailRecords = Record.readInt();
-    for (unsigned i = 0; i != NumDetailRecords; ++i) {
-      Expr *ConstraintExpr = Record.readExpr();
-      bool IsDiagnostic = Record.readInt();
-      if (IsDiagnostic) {
-        SourceLocation DiagLocation = Record.readSourceLocation();
-        std::string DiagMessage = Record.readString();
-        Satisfaction.Details.emplace_back(
-            ConstraintExpr, new (Record.getContext())
-                                ConstraintSatisfaction::SubstitutionDiagnostic{
-                                    DiagLocation, DiagMessage});
-      } else
-        Satisfaction.Details.emplace_back(ConstraintExpr, Record.readExpr());
+  E->Satisfaction = E->isValueDependent() ? nullptr :
+      ASTConstraintSatisfaction::Create(Record.getContext(),
+                                        readConstraintSatisfaction(Record));
+}
+
+static concepts::Requirement::SubstitutionDiagnostic *
+readSubstitutionDiagnostic(ASTRecordReader &Record) {
+  std::string SubstitutedEntity = Record.readString();
+  SourceLocation DiagLoc = Record.readSourceLocation();
+  std::string DiagMessage = Record.readString();
+  return new (Record.getContext())
+      concepts::Requirement::SubstitutionDiagnostic{SubstitutedEntity, DiagLoc,
+                                                    DiagMessage};
+}
+
+void ASTStmtReader::VisitRequiresExpr(RequiresExpr *E) {
+  VisitExpr(E);
+  unsigned NumLocalParameters = Record.readInt();
+  unsigned NumRequirements = Record.readInt();
+  E->RequiresExprBits.RequiresKWLoc = Record.readSourceLocation();
+  E->RequiresExprBits.IsSatisfied = Record.readInt();
+  E->Body = Record.readDeclAs<RequiresExprBodyDecl>();
+  llvm::SmallVector<ParmVarDecl *, 4> LocalParameters;
+  for (unsigned i = 0; i < NumLocalParameters; ++i)
+    LocalParameters.push_back(cast<ParmVarDecl>(Record.readDecl()));
+  std::copy(LocalParameters.begin(), LocalParameters.end(),
+            E->getTrailingObjects<ParmVarDecl *>());
+  llvm::SmallVector<concepts::Requirement *, 4> Requirements;
+  for (unsigned i = 0; i < NumRequirements; ++i) {
+    auto RK =
+        static_cast<concepts::Requirement::RequirementKind>(Record.readInt());
+    concepts::Requirement *R = nullptr;
+    switch (RK) {
+      case concepts::Requirement::RK_Type: {
+        auto Status =
+            static_cast<concepts::TypeRequirement::SatisfactionStatus>(
+                Record.readInt());
+        if (Status == concepts::TypeRequirement::SS_SubstitutionFailure)
+          R = new (Record.getContext())
+              concepts::TypeRequirement(readSubstitutionDiagnostic(Record));
+        else
+          R = new (Record.getContext())
+              concepts::TypeRequirement(Record.readTypeSourceInfo());
+      } break;
+      case concepts::Requirement::RK_Simple:
+      case concepts::Requirement::RK_Compound: {
+        auto Status =
+            static_cast<concepts::ExprRequirement::SatisfactionStatus>(
+                Record.readInt());
+        llvm::PointerUnion<concepts::Requirement::SubstitutionDiagnostic *,
+                           Expr *> E;
+        if (Status == concepts::ExprRequirement::SS_ExprSubstitutionFailure) {
+          E = readSubstitutionDiagnostic(Record);
+        } else
+          E = Record.readExpr();
+
+        llvm::Optional<concepts::ExprRequirement::ReturnTypeRequirement> Req;
+        ConceptSpecializationExpr *SubstitutedConstraintExpr = nullptr;
+        SourceLocation NoexceptLoc;
+        if (RK == concepts::Requirement::RK_Simple) {
+          Req.emplace();
+        } else {
+          NoexceptLoc = Record.readSourceLocation();
+          switch (auto returnTypeRequirementKind = Record.readInt()) {
+            case 0:
+              // No return type requirement.
+              Req.emplace();
+              break;
+            case 1: {
+              // type-constraint
+              TemplateParameterList *TPL = Record.readTemplateParameterList();
+              if (Status >=
+                  concepts::ExprRequirement::SS_ConstraintsNotSatisfied)
+                SubstitutedConstraintExpr =
+                    cast<ConceptSpecializationExpr>(Record.readExpr());
+              Req.emplace(TPL);
+            } break;
+            case 2:
+              // Substitution failure
+              Req.emplace(readSubstitutionDiagnostic(Record));
+              break;
+          }
+        }
+        if (Expr *Ex = E.dyn_cast<Expr *>())
+          R = new (Record.getContext()) concepts::ExprRequirement(
+                  Ex, RK == concepts::Requirement::RK_Simple, NoexceptLoc,
+                  std::move(*Req), Status, SubstitutedConstraintExpr);
+        else
+          R = new (Record.getContext()) concepts::ExprRequirement(
+                  E.get<concepts::Requirement::SubstitutionDiagnostic *>(),
+                  RK == concepts::Requirement::RK_Simple, NoexceptLoc,
+                  std::move(*Req));
+      } break;
+      case concepts::Requirement::RK_Nested: {
+        if (bool IsSubstitutionDiagnostic = Record.readInt()) {
+          R = new (Record.getContext()) concepts::NestedRequirement(
+              readSubstitutionDiagnostic(Record));
+          break;
+        }
+        Expr *E = Record.readExpr();
+        if (E->isInstantiationDependent())
+          R = new (Record.getContext()) concepts::NestedRequirement(E);
+        else
+          R = new (Record.getContext())
+              concepts::NestedRequirement(Record.getContext(), E,
+                                          readConstraintSatisfaction(Record));
+      } break;
     }
+    if (!R)
+      continue;
+    Requirements.push_back(R);
   }
-  E->Satisfaction = ASTConstraintSatisfaction::Create(Record.getContext(),
-                                                      Satisfaction);
+  std::copy(Requirements.begin(), Requirements.end(),
+            E->getTrailingObjects<concepts::Requirement *>());
+  E->RBraceLoc = Record.readSourceLocation();
 }
 
 void ASTStmtReader::VisitArraySubscriptExpr(ArraySubscriptExpr *E) {
@@ -3566,11 +3683,18 @@ Stmt *ASTReader::ReadStmtFromStream(ModuleFile &F) {
       S = new (Context) DependentCoawaitExpr(Empty);
       break;
 
-    case EXPR_CONCEPT_SPECIALIZATION:
+    case EXPR_CONCEPT_SPECIALIZATION: {
       unsigned numTemplateArgs = Record[ASTStmtReader::NumExprFields];
       S = ConceptSpecializationExpr::Create(Context, Empty, numTemplateArgs);
       break;
-      
+    }
+
+    case EXPR_REQUIRES:
+      unsigned numLocalParameters = Record[ASTStmtReader::NumExprFields];
+      unsigned numRequirement = Record[ASTStmtReader::NumExprFields + 1];
+      S = RequiresExpr::Create(Context, Empty, numLocalParameters,
+                               numRequirement);
+      break;
     }
 
     // We hit a STMT_STOP, so we're done with this expression.

clang/lib/Serialization/ASTWriter.cpp

@@ -885,6 +885,7 @@ void ASTWriter::WriteBlockInfoBlock() {
   RECORD(DECL_NON_TYPE_TEMPLATE_PARM);
   RECORD(DECL_TEMPLATE_TEMPLATE_PARM);
   RECORD(DECL_CONCEPT);
+  RECORD(DECL_REQUIRES_EXPR_BODY);
   RECORD(DECL_TYPE_ALIAS_TEMPLATE);
   RECORD(DECL_STATIC_ASSERT);
   RECORD(DECL_CXX_BASE_SPECIFIERS);

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -104,6 +104,7 @@ namespace clang {
     void VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D);
     void VisitTemplateDecl(TemplateDecl *D);
     void VisitConceptDecl(ConceptDecl *D);
+    void VisitRequiresExprBodyDecl(RequiresExprBodyDecl *D);
     void VisitRedeclarableTemplateDecl(RedeclarableTemplateDecl *D);
     void VisitClassTemplateDecl(ClassTemplateDecl *D);
     void VisitVarTemplateDecl(VarTemplateDecl *D);
@@ -1481,6 +1482,10 @@ void ASTDeclWriter::VisitConceptDecl(ConceptDecl *D) {
   Code = serialization::DECL_CONCEPT;
 }
 
+void ASTDeclWriter::VisitRequiresExprBodyDecl(RequiresExprBodyDecl *D) {
+  Code = serialization::DECL_REQUIRES_EXPR_BODY;
+}
+
 void ASTDeclWriter::VisitRedeclarableTemplateDecl(RedeclarableTemplateDecl *D) {
   VisitRedeclarable(D);
 

clang/lib/Serialization/ASTWriterStmt.cpp

@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/Serialization/ASTRecordWriter.h"
+#include "clang/Sema/DeclSpec.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclObjC.h"
@@ -388,19 +389,9 @@ void ASTStmtWriter::VisitDependentCoawaitExpr(DependentCoawaitExpr *E) {
   Code = serialization::EXPR_DEPENDENT_COAWAIT;
 }
 
-void ASTStmtWriter::VisitConceptSpecializationExpr(
-        ConceptSpecializationExpr *E) {
-  VisitExpr(E);
-  ArrayRef<TemplateArgument> TemplateArgs = E->getTemplateArguments();
-  Record.push_back(TemplateArgs.size());
-  Record.AddNestedNameSpecifierLoc(E->getNestedNameSpecifierLoc());
-  Record.AddSourceLocation(E->getTemplateKWLoc());
-  Record.AddDeclarationNameInfo(E->getConceptNameInfo());
-  Record.AddDeclRef(E->getNamedConcept());
-  Record.AddASTTemplateArgumentListInfo(E->getTemplateArgsAsWritten());
-  for (const TemplateArgument &Arg : TemplateArgs)
-    Record.AddTemplateArgument(Arg);
-  const ASTConstraintSatisfaction &Satisfaction = E->getSatisfaction();
+static void
+addConstraintSatisfaction(ASTRecordWriter &Record,
+                          const ASTConstraintSatisfaction &Satisfaction) {
   Record.push_back(Satisfaction.IsSatisfied);
   if (!Satisfaction.IsSatisfied) {
     Record.push_back(Satisfaction.NumRecords);
@@ -418,10 +409,98 @@ void ASTStmtWriter::VisitConceptSpecializationExpr(
       }
     }
   }
+}
+
+static void
+addSubstitutionDiagnostic(
+    ASTRecordWriter &Record,
+    const concepts::Requirement::SubstitutionDiagnostic *D) {
+  Record.AddString(D->SubstitutedEntity);
+  Record.AddSourceLocation(D->DiagLoc);
+  Record.AddString(D->DiagMessage);
+}
+
+void ASTStmtWriter::VisitConceptSpecializationExpr(
+        ConceptSpecializationExpr *E) {
+  VisitExpr(E);
+  ArrayRef<TemplateArgument> TemplateArgs = E->getTemplateArguments();
+  Record.push_back(TemplateArgs.size());
+  Record.AddNestedNameSpecifierLoc(E->getNestedNameSpecifierLoc());
+  Record.AddSourceLocation(E->getTemplateKWLoc());
+  Record.AddDeclarationNameInfo(E->getConceptNameInfo());
+  Record.AddDeclRef(E->getNamedConcept());
+  Record.AddASTTemplateArgumentListInfo(E->getTemplateArgsAsWritten());
+  for (const TemplateArgument &Arg : TemplateArgs)
+    Record.AddTemplateArgument(Arg);
+  if (!E->isValueDependent())
+    addConstraintSatisfaction(Record, E->getSatisfaction());
 
   Code = serialization::EXPR_CONCEPT_SPECIALIZATION;
 }
 
+void ASTStmtWriter::VisitRequiresExpr(RequiresExpr *E) {
+  VisitExpr(E);
+  Record.push_back(E->getLocalParameters().size());
+  Record.push_back(E->getRequirements().size());
+  Record.AddSourceLocation(E->RequiresExprBits.RequiresKWLoc);
+  Record.push_back(E->RequiresExprBits.IsSatisfied);
+  Record.AddDeclRef(E->getBody());
+  for (ParmVarDecl *P : E->getLocalParameters())
+    Record.AddDeclRef(P);
+  for (concepts::Requirement *R : E->getRequirements()) {
+    if (auto *TypeReq = dyn_cast<concepts::TypeRequirement>(R)) {
+      Record.push_back(concepts::Requirement::RK_Type);
+      Record.push_back(TypeReq->Status);
+      if (TypeReq->Status == concepts::TypeRequirement::SS_SubstitutionFailure)
+        addSubstitutionDiagnostic(Record, TypeReq->getSubstitutionDiagnostic());
+      else
+        Record.AddTypeSourceInfo(TypeReq->getType());
+    } else if (auto *ExprReq = dyn_cast<concepts::ExprRequirement>(R)) {
+      Record.push_back(ExprReq->getKind());
+      Record.push_back(ExprReq->Status);
+      if (ExprReq->isExprSubstitutionFailure()) {
+        addSubstitutionDiagnostic(Record,
+         ExprReq->Value.get<concepts::Requirement::SubstitutionDiagnostic *>());
+      } else
+        Record.AddStmt(ExprReq->Value.get<Expr *>());
+      if (ExprReq->getKind() == concepts::Requirement::RK_Compound) {
+        Record.AddSourceLocation(ExprReq->NoexceptLoc);
+        const auto &RetReq = ExprReq->getReturnTypeRequirement();
+        if (RetReq.isSubstitutionFailure()) {
+          Record.push_back(2);
+          addSubstitutionDiagnostic(Record, RetReq.getSubstitutionDiagnostic());
+        } else if (RetReq.isTypeConstraint()) {
+          Record.push_back(1);
+          Record.AddTemplateParameterList(
+              RetReq.getTypeConstraintTemplateParameterList());
+          if (ExprReq->Status >=
+              concepts::ExprRequirement::SS_ConstraintsNotSatisfied)
+            Record.AddStmt(
+                ExprReq->getReturnTypeRequirementSubstitutedConstraintExpr());
+        } else {
+          assert(RetReq.isEmpty());
+          Record.push_back(0);
+        }
+      }
+    } else {
+      auto *NestedReq = cast<concepts::NestedRequirement>(R);
+      Record.push_back(concepts::Requirement::RK_Nested);
+      Record.push_back(NestedReq->isSubstitutionFailure());
+      if (NestedReq->isSubstitutionFailure()){
+        addSubstitutionDiagnostic(Record,
+                                  NestedReq->getSubstitutionDiagnostic());
+      } else {
+        Record.AddStmt(NestedReq->Value.get<Expr *>());
+        if (!NestedReq->isDependent())
+          addConstraintSatisfaction(Record, *NestedReq->Satisfaction);
+      }
+    }
+  }
+  Record.AddSourceLocation(E->getEndLoc());
+
+  Code = serialization::EXPR_REQUIRES;
+}
+
 
 void ASTStmtWriter::VisitCapturedStmt(CapturedStmt *S) {
   VisitStmt(S);

clang/lib/StaticAnalyzer/Core/ExprEngine.cpp

@@ -1386,6 +1386,7 @@ void ExprEngine::Visit(const Stmt *S, ExplodedNode *Pred,
     case Stmt::AsTypeExprClass:
     case Stmt::ConceptSpecializationExprClass:
     case Stmt::CXXRewrittenBinaryOperatorClass:
+    case Stmt::RequiresExprClass:
       // Fall through.
 
     // Cases we intentionally don't evaluate, since they don't need

clang/test/CXX/expr/expr.prim/expr.prim.req/compound-requirement.cpp

@@ -0,0 +1,175 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ %s -verify
+
+static_assert(requires { { 0 }; });
+static_assert(requires { { "aaaa" }; });
+static_assert(requires { { (0).da }; }); // expected-error{{member reference base type 'int' is not a structure or union}}
+
+void foo() {}
+static_assert(requires { { foo() }; });
+
+// Substitution failure in expression
+
+struct A {};
+struct B {
+    B operator+(const B &other) const { return other; }
+};
+struct C {
+    C operator+(C &other) const { return other; }
+};
+
+template<typename T> requires requires (T a, const T& b) { { a + b }; } // expected-note{{because 'a + b' would be invalid: invalid operands to binary expression ('A' and 'const A')}} expected-note{{because 'a + b' would be invalid: invalid operands to binary expression ('C' and 'const C')}}
+struct r1 {};
+
+using r1i1 = r1<int>;
+using r1i2 = r1<A>; // expected-error{{constraints not satisfied for class template 'r1' [with T = A]}}
+using r1i3 = r1<B>;
+using r1i4 = r1<C>; // expected-error{{constraints not satisfied for class template 'r1' [with T = C]}}
+
+struct D { void foo() {} };
+
+template<typename T> requires requires (T a) { { a.foo() }; } // expected-note{{because 'a.foo()' would be invalid: no member named 'foo' in 'A'}} expected-note{{because 'a.foo()' would be invalid: member reference base type 'int' is not a structure or union}} expected-note{{because 'a.foo()' would be invalid: 'this' argument to member function 'foo' has type 'const D', but function is not marked const}}
+struct r2 {};
+
+using r2i1 = r2<int>; // expected-error{{constraints not satisfied for class template 'r2' [with T = int]}}
+using r2i2 = r2<A>; // expected-error{{constraints not satisfied for class template 'r2' [with T = A]}}
+using r2i3 = r2<D>;
+using r2i4 = r2<const D>; // expected-error{{constraints not satisfied for class template 'r2' [with T = const D]}}
+
+template<typename T> requires requires { { sizeof(T) }; } // expected-note{{because 'sizeof(T)' would be invalid: invalid application of 'sizeof' to an incomplete type 'void'}} expected-note{{because 'sizeof(T)' would be invalid: invalid application of 'sizeof' to an incomplete type 'nonexistent'}}
+struct r3 {};
+
+using r3i1 = r3<int>;
+using r3i2 = r3<A>;
+using r3i3 = r3<A &>;
+using r3i4 = r3<void>; // expected-error{{constraints not satisfied for class template 'r3' [with T = void]}}
+using r3i4 = r3<class nonexistent>; // expected-error{{constraints not satisfied for class template 'r3' [with T = nonexistent]}}
+
+// Non-dependent expressions
+
+template<typename T> requires requires (T t) { { 0 }; { "a" }; { (void)'a' }; }
+struct r4 {};
+
+using r4i1 = r4<int>;
+using r4i2 = r4<int[10]>;
+using r4i3 = r4<int(int)>;
+
+// Noexcept requirement
+void maythrow() { }
+static_assert(!requires { { maythrow() } noexcept; });
+static_assert(requires { { 1 } noexcept; });
+
+struct E { void operator++(int) noexcept; };
+struct F { void operator++(int); };
+
+template<typename T> requires requires (T t) { { t++ } noexcept; } // expected-note{{because 't ++' may throw an exception}}
+struct r5 {};
+
+using r5i1 = r5<int>;
+using r5i2 = r5<E>;
+using r5i2 = r5<F>; // expected-error{{constraints not satisfied for class template 'r5' [with T = F]}}
+
+template<typename T> requires requires (T t) { { t.foo() } noexcept; } // expected-note{{because 't.foo()' would be invalid: no member named 'foo' in 'E'}}
+struct r6 {};
+
+using r6i = r6<E>; // expected-error{{constraints not satisfied for class template 'r6' [with T = E]}}
+
+template<typename T, typename U>
+constexpr bool is_same_v = false;
+
+template<typename T>
+constexpr bool is_same_v<T, T> = true;
+
+template<typename T, typename U>
+concept Same = is_same_v<T, U>;
+
+template<typename T>
+concept Large = sizeof(T) >= 4; // expected-note{{because 'sizeof(short) >= 4' (2 >= 4) evaluated to false}}
+
+template<typename T> requires requires (T t) { { t } -> Large; } // expected-note{{because 'decltype(t)' (aka 'short') does not satisfy 'Large':}}
+struct r7 {};
+
+using r7i1 = r7<int>;
+using r7i2 = r7<short>; // expected-error{{constraints not satisfied for class template 'r7' [with T = short]}}
+
+template<typename T> requires requires (T t) { { t } -> Same<T>; }
+struct r8 {};
+
+using r8i1 = r8<int>;
+using r8i2 = r8<short*>;
+
+// Substitution failure in type constraint
+
+template<typename T> requires requires (T t) { { t } -> Same<typename T::type>; } // expected-note{{because 'Same<expr-type, typename T::type>' would be invalid: type 'int' cannot be used prior to '::' because it has no members}}
+struct r9 {};
+
+struct M { using type = M; };
+
+using r9i1 = r9<M>;
+using r9i2 = r9<int>; // expected-error{{constraints not satisfied for class template 'r9' [with T = int]}}
+
+// Substitution failure in both expression and return type requirement
+
+template<typename T> requires requires (T t) { { t.foo() } -> Same<typename T::type>; } // expected-note{{because 't.foo()' would be invalid: member reference base type 'int' is not a structure or union}}
+struct r10 {};
+
+using r10i = r10<int>; // expected-error{{constraints not satisfied for class template 'r10' [with T = int]}}
+
+// Non-type concept in type constraint
+
+template<int T>
+concept IsEven = (T % 2) == 0;
+
+template<typename T> requires requires (T t) { { t } -> IsEven; } // expected-error{{concept named in type constraint is not a type concept}}
+struct r11 {};
+
+// C++ [expr.prim.req.compound] Example
+namespace std_example {
+  template<typename T> concept C1 =
+    requires(T x) {
+      {x++};
+    };
+
+  template<typename T, typename U> constexpr bool is_same_v = false;
+  template<typename T> constexpr bool is_same_v<T, T> = true;
+
+  template<typename T, typename U> concept same_as = is_same_v<T, U>;
+  // expected-note@-1 {{because 'is_same_v<int, int *>' evaluated to false}}
+
+  static_assert(C1<int>);
+  static_assert(C1<int*>);
+  template<C1 T> struct C1_check {};
+  using c1c1 = C1_check<int>;
+  using c1c2 = C1_check<int[10]>;
+
+  template<typename T> concept C2 =
+    requires(T x) {
+      {*x} -> same_as<typename T::inner>;
+      // expected-note@-1{{because type constraint 'same_as<int, typename T2::inner>' was not satisfied:}}
+      // expected-note@-2{{because '*x' would be invalid: indirection requires pointer operand ('int' invalid)}}
+    };
+
+  struct T1 {
+    using inner = int;
+    inner operator *() { return 0; }
+  };
+  struct T2 {
+    using inner = int *;
+    int operator *() { return 0; }
+  };
+  static_assert(C2<T1>);
+  template<C2 T> struct C2_check {}; // expected-note{{because 'int' does not satisfy 'C2'}} expected-note{{because 'std_example::T2' does not satisfy 'C2'}}
+  using c2c1 = C2_check<int>; // expected-error{{constraints not satisfied for class template 'C2_check' [with T = int]}}
+  using c2c2 = C2_check<T2>; // expected-error{{constraints not satisfied for class template 'C2_check' [with T = std_example::T2]}}
+
+  template<typename T>
+  void g(T t) noexcept(sizeof(T) == 1) {}
+
+  template<typename T> concept C5 =
+    requires(T x) {
+      {g(x)} noexcept; // expected-note{{because 'g(x)' may throw an exception}}
+    };
+
+  static_assert(C5<char>);
+  template<C5 T> struct C5_check {}; // expected-note{{because 'short' does not satisfy 'C5'}}
+  using c5 = C5_check<short>; // expected-error{{constraints not satisfied for class template 'C5_check' [with T = short]}}
+}

clang/test/CXX/expr/expr.prim/expr.prim.req/equivalence.cpp

@@ -0,0 +1,125 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ %s -verify
+
+template<typename T, typename U> constexpr bool is_same_v = false;
+template<typename T> constexpr bool is_same_v<T, T> = true;
+
+template<typename T> struct identity { using type = T; };
+template<typename T> using identity_t = T;
+
+// Type requirements
+template<typename T> requires requires { typename identity_t<T>; }
+struct r1;
+template<typename U> requires requires { typename identity_t<U>; } // expected-note{{previous template declaration is here}}
+struct r1;
+template<typename T> requires requires { typename identity_t<T*>; } // expected-error{{requires clause differs in template redeclaration}}
+struct r1;
+template<typename T> requires requires { typename ::identity_t<T>; }
+struct r1;
+
+template<typename Y> requires requires { typename identity<Y>::type; }
+struct r2;
+template<typename U> requires requires { typename identity<U>::type; }
+struct r2;
+template<typename T> requires requires { typename ::identity<T>::type; } // expected-note 2{{previous template declaration is here}}
+struct r2;
+template<typename T> requires requires { typename identity<T>::typr; } // expected-error{{requires clause differs in template redeclaration}}
+struct r2;
+namespace ns {
+  template<typename T> struct identity { using type = T; };
+}
+template<typename T> requires requires { typename ns::identity<T>::type; } // expected-error{{requires clause differs in template redeclaration}}
+struct r2;
+
+template<typename T> requires requires { typename T::template identity<T>::type; }
+struct r3;
+template<typename U> requires requires { typename U::template identity<U>::type; } // expected-note{{previous template declaration is here}}
+struct r3;
+template<typename T> requires requires { typename T::template identitr<T>::type; } // expected-error{{requires clause differs in template redeclaration}}
+struct r3;
+
+template<typename T> requires requires { typename T::template temp<>; }
+struct r4;
+template<typename U> requires requires { typename U::template temp<>; }
+struct r4;
+
+// Expr requirements
+template<typename T> requires requires { 0; } // expected-note{{previous template declaration is here}}
+struct r5;
+template<typename T> requires requires { 1; } // expected-error{{requires clause differs in template redeclaration}}
+struct r5;
+
+template<typename T>
+concept C1 = true;
+
+template<typename T> requires requires { sizeof(T); }
+struct r6;
+template<typename U> requires requires { sizeof(U); } // expected-note{{previous template declaration is here}}
+struct r6;
+template<typename U> requires requires { sizeof(U) - 1; } // expected-error{{requires clause differs in template redeclaration}}
+struct r6;
+template<typename T> requires requires { { sizeof(T) }; } // expected-note 2{{previous template declaration is here}}
+struct r6;
+template<typename T> requires requires { { sizeof(T) } noexcept; } // expected-error{{requires clause differs in template redeclaration}}
+struct r6;
+template<typename T> requires requires { { sizeof(T) } -> C1; } // expected-error{{requires clause differs in template redeclaration}}
+struct r6;
+
+template<typename T> requires requires { { sizeof(T) } -> C1; }
+struct r7;
+template<typename U> requires requires { { sizeof(U) } -> C1; }
+struct r7;
+template<typename T> requires requires { { sizeof(T) } -> C1<>; } // expected-note {{previous template declaration is here}}
+struct r7;
+template<typename U> requires requires { { sizeof(U) }; } // expected-error{{requires clause differs in template redeclaration}}
+struct r7;
+
+template<typename T, typename U>
+concept C2 = true;
+
+template<typename T> requires requires { { sizeof(T) } -> C2<T>; }
+struct r8;
+template<typename U> requires requires { { sizeof(U) } -> C2<U>; } // expected-note{{previous template declaration is here}}
+struct r8;
+template<typename T> requires requires { { sizeof(T) } -> C2<T*>; } // expected-error{{requires clause differs in template redeclaration}}
+struct r8;
+
+// Nested requirements
+template<typename T> requires requires { requires sizeof(T) == 0; }
+struct r9;
+template<typename U> requires requires { requires sizeof(U) == 0; } // expected-note{{previous template declaration is here}}
+struct r9;
+template<typename T> requires requires { requires sizeof(T) == 1; } // expected-error{{requires clause differs in template redeclaration}}
+struct r9;
+
+// Parameter list
+template<typename T> requires requires { requires true; }
+struct r10;
+template<typename T> requires requires() { requires true; } // expected-note{{previous template declaration is here}}
+struct r10;
+template<typename T> requires requires(T i) { requires true; } // expected-error{{requires clause differs in template redeclaration}}
+struct r10;
+
+template<typename T> requires requires(T i, T *j) { requires true; } // expected-note 2{{previous template declaration is here}}
+struct r11;
+template<typename T> requires requires(T i) { requires true; } // expected-error{{requires clause differs in template redeclaration}}
+struct r11;
+template<typename T> requires requires(T i, T *j, T &k) { requires true; } // expected-error{{requires clause differs in template redeclaration}}
+struct r11;
+
+// Parameter names
+template<typename T> requires requires(int i) { requires sizeof(i) == 1; }
+struct r12;
+template<typename T> requires requires(int j) { requires sizeof(j) == 1; } // expected-note 2{{previous template declaration is here}}
+struct r12;
+template<typename T> requires requires(int k) { requires sizeof(k) == 2; } // expected-error{{requires clause differs in template redeclaration}}
+struct r12;
+template<typename T> requires requires(const int k) { requires sizeof(k) == 1; } // expected-error{{requires clause differs in template redeclaration}}
+struct r12;
+
+// Order of requirements
+template<typename T> requires requires { requires true; 0; }
+struct r13;
+template<typename T> requires requires { requires true; 0; } // expected-note{{previous template declaration is here}}
+struct r13;
+template<typename T> requires requires { 0; requires true; } // expected-error{{requires clause differs in template redeclaration}}
+struct r13;

clang/test/CXX/expr/expr.prim/expr.prim.req/nested-requirement.cpp

@@ -0,0 +1,46 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ %s -verify
+
+static_assert(requires { requires true; });
+
+template<typename T> requires requires { requires false; } // expected-note{{because 'false' evaluated to false}}
+struct r1 {};
+
+using r1i = r1<int>; // expected-error{{constraints not satisfied for class template 'r1' [with T = int]}}
+
+template<typename T> requires requires { requires sizeof(T) == 0; } // expected-note{{because 'sizeof(int) == 0' (4 == 0) evaluated to false}}
+struct r2 {};
+
+using r2i = r2<int>; // expected-error{{constraints not satisfied for class template 'r2' [with T = int]}}
+
+template<typename T> requires requires (T t) { requires sizeof(t) == 0; } // expected-note{{because 'sizeof (t) == 0' (4 == 0) evaluated to false}}
+struct r3 {};
+
+using r3i = r3<int>; // expected-error{{constraints not satisfied for class template 'r3' [with T = int]}}
+
+template<typename T>
+struct X {
+    template<typename U> requires requires (U u) { requires sizeof(u) == sizeof(T); } // expected-note{{because 'sizeof (u) == sizeof(T)' would be invalid: invalid application of 'sizeof' to an incomplete type 'void'}}
+    struct r4 {};
+};
+
+using r4i = X<void>::r4<int>; // expected-error{{constraints not satisfied for class template 'r4' [with U = int]}}
+
+// C++ [expr.prim.req.nested] Examples
+namespace std_example {
+  template<typename U> concept C1 = sizeof(U) == 1; // expected-note{{because 'sizeof(int) == 1' (4 == 1) evaluated to false}}
+  template<typename T> concept D =
+    requires (T t) {
+      requires C1<decltype (+t)>; // expected-note{{because 'decltype(+t)' (aka 'int') does not satisfy 'C1'}}
+  };
+
+  struct T1 { char operator+() { return 'a'; } };
+  static_assert(D<T1>);
+  template<D T> struct D_check {}; // expected-note{{because 'short' does not satisfy 'D'}}
+  using dc1 = D_check<short>; // expected-error{{constraints not satisfied for class template 'D_check' [with T = short]}}
+
+  template<typename T>
+  concept C2 = requires (T a) { // expected-note{{'a' declared here}}
+      requires sizeof(a) == 4; // OK
+      requires a == 0; // expected-error{{constraint variable 'a' cannot be used in an evaluated context}}
+    };
+}

clang/test/CXX/expr/expr.prim/expr.prim.req/p3.cpp

@@ -0,0 +1,37 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ %s -verify
+
+// Examples from standard
+
+template<typename T, typename U>
+concept convertible_to = requires(T t) { U(t); };
+
+template<typename T>
+concept R = requires (T i) {
+  typename T::type;
+  {*i} -> convertible_to<const typename T::type&>;
+};
+
+template<typename T> requires R<T> struct S {};
+
+struct T {
+  using type = int;
+  type i;
+  const type &operator*() { return i; }
+};
+
+using si = S<T>;
+
+template<typename T>
+requires requires (T x) { x + x; } // expected-note{{because 'x + x' would be invalid: invalid operands to binary expression ('T' and 'T')}}
+T add(T a, T b) { return a + b; } // expected-note{{candidate template ignored: constraints not satisfied [with T = T]}}
+
+int x = add(1, 2);
+int y = add(T{}, T{}); // expected-error{{no matching function for call to 'add'}}
+
+template<typename T>
+concept C = requires (T x) { x + x; }; // expected-note{{because 'x + x' would be invalid: invalid operands to binary expression ('T' and 'T')}}
+template<typename T> requires C<T> // expected-note{{because 'T' does not satisfy 'C'}}
+T add2(T a, T b) { return a + b; } // expected-note{{candidate template ignored: constraints not satisfied [with T = T]}}
+
+int z = add2(1, 2);
+int w = add2(T{}, T{}); // expected-error{{no matching function for call to 'add2'}}

clang/test/CXX/expr/expr.prim/expr.prim.req/requires-expr.cpp

@@ -0,0 +1,68 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ %s -verify
+
+using A = int;
+
+template<typename T, typename U>
+constexpr bool is_same_v = false;
+
+template<typename T>
+constexpr bool is_same_v<T, T> = true;
+
+template<typename T, typename U>
+concept same_as = is_same_v<T, U>;
+
+static_assert(requires { requires true; 0; typename A;
+                         { 0 } -> same_as<int>; });
+static_assert(is_same_v<bool, decltype(requires { requires false; })>);
+
+// Check that requires expr is an unevaluated context.
+struct Y {
+  int i;
+  static constexpr bool r = requires { i; };
+};
+
+template<typename T> requires requires (T t) {
+  requires false; // expected-note{{because 'false' evaluated to false}}
+  requires false;
+  requires requires {
+    requires false;
+  };
+}
+struct r1 { };
+
+using r1i = r1<int>;
+// expected-error@-1 {{constraints not satisfied for class template 'r1' [with T = int]}}
+
+template<typename T> requires requires (T t) {
+  requires requires {
+    requires false; // expected-note{{because 'false' evaluated to false}}
+  };
+}
+struct r2 { };
+
+using r2i = r2<int>;
+// expected-error@-1 {{constraints not satisfied for class template 'r2' [with T = int]}}
+
+template<typename T> requires requires (T t) {
+  requires requires {
+    requires true;
+  };
+  requires true;
+  requires requires {
+    requires false; // expected-note{{because 'false' evaluated to false}}
+  };
+}
+struct r3 { };
+
+using r3i = r3<int>;
+// expected-error@-1 {{constraints not satisfied for class template 'r3' [with T = int]}}
+
+template<typename T>
+struct S { static const int s = T::value; };
+
+template<typename T> requires requires { T::value; S<T>::s; }
+// expected-note@-1 {{because 'T::value' would be invalid: type 'int' cannot be used prior to '::' because it has no members}}
+struct r4 { };
+
+using r4i = r4<int>;
+// expected-error@-1 {{constraints not satisfied for class template 'r4' [with T = int]}}

clang/test/CXX/expr/expr.prim/expr.prim.req/simple-requirement.cpp

@@ -0,0 +1,106 @@
+// RUN: %clang_cc1 %s -I%S -std=c++2a -fconcepts-ts -verify
+
+namespace std { struct type_info; }
+
+static_assert(requires { 0; });
+static_assert(requires { "aaaa"; });
+static_assert(requires { (0).da; }); // expected-error{{member reference base type 'int' is not a structure or union}}
+
+struct A {};
+struct B {
+    B operator+(const B &other) const { return other; }
+};
+struct C {
+    C operator+(C &other) const { return other; }
+};
+
+template<typename T> requires requires (T a, const T& b) { a + b; }
+// expected-note@-1{{because 'a + b' would be invalid: invalid operands to binary expression ('A' and 'const A')}}
+// expected-note@-2{{because 'a + b' would be invalid: invalid operands to binary expression ('C' and 'const C')}}
+struct r1 {};
+
+using r1i1 = r1<int>;
+using r1i2 = r1<A>; // expected-error{{constraints not satisfied for class template 'r1' [with T = A]}}
+using r1i3 = r1<B>;
+using r1i4 = r1<C>; // expected-error{{constraints not satisfied for class template 'r1' [with T = C]}}
+
+struct D { void foo() {} };
+
+template<typename T> requires requires (T a) { a.foo(); }
+// expected-note@-1{{because 'a.foo()' would be invalid: no member named 'foo' in 'A'}}
+// expected-note@-2{{because 'a.foo()' would be invalid: member reference base type 'int' is not a structure or union}}
+// expected-note@-3{{because 'a.foo()' would be invalid: 'this' argument to member function 'foo' has type 'const D', but function is not marked const}}
+struct r2 {};
+
+using r2i1 = r2<int>; // expected-error{{constraints not satisfied for class template 'r2' [with T = int]}}
+using r2i2 = r2<A>; // expected-error{{constraints not satisfied for class template 'r2' [with T = A]}}
+using r2i3 = r2<D>;
+using r2i4 = r2<const D>; // expected-error{{constraints not satisfied for class template 'r2' [with T = const D]}}
+
+template<typename T> requires requires { sizeof(T); }
+// expected-note@-1{{because 'sizeof(T)' would be invalid: invalid application of 'sizeof' to an incomplete type 'void'}}
+// expected-note@-2{{because 'sizeof(T)' would be invalid: invalid application of 'sizeof' to an incomplete type 'nonexistent'}}
+struct r3 {};
+
+using r3i1 = r3<int>;
+using r3i2 = r3<A>;
+using r3i3 = r3<A &>;
+using r3i4 = r3<void>; // expected-error{{constraints not satisfied for class template 'r3' [with T = void]}}
+using r3i4 = r3<class nonexistent>; // expected-error{{constraints not satisfied for class template 'r3' [with T = nonexistent]}}
+
+template<typename T> requires requires (T t) { 0; "a"; (void)'a'; }
+struct r4 {};
+
+using r4i1 = r4<int>;
+using r4i2 = r4<int[10]>;
+using r4i3 = r4<int(int)>;
+
+template<class T> void f(T) = delete;
+template<class T> requires (sizeof(T) == 1) void f(T) { }
+
+template<typename T> requires requires(T t) { f(t); }
+// expected-note@-1{{because 'f(t)' would be invalid: call to deleted function 'f'}}
+struct r5 {};
+
+using r5i1 = r5<int>;
+// expected-error@-1 {{constraints not satisfied for class template 'r5' [with T = int]}}
+using r5i2 = r5<char>;
+
+template<typename T>
+struct E {
+  struct non_default_constructible { non_default_constructible(T t) { } };
+};
+
+template<typename T> requires requires(T t) { typename E<T>::non_default_constructible{}; }
+// expected-note@-1 {{because 'typename E<T>::non_default_constructible({})' would be invalid: no matching constructor for initialization of 'typename E<int>::non_default_constructible'}}
+struct r6 {};
+
+using r6i1 = r6<int>;
+// expected-error@-1 {{constraints not satisfied for class template 'r6' [with T = int]}}
+
+template<typename T> requires requires(T t) { typename E<T>::non_default_constructible(); }
+// expected-note@-1 {{because 'typename E<T>::non_default_constructible()' would be invalid: no matching constructor for initialization of 'typename E<int>::non_default_constructible'}}
+struct r7 {};
+
+using r7i1 = r7<int>;
+// expected-error@-1 {{constraints not satisfied for class template 'r7' [with T = int]}}
+
+// C++ [expr.prim.req.simple] Example
+namespace std_example {
+  template<typename T> concept C =
+    requires (T a, T b) { // expected-note{{because substituted constraint expression is ill-formed: argument may not have 'void' type}}
+      a + b; // expected-note{{because 'a + b' would be invalid: invalid operands to binary expression ('int *' and 'int *')}}
+    };
+
+  static_assert(C<int>);
+  template<C T> struct C_check {}; // expected-note{{because 'void' does not satisfy 'C'}} expected-note{{because 'int *' does not satisfy 'C'}}
+  using c1c1 = C_check<void>; // expected-error{{constraints not satisfied for class template 'C_check' [with T = void]}}
+  using c1c2 = C_check<int *>; // expected-error{{constraints not satisfied for class template 'C_check' [with T = int *]}}
+}
+
+// typeid() of an expression becomes potentially evaluated if the expression is
+// of a polymorphic type.
+class X { virtual ~X(); };
+constexpr bool b = requires (X &x) { static_cast<int(*)[(typeid(x), 0)]>(nullptr); };
+// expected-error@-1{{constraint variable 'x' cannot be used in an evaluated context}}
+// expected-note@-2{{'x' declared here}}

clang/test/CXX/expr/expr.prim/expr.prim.req/type-requirement.cpp

@@ -0,0 +1,194 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ %s -verify
+
+using A = int;
+
+template<typename T> using identity_t = T; // expected-note 4{{template is declared here}}}
+
+template<typename T> struct identity { using type = T; };
+// expected-note@-1 2{{template is declared here}}
+
+struct C {};
+
+struct D { static int type; }; // expected-note{{referenced member 'type' is declared here}}
+
+// Basic unqualified and global-qualified lookups
+
+static_assert(requires { typename A; typename ::A; });
+static_assert(requires { typename identity_t<A>; typename ::identity_t<A>; });
+static_assert(!requires { typename identity_t<A, A>; }); // expected-error{{too many template arguments for alias template 'identity_t'}}
+static_assert(!requires { typename ::identity_t<A, A>; }); // expected-error{{too many template arguments for alias template 'identity_t'}}
+static_assert(requires { typename identity<A>; });
+static_assert(!requires { typename identity; });
+// expected-error@-1 {{typename specifier refers to class template; argument deduction not allowed here}}
+static_assert(!requires { typename ::identity; });
+// expected-error@-1 {{typename specifier refers to class template; argument deduction not allowed here}}
+static_assert(!requires { typename identity_t; });
+// expected-error@-1 {{typename specifier refers to alias template; argument deduction not allowed here}}
+static_assert(!requires { typename ::identity_t; });
+// expected-error@-1 {{typename specifier refers to alias template; argument deduction not allowed here}}
+
+namespace ns {
+  using B = int;
+  int C = 0;
+  // expected-note@-1 {{referenced 'C' is declared here}}
+  static_assert(requires { typename A; typename B; typename ::A; });
+  static_assert(!requires { typename ns::A; }); // expected-error{{no type named 'A' in namespace 'ns'}}
+  static_assert(!requires { typename ::B; }); // expected-error{{no type named 'B' in the global namespace}}
+  static_assert(requires { typename C; });
+  // expected-error@-1 {{typename specifier refers to non-type 'C'}}
+}
+
+// member type lookups
+
+static_assert(requires { typename identity<int>::type; typename ::identity<int>::type; });
+static_assert(!requires { typename identity<int>::typr; }); // expected-error{{no type named 'typr' in 'identity<int>'}}
+static_assert(!requires { typename ::identity<int>::typr; }); // expected-error{{no type named 'typr' in 'identity<int>'}}
+
+template<typename T> requires requires { typename T::type; }
+// expected-note@-1 {{because 'typename T::type' would be invalid: type 'int' cannot be used prior to '::' because it has no members}}
+// expected-note@-2 {{because 'typename T::type' would be invalid: no type named 'type' in 'C'}}
+// expected-note@-3 {{because 'typename T::type' would be invalid: typename specifier refers to non-type member 'type' in 'D'}}
+// expected-note@-4 {{in instantiation of template class 'invalid<D>' requested here}}
+// expected-note@-5 {{in instantiation of requirement here}}
+// expected-note@-6 {{while substituting template arguments into constraint expression here}}
+// expected-note@-7 {{because 'typename T::type' would be invalid}}
+struct r1 {};
+
+using r1i1 = r1<identity<int>>;
+using r1i2 = r1<int>; // expected-error{{constraints not satisfied for class template 'r1' [with T = int]}}
+using r1i3 = r1<C>; // expected-error{{constraints not satisfied for class template 'r1' [with T = C]}}
+using r1i4 = r1<D>; // expected-error{{constraints not satisfied for class template 'r1' [with T = D]}}
+
+template<typename T> struct invalid { typename T::type x; };
+// expected-error@-1 {{typename specifier refers to non-type member 'type' in 'D'}}
+using r1i5 = r1<invalid<D>>;
+// expected-error@-1 {{constraints not satisfied for class template 'r1' [with T = invalid<D>]}}
+// expected-note@-2 {{while checking constraint satisfaction for template 'r1<invalid<D> >' required here}}
+
+// mismatching template arguments
+
+template<typename... Ts> requires requires { typename identity<Ts...>; } // expected-note{{because 'typename identity<Ts...>' would be invalid: too many template arguments for class template 'identity'}}
+struct r2 {};
+
+using r2i1 = r2<int>;
+using r2i2 = r2<void>;
+using r2i3 = r2<int, int>; // expected-error{{constraints not satisfied for class template 'r2' [with Ts = <int, int>]}}
+
+namespace ns2 {
+  template<typename T, typename U> struct identity {};
+
+  template<typename... Ts> requires requires { typename identity<Ts...>; } // expected-note 2{{because 'typename identity<Ts...>' would be invalid: too few template arguments for class template 'identity'}}
+  struct r4 {};
+
+  using r4i1 = r4<int>; // expected-error{{constraints not satisfied for class template 'r4' [with Ts = <int>]}}
+}
+
+using r4i2 = ns2::r4<int>; // expected-error{{constraints not satisfied for class template 'r4' [with Ts = <int>]}}
+
+using E = int;
+template<typename T> requires requires { typename E<T>; } // expected-error{{expected ';' at end of requirement}}
+struct r5v1 {};
+template<typename T> requires requires { typename ::E<T>; } // expected-error{{expected ';' at end of requirement}}
+struct r5v2 {};
+
+template<typename T> requires (sizeof(T) == 1)
+struct chars_only {};
+
+template<typename T> requires requires { typename chars_only<T>; } // expected-note{{because 'typename chars_only<T>' would be invalid: constraints not satisfied for class template 'chars_only' [with T = int]}}
+struct r6 {};
+
+using r6i = r6<int>; // expected-error{{constraints not satisfied for class template 'r6' [with T = int]}}
+
+template<typename T> int F = 0; // expected-note 2{{variable template 'F' declared here}}
+
+static_assert(!requires { typename F<int>; });
+// expected-error@-1{{template name refers to non-type template 'F'}}
+static_assert(!requires { typename ::F<int>; });
+// expected-error@-1{{template name refers to non-type template '::F'}}
+
+struct G { template<typename T> static T temp; };
+
+template<typename T> requires requires { typename T::template temp<int>; }
+// expected-note@-1{{because 'typename T::temp<int>' would be invalid: type 'int' cannot be used prior to '::' because it has no members}}
+// expected-note@-2{{because 'typename T::temp<int>' would be invalid: no member named 'temp' in 'D'}}
+// expected-note@-3{{because 'typename T::temp<int>' would be invalid: template name refers to non-type template 'G::template temp'}}
+struct r7 {};
+
+using r7i1 = r7<int>; // expected-error{{constraints not satisfied for class template 'r7' [with T = int]}}
+using r7i2 = r7<D>; // expected-error{{constraints not satisfied for class template 'r7' [with T = D]}}
+using r7i3 = r7<G>; // expected-error{{constraints not satisfied for class template 'r7' [with T = G]}}
+
+template<typename T> struct H;
+
+template<typename T> requires requires { typename H<T>; }
+struct r8 {};
+
+using r8i = r8<int>;
+
+template<typename T> struct I { struct incomplete; }; // expected-note{{member is declared here}}
+
+static_assert(!requires { I<int>::incomplete::inner; }); // expected-error{{implicit instantiation of undefined member 'I<int>::incomplete'}}
+
+template<typename T> requires requires { typename I<T>::incomplete::inner; } // expected-note{{because 'typename I<T>::incomplete::inner' would be invalid: implicit instantiation of undefined member 'I<int>::incomplete'}}
+struct r9 {};
+
+using r9i = r9<int>; // expected-error{{constraints not satisfied for class template 'r9' [with T = int]}}
+
+namespace ns3 {
+  struct X { }; // expected-note 2{{candidate found by name lookup is 'ns3::X'}}
+}
+
+struct X { using inner = int; }; // expected-note 2{{candidate found by name lookup is 'X'}}
+
+using namespace ns3;
+static_assert(requires { typename X; }); // expected-error{{reference to 'X' is ambiguous}}
+static_assert(requires { typename X::inner; }); // expected-error{{reference to 'X' is ambiguous}}
+// expected-error@-1{{unknown type name 'inner'}}
+
+// naming a type template specialization in a type requirement does not require
+// it to be complete and should not care about partial specializations.
+
+template<typename T>
+struct Z;
+
+template<typename T> requires (sizeof(T) >= 1)
+struct Z<T> {}; // expected-note{{partial specialization matches [with T = int]}}
+
+template<typename T> requires (sizeof(T) <= 4)
+struct Z<T> {}; // expected-note{{partial specialization matches [with T = int]}}
+
+Z<int> x; // expected-error{{ambiguous partial specializations of 'Z<int>'}}
+
+static_assert(requires { typename Z<int>; });
+
+// C++ [expr.prim.req.type] Example
+namespace std_example {
+  template<typename T, typename T::type = 0> struct S;
+  // expected-note@-1 {{because 'typename S<T>' would be invalid: no type named 'type' in 'std_example::has_inner}}
+  template<typename T> using Ref = T&; // expected-note{{because 'typename Ref<T>' would be invalid: cannot form a reference to 'void'}}
+  template<typename T> concept C1 =
+    requires {
+      typename T::inner;
+      // expected-note@-1 {{because 'typename T::inner' would be invalid: type 'int' cannot be used prior to '::' because it has no members}}
+      // expected-note@-2 {{because 'typename T::inner' would be invalid: no type named 'inner' in 'std_example::has_type'}}
+    };
+  template<typename T> concept C2 = requires { typename S<T>; };
+  template<typename T> concept C3 = requires { typename Ref<T>; };
+
+  struct has_inner { using inner = int;};
+  struct has_type { using type = int; };
+  struct has_inner_and_type { using inner = int; using type = int; };
+
+  static_assert(C1<has_inner_and_type> && C2<has_inner_and_type> && C3<has_inner_and_type>);
+  template<C1 T> struct C1_check {};
+  // expected-note@-1 {{because 'int' does not satisfy 'C1'}}
+  // expected-note@-2 {{because 'std_example::has_type' does not satisfy 'C1'}}
+  template<C2 T> struct C2_check {};
+  // expected-note@-1 {{because 'std_example::has_inner' does not satisfy 'C2'}}
+  template<C3 T> struct C3_check {};
+  // expected-note@-1 {{because 'void' does not satisfy 'C3'}}
+  using c1 = C1_check<int>; // expected-error{{constraints not satisfied for class template 'C1_check' [with T = int]}}
+  using c2 = C1_check<has_type>; // expected-error{{constraints not satisfied for class template 'C1_check' [with T = std_example::has_type]}}
+  using c3 = C2_check<has_inner>; // expected-error{{constraints not satisfied for class template 'C2_check' [with T = std_example::has_inner]}}
+  using c4 = C3_check<void>; // expected-error{{constraints not satisfied for class template 'C3_check' [with T = void]}}
+}

clang/test/PCH/cxx2a-requires-expr.cpp

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -emit-pch -std=c++2a -fconcepts-ts -o %t %s
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x ast -ast-print %t | FileCheck %s
+
+template<typename T>
+concept C = true;
+
+template<typename T, typename U>
+concept C2 = true;
+
+template<typename T>
+bool f() {
+  // CHECK: requires (T t) { t++; { t++ } noexcept -> C; { t++ } -> C2<int>; typename T::a; requires T::val; };
+  return requires (T t) {
+    t++;
+    { t++ } noexcept -> C;
+    { t++ } -> C2<int>;
+    typename T::a;
+    requires T::val;
+  };
+}

clang/test/Parser/cxx2a-concepts-requires-expr.cpp

@@ -0,0 +1,141 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ %s -verify
+
+bool r1 = requires () {};
+// expected-error@-1 {{a requires expression must contain at least one requirement}}
+
+bool r2 = requires { requires true; };
+
+bool r3 = requires (int a, ...) { requires true; };
+// expected-error@-1 {{varargs not allowed in requires expression}}
+
+template<typename... T>
+bool r4 = requires (T... ts) { requires true; };
+
+bool r5 = requires (bool c, int d) { c; d; };
+
+bool r6 = requires (bool c, int d) { c; d; } && decltype(d){};
+// expected-error@-1 {{use of undeclared identifier 'd'}}
+
+bool r7 = requires (bool c) { c; (requires (int d) { c; d; }); d; } && decltype(c){} && decltype(d){};
+// expected-error@-1 2{{use of undeclared identifier 'd'}}
+// expected-error@-2 {{use of undeclared identifier 'c'}}
+
+bool r8 = requires (bool, int) { requires true; };
+
+bool r9 = requires (bool a, int a) { requires true; };
+// expected-error@-1 {{redefinition of parameter 'a'}}
+// expected-note@-2 {{previous declaration is here}}
+
+bool r10 = requires (struct new_struct { int x; } s) { requires true; };
+// expected-error@-1 {{'new_struct' cannot be defined in a parameter type}}
+
+bool r11 = requires (int x(1)) { requires true; };
+// expected-error@-1 {{expected parameter declarator}}
+// expected-error@-2 {{expected ')'}}
+// expected-note@-3 {{to match this '('}}
+
+bool r12 = requires (int x = 10) { requires true; };
+// expected-error@-1 {{default arguments not allowed for parameters of a requires expression}}
+
+bool r13 = requires (int f(int)) { requires true; };
+
+bool r14 = requires (int (*f)(int)) { requires true; };
+
+bool r15 = requires (10) { requires true; };
+// expected-error@-1 {{expected parameter declarator}}
+
+bool r16 = requires (auto x) { requires true; };
+// expected-error@-1 {{'auto' not allowed in requires expression parameter}}
+
+bool r17 = requires (auto [x, y]) { requires true; };
+// expected-error@-1 {{'auto' not allowed in requires expression parameter}}
+// expected-error@-2 {{use of undeclared identifier 'x'}}
+
+using a = int;
+
+bool r18 = requires { typename a; };
+
+bool r19 = requires { typename ::a; };
+
+template<typename T> struct identity { using type = T; };
+
+template<typename T> using identity_t = T;
+
+bool r20 = requires {
+    typename identity<int>::type;
+    typename identity<int>;
+    typename ::identity_t<int>;
+};
+
+struct s { bool operator==(const s&); ~s(); };
+
+bool r21 = requires { typename s::operator==; };
+// expected-error@-1 {{expected an identifier or template-id after '::'}}
+
+bool r22 = requires { typename s::~s; };
+// expected-error@-1 {{expected an identifier or template-id after '::'}}
+
+template<typename T>
+bool r23 = requires { typename identity<T>::temp<T>; };
+// expected-error@-1 {{use 'template' keyword to treat 'temp' as a dependent template name}}
+
+template<typename T>
+bool r24 = requires {
+    typename identity<T>::template temp<T>;
+    typename identity<T>::template temp; // expected-error{{expected an identifier or template-id after '::'}}
+};
+
+bool r25 = requires { ; };
+// expected-error@-1 {{expected expression}}
+
+bool r26 = requires { {}; };
+// expected-error@-1 {{expected expression}}
+
+bool r27 = requires { { 0 } noexcept; };
+
+bool r28 = requires { { 0 } noexcept noexcept; };
+// expected-error@-1 {{expected '->' before expression type requirement}}
+// expected-error@-2 {{expected concept name with optional arguments}}
+
+template<typename T>
+concept C1 = true;
+
+template<typename T, typename U>
+concept C2 = true;
+
+bool r29 = requires { { 0 } noexcept C1; };
+// expected-error@-1 {{expected '->' before expression type requirement}}
+
+bool r30 = requires { { 0 } noexcept -> C2<int>; };
+
+template<typename T>
+T i1 = 0;
+
+bool r31 = requires { requires false, 1; };
+// expected-error@-1 {{expected ';' at end of requirement}}
+
+bool r32 = requires { 0 noexcept; };
+// expected-error@-1 {{'noexcept' can only be used in a compound requirement (with '{' '}' around the expression)}}
+
+bool r33 = requires { 0 int; };
+// expected-error@-1 {{expected ';' at end of requirement}}
+
+bool r34 = requires { requires true };
+// expected-error@-1 {{expected ';' at end of requirement}}
+
+bool r35 = requires (bool b) { requires sizeof(b) == 1; };
+
+void r36(bool b) requires requires { 1 } {}
+// expected-error@-1 {{expected ';' at end of requirement}}
+
+bool r37 = requires { requires { 1; }; };
+// expected-warning@-1 {{this requires expression will only be checked for syntactic validity; did you intend to place it in a nested requirement? (add another 'requires' before the expression)}}
+
+bool r38 = requires { requires () { 1; }; };
+// expected-warning@-1 {{this requires expression will only be checked for syntactic validity; did you intend to place it in a nested requirement? (add another 'requires' before the expression)}}
+
+bool r39 = requires { requires (int i) { i; }; };
+// expected-warning@-1 {{this requires expression will only be checked for syntactic validity; did you intend to place it in a nested requirement? (add another 'requires' before the expression)}}
+
+bool r40 = requires { requires (); };
+// expected-error@-1 {{expected expression}}

clang/test/SemaTemplate/instantiate-requires-expr.cpp

@@ -0,0 +1,216 @@
+// RUN: %clang_cc1 -std=c++2a -fconcepts-ts -x c++ %s -verify -Wno-unused-value
+
+template<typename T, typename U>
+constexpr bool is_same_v = false;
+
+template<typename T>
+constexpr bool is_same_v<T, T> = true;
+
+// We use a hack in this file to make the compiler print out the requires
+// expression after it has been instantiated - we put false_v<requires {...}> as
+// the requires clause of a class template, then instantiate the template.
+// The requirement will not be satisfied, and the explaining diagnostic will
+// print out false_v<requires {...}> in its raw form (the false_v serves to
+// prevent the diagnostic from elaborating on why the requires expr wasn't
+// satisfied).
+
+template<bool v>
+constexpr bool false_v = false;
+
+template<typename... Ts>
+using void_t = void;
+
+// Check that requires parameters are instantiated correctly.
+
+template<typename T> requires
+false_v<requires (T t) { requires is_same_v<decltype(t), int>; }>
+// expected-note@-1 {{because 'false_v<requires (int t) { requires is_same_v<decltype(t), int>; }>' evaluated to false}}
+// expected-note@-2 {{because 'false_v<requires (char t) { requires is_same_v<decltype(t), int>; }>' evaluated to false}}
+struct r1 {};
+
+using r1i1 = r1<int>; // expected-error {{constraints not satisfied for class template 'r1' [with T = int]}}
+using r1i2 = r1<char>; // expected-error {{constraints not satisfied for class template 'r1' [with T = char]}}
+
+// Check that parameter packs work.
+
+template<typename... Ts> requires
+false_v<requires (Ts... ts) {requires ((sizeof(ts) == 2) && ...);}>
+// expected-note@-1 {{because 'false_v<requires (short ts, unsigned short ts) { requires (sizeof (ts) == 2) && (sizeof (ts) == 2); }>'}}
+// expected-note@-2 {{because 'false_v<requires (short ts) { requires (sizeof (ts) == 2); }>' evaluated to false}}
+struct r2 {};
+
+using r2i1 = r2<short, unsigned short>; // expected-error {{constraints not satisfied for class template 'r2' [with Ts = <short, unsigned short>]}}
+using r2i2 = r2<short>; // expected-error {{constraints not satisfied for class template 'r2' [with Ts = <short>]}}
+
+template<typename... Ts> requires
+false_v<(requires (Ts ts) {requires sizeof(ts) != 0;} && ...)>
+// expected-note@-1 {{because 'false_v<requires (short ts) { requires sizeof (ts) != 0; } && requires (unsigned short ts) { requires sizeof (ts) != 0; }>' evaluated to false}}
+// expected-note@-2 {{because 'false_v<requires (short ts) { requires sizeof (ts) != 0; }>' evaluated to false}}
+struct r3 {};
+
+using r3i1 = r3<short, unsigned short>; // expected-error {{constraints not satisfied for class template 'r3' [with Ts = <short, unsigned short>]}}
+using r3i2 = r3<short>; // expected-error {{constraints not satisfied for class template 'r3' [with Ts = <short>]}}
+
+template<typename T>
+struct identity { using type = T; };
+
+namespace type_requirement {
+  struct A {};
+
+  // check that nested name specifier is instantiated correctly.
+  template<typename T> requires false_v<requires { typename T::type; }> // expected-note{{because 'false_v<requires { typename identity<int>::type; }>' evaluated to false}}
+  struct r1 {};
+
+  using r1i = r1<identity<int>>; // expected-error{{constraints not satisfied for class template 'r1' [with T = identity<int>]}}
+
+  // check that template argument list is instantiated correctly.
+  template<typename T>
+  struct contains_template {
+      template<typename U> requires is_same_v<contains_template<T>, U>
+      using temp = int;
+  };
+
+  template<typename T> requires
+  false_v<requires { typename T::template temp<T>; }>
+  // expected-note@-1 {{because 'false_v<requires { typename contains_template<int>::temp<contains_template<int> >; }>' evaluated to false}}
+  // expected-note@-2 {{because 'false_v<requires { typename contains_template<short>::temp<contains_template<short> >; }>' evaluated to false}}
+  struct r2 {};
+
+  using r2i1 = r2<contains_template<int>>; // expected-error{{constraints not satisfied for class template 'r2' [with T = type_requirement::contains_template<int>]}}
+  using r2i2 = r2<contains_template<short>>; // expected-error{{constraints not satisfied for class template 'r2' [with T = type_requirement::contains_template<short>]}}
+
+  // substitution error occurs, then requires expr is instantiated again
+
+  template<typename T>
+  struct a {
+      template<typename U> requires (requires { typename T::a::a; }, false)
+      // expected-note@-1{{because 'requires { <<error-type>>; } , false' evaluated to false}}
+      struct r {};
+  };
+
+  using ari = a<int>::r<short>; // expected-error{{constraints not satisfied for class template 'r' [with U = short]}}
+
+  // Parameter pack inside expr
+  template<typename... Ts> requires
+  false_v<(requires { typename Ts::type; } && ...)>
+  // expected-note@-1 {{because 'false_v<requires { typename identity<short>::type; } && requires { typename identity<int>::type; } && requires { <<error-type>>; }>' evaluated to false}}
+  struct r5 {};
+
+  using r5i = r5<identity<short>, identity<int>, short>; // expected-error{{constraints not satisfied for class template 'r5' [with Ts = <identity<short>, identity<int>, short>]}}
+  template<typename... Ts> requires
+  false_v<(requires { typename void_t<Ts>; } && ...)> // expected-note{{because 'false_v<requires { typename void_t<int>; } && requires { typename void_t<short>; }>' evaluated to false}}
+  struct r6 {};
+
+  using r6i = r6<int, short>; // expected-error{{constraints not satisfied for class template 'r6' [with Ts = <int, short>]}}
+
+  template<typename... Ts> requires
+  false_v<(requires { typename Ts::template aaa<Ts>; } && ...)>
+  // expected-note@-1 {{because 'false_v<requires { <<error-type>>; } && requires { <<error-type>>; }>' evaluated to false}}
+  struct r7 {};
+
+  using r7i = r7<int, A>; // expected-error{{constraints not satisfied for class template 'r7' [with Ts = <int, type_requirement::A>]}}
+}
+
+namespace expr_requirement {
+  // check that compound/simple requirements are instantiated correctly.
+
+  template<typename T> requires false_v<requires { sizeof(T); { sizeof(T) }; }>
+  // expected-note@-1 {{because 'false_v<requires { sizeof(int); { sizeof(int) }; }>' evaluated to false}}
+  // expected-note@-2 {{because 'false_v<requires { <<error-expression>>; { sizeof(T) }; }>' evaluated to false}}
+  struct r1 {};
+
+  using r1i1 = r1<int>; // expected-error{{constraints not satisfied for class template 'r1' [with T = int]}}
+  using r1i2 = r1<void>; // expected-error{{constraints not satisfied for class template 'r1' [with T = void]}}
+
+  // substitution error occurs in expr, then expr is instantiated again.
+
+  template<typename T>
+  struct a {
+      template<typename U> requires (requires { sizeof(T::a); }, false) // expected-note{{because 'requires { <<error-expression>>; } , false' evaluated to false}}
+      struct r {};
+  };
+
+  using ari = a<int>::r<short>; // expected-error{{constraints not satisfied for class template 'r' [with U = short]}}
+
+  // check that the return-type-requirement is instantiated correctly.
+
+  template<typename T, typename U = int>
+  concept C1 = is_same_v<T, U>;
+
+  template<typename T> requires false_v<requires(T t) { { t } -> C1<T>; }>
+  // expected-note@-1 {{because 'false_v<requires (int t) { { t } -> C1<int>; }>' evaluated to false}}
+  // expected-note@-2 {{because 'false_v<requires (double t) { { t } -> C1<double>; }>' evaluated to false}}
+  struct r2 {};
+
+  using r2i1 = r2<int>; // expected-error{{constraints not satisfied for class template 'r2' [with T = int]}}
+  using r2i2 = r2<double>; // expected-error{{constraints not satisfied for class template 'r2' [with T = double]}}
+
+
+  // substitution error occurs in return type requirement, then requires expr is
+  // instantiated again.
+
+  template<typename T>
+  struct b {
+      template<typename U> requires (requires { { 0 } -> C1<typename T::a>; }, false) // expected-note{{because 'requires { { 0 } -> <<error-type>>; } , false' evaluated to false}}
+      struct r {};
+  };
+
+  using bri = b<int>::r<short>; // expected-error{{constraints not satisfied for class template 'r' [with U = short]}}
+
+
+  template<typename... Ts> requires
+  false_v<(requires { { 0 } noexcept -> C1<Ts>; } && ...)>
+  // expected-note@-1 {{because 'false_v<requires { { 0 } noexcept -> C1<int>; } && requires { { 0 } noexcept -> C1<unsigned int>; }>' evaluated to false}}
+  struct r3 {};
+
+  using r3i = r3<int, unsigned int>; // expected-error{{constraints not satisfied for class template 'r3' [with Ts = <int, unsigned int>]}}
+}
+
+namespace nested_requirement {
+  // check that constraint expression is instantiated correctly
+  template<typename T> requires false_v<requires { requires sizeof(T) == 2; }> // expected-note{{because 'false_v<requires { requires sizeof(int) == 2; }>' evaluated to false}}
+  struct r1 {};
+
+  using r1i = r1<int>; // expected-error{{constraints not satisfied for class template 'r1' [with T = int]}}
+
+  // substitution error occurs in expr, then expr is instantiated again.
+  template<typename T>
+  struct a {
+      template<typename U> requires
+      (requires { requires sizeof(T::a) == 0; }, false) // expected-note{{because 'requires { requires <<error-expression>>; } , false' evaluated to false}}
+      struct r {};
+  };
+
+  using ari = a<int>::r<short>; // expected-error{{constraints not satisfied for class template 'r' [with U = short]}}
+
+  // Parameter pack inside expr
+  template<typename... Ts> requires
+  false_v<(requires { requires sizeof(Ts) == 0; } && ...)>
+  // expected-note@-1 {{because 'false_v<requires { requires sizeof(int) == 0; } && requires { requires sizeof(short) == 0; }>' evaluated to false}}
+  struct r2 {};
+
+  using r2i = r2<int, short>; // expected-error{{constraints not satisfied for class template 'r2' [with Ts = <int, short>]}}
+}
+
+// Parameter pack inside multiple requirements
+template<typename... Ts> requires
+false_v<(requires { requires sizeof(Ts) == 0; sizeof(Ts); } && ...)>
+// expected-note@-1 {{because 'false_v<requires { requires sizeof(int) == 0; sizeof(Ts); } && requires { requires sizeof(short) == 0; sizeof(Ts); }>' evaluated to false}}
+struct r4 {};
+
+using r4i = r4<int, short>; // expected-error{{constraints not satisfied for class template 'r4' [with Ts = <int, short>]}}
+
+template<typename... Ts> requires
+false_v<(requires(Ts t) { requires sizeof(t) == 0; t++; } && ...)>
+// expected-note@-1 {{because 'false_v<requires (int t) { requires sizeof (t) == 0; t++; } && requires (short t) { requires sizeof (t) == 0; t++; }>' evaluated to false}}
+struct r5 {};
+
+using r5i = r5<int, short>; // expected-error{{constraints not satisfied for class template 'r5' [with Ts = <int, short>]}}
+
+template<typename T> requires
+false_v<(requires(T t) { T{t}; })> // T{t} creates an "UnevaluatedList" context.
+// expected-note@-1 {{because 'false_v<(requires (int t) { int{t}; })>' evaluated to false}}
+struct r6 {};
+
+using r6i = r6<int>;
+// expected-error@-1 {{constraints not satisfied for class template 'r6' [with T = int]}}

clang/tools/libclang/CIndex.cpp

@@ -6340,6 +6340,7 @@ CXCursor clang_getCursorDefinition(CXCursor C) {
   case Decl::UsingPack:
   case Decl::Concept:
   case Decl::LifetimeExtendedTemporary:
+  case Decl::RequiresExprBody:
     return C;
 
   // Declaration kinds that don't make any sense here, but are

clang/tools/libclang/CXCursor.cpp

@@ -257,6 +257,7 @@ CXCursor cxcursor::MakeCXCursor(const Stmt *S, const Decl *Parent,
   case Stmt::TypeTraitExprClass:
   case Stmt::CoawaitExprClass:
   case Stmt::ConceptSpecializationExprClass:
+  case Stmt::RequiresExprClass:
   case Stmt::DependentCoawaitExprClass:
   case Stmt::CoyieldExprClass:
   case Stmt::CXXBindTemporaryExprClass: