[Clang] Do not change the type of captured vars when checking lambda constraints

When checking the constraint of a lambda, we need to respect the constness
of the call operator when establishing the type of capture variables.

In D124351, this was done by adding const to the captured variable...
However, that would change the type of the variable outside of the scope
of the lambda, which is clearly not the desired outcome.

Instead, to ensure const-correctness, we need to populate
a LambdaScopeInfo with the capture variables before checking the
constraints of a generic lambda.

There is no changelog as I'd like to tentatively propose we backport
this change to RC3 as it is a regression introduced in the Clang 17
cycle.

Fixes #61267

Reviewed By: aaron.ballman, #clang-language-wg

Differential Revision: https://reviews.llvm.org/D158433

clang/include/clang/Sema/Sema.h

@@ -7343,6 +7343,8 @@ public:
                                            CXXConversionDecl *Conv,
                                            Expr *Src);
 
+  sema::LambdaScopeInfo *RebuildLambdaScopeInfo(CXXMethodDecl *CallOperator);
+
   /// Check whether the given expression is a valid constraint expression.
   /// A diagnostic is emitted if it is not, false is returned, and
   /// PossibleNonPrimary will be set to true if the failure might be due to a

clang/lib/Sema/SemaConcept.cpp

@@ -13,12 +13,14 @@
 #include "clang/Sema/SemaConcept.h"
 #include "TreeTransform.h"
 #include "clang/AST/ASTLambda.h"
+#include "clang/AST/DeclCXX.h"
 #include "clang/AST/ExprConcepts.h"
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/Basic/OperatorPrecedence.h"
 #include "clang/Sema/EnterExpressionEvaluationContext.h"
 #include "clang/Sema/Initialization.h"
 #include "clang/Sema/Overload.h"
+#include "clang/Sema/ScopeInfo.h"
 #include "clang/Sema/Sema.h"
 #include "clang/Sema/SemaDiagnostic.h"
 #include "clang/Sema/SemaInternal.h"
@@ -540,11 +542,6 @@ bool Sema::addInstantiatedCapturesToScope(
   auto AddSingleCapture = [&](const ValueDecl *CapturedPattern,
                               unsigned Index) {
     ValueDecl *CapturedVar = LambdaClass->getCapture(Index)->getCapturedVar();
-    if (cast<CXXMethodDecl>(Function)->isConst()) {
-      QualType T = CapturedVar->getType();
-      T.addConst();
-      CapturedVar->setType(T);
-    }
     if (CapturedVar->isInitCapture())
       Scope.InstantiatedLocal(CapturedPattern, CapturedVar);
   };
@@ -714,6 +711,22 @@ bool Sema::CheckFunctionConstraints(const FunctionDecl *FD,
     Record = const_cast<CXXRecordDecl *>(Method->getParent());
   }
   CXXThisScopeRAII ThisScope(*this, Record, ThisQuals, Record != nullptr);
+
+  // When checking the constraints of a lambda, we need to restore a
+  // LambdaScopeInfo populated with correct capture information so that the type
+  // of a variable referring to a capture is correctly const-adjusted.
+  FunctionScopeRAII FuncScope(*this);
+  if (isLambdaCallOperator(FD)) {
+    LambdaScopeInfo *LSI = RebuildLambdaScopeInfo(
+        const_cast<CXXMethodDecl *>(cast<CXXMethodDecl>(FD)));
+    // Constraints are checked from the parent context of the lambda, so we set
+    // AfterParameterList to false, so that `tryCaptureVariable` finds
+    // explicit captures in the appropriate context.
+    LSI->AfterParameterList = false;
+  } else {
+    FuncScope.disable();
+  }
+
   return CheckConstraintSatisfaction(
       FD, {FD->getTrailingRequiresClause()}, *MLTAL,
       SourceRange(UsageLoc.isValid() ? UsageLoc : FD->getLocation()),
@@ -902,10 +915,13 @@ bool Sema::CheckInstantiatedFunctionTemplateConstraints(
   }
   CXXThisScopeRAII ThisScope(*this, Record, ThisQuals, Record != nullptr);
   FunctionScopeRAII FuncScope(*this);
-  if (isLambdaCallOperator(Decl))
-    PushLambdaScope();
-  else
+
+  if (isLambdaCallOperator(Decl)) {
+    LambdaScopeInfo *LSI = RebuildLambdaScopeInfo(cast<CXXMethodDecl>(Decl));
+    LSI->AfterParameterList = false;
+  } else {
     FuncScope.disable();
+  }
 
   llvm::SmallVector<Expr *, 1> Converted;
   return CheckConstraintSatisfaction(Template, TemplateAC, Converted, *MLTAL,

clang/lib/Sema/SemaDecl.cpp

@@ -15289,11 +15289,10 @@ Sema::CheckForFunctionRedefinition(FunctionDecl *FD,
   FD->setInvalidDecl();
 }
 
-static void RebuildLambdaScopeInfo(CXXMethodDecl *CallOperator,
-                                   Sema &S) {
-  CXXRecordDecl *const LambdaClass = CallOperator->getParent();
+LambdaScopeInfo *Sema::RebuildLambdaScopeInfo(CXXMethodDecl *CallOperator) {
+  CXXRecordDecl *LambdaClass = CallOperator->getParent();
 
-  LambdaScopeInfo *LSI = S.PushLambdaScope();
+  LambdaScopeInfo *LSI = PushLambdaScope();
   LSI->CallOperator = CallOperator;
   LSI->Lambda = LambdaClass;
   LSI->ReturnType = CallOperator->getReturnType();
@@ -15317,7 +15316,7 @@ static void RebuildLambdaScopeInfo(CXXMethodDecl *CallOperator,
     if (C.capturesVariable()) {
       ValueDecl *VD = C.getCapturedVar();
       if (VD->isInitCapture())
-        S.CurrentInstantiationScope->InstantiatedLocal(VD, VD);
+        CurrentInstantiationScope->InstantiatedLocal(VD, VD);
       const bool ByRef = C.getCaptureKind() == LCK_ByRef;
       LSI->addCapture(VD, /*IsBlock*/false, ByRef,
           /*RefersToEnclosingVariableOrCapture*/true, C.getLocation(),
@@ -15334,6 +15333,7 @@ static void RebuildLambdaScopeInfo(CXXMethodDecl *CallOperator,
     }
     ++I;
   }
+  return LSI;
 }
 
 Decl *Sema::ActOnStartOfFunctionDef(Scope *FnBodyScope, Decl *D,
@@ -15437,7 +15437,7 @@ Decl *Sema::ActOnStartOfFunctionDef(Scope *FnBodyScope, Decl *D,
     assert(inTemplateInstantiation() &&
            "There should be an active template instantiation on the stack "
            "when instantiating a generic lambda!");
-    RebuildLambdaScopeInfo(cast<CXXMethodDecl>(D), *this);
+    RebuildLambdaScopeInfo(cast<CXXMethodDecl>(D));
   } else {
     // Enter a new function scope
     PushFunctionScope();

clang/lib/Sema/SemaExpr.cpp

@@ -19722,13 +19722,6 @@ bool Sema::tryCaptureVariable(
         FunctionScopesIndex == MaxFunctionScopesIndex && VarDC == DC)
       return true;
 
-    // When evaluating some attributes (like enable_if) we might refer to a
-    // function parameter appertaining to the same declaration as that
-    // attribute.
-    if (const auto *Parm = dyn_cast<ParmVarDecl>(Var);
-        Parm && Parm->getDeclContext() == DC)
-      return true;
-
     // Only block literals, captured statements, and lambda expressions can
     // capture; other scopes don't work.
     DeclContext *ParentDC =
@@ -19756,6 +19749,14 @@ bool Sema::tryCaptureVariable(
       CSI->getCapture(Var).markUsed(BuildAndDiagnose);
       break;
     }
+
+    // When evaluating some attributes (like enable_if) we might refer to a
+    // function parameter appertaining to the same declaration as that
+    // attribute.
+    if (const auto *Parm = dyn_cast<ParmVarDecl>(Var);
+        Parm && Parm->getDeclContext() == DC)
+      return true;
+
     // If we are instantiating a generic lambda call operator body,
     // we do not want to capture new variables.  What was captured
     // during either a lambdas transformation or initial parsing

clang/test/SemaCXX/lambda-capture-type-deduction.cpp

@@ -246,3 +246,17 @@ void check_params_tpl() {
     static_assert(is_same<int&, decltype((ap))>);
   };
 }
+
+namespace GH61267 {
+template <typename> concept C = true;
+
+template<typename>
+void f(int) {
+  int i;
+  [i]<C P>(P) {}(0);
+  i = 4;
+}
+
+void test() { f<int>(0);  }
+
+}