[C++20] [Modules] Don't perform ODR checks in GMF

Close https://github.com/llvm/llvm-project/issues/79240.

See the linked issue for details. Given the frequency of issue reporting
about false positive ODR checks (I received private issue reports too),
I'd like to backport this to 18.x too.

clang/docs/ReleaseNotes.rst

@@ -188,6 +188,11 @@ C++20 Feature Support
   This feature is still experimental. Accordingly, ``__cpp_nontype_template_args`` was not updated.
   However, its support can be tested with ``__has_extension(cxx_generalized_nttp)``.
 
+- Clang won't perform ODR checks for decls in the global module fragment any
+  more to ease the implementation and improve the user's using experience.
+  This follows the MSVC's behavior.
+  (`#79240 <https://github.com/llvm/llvm-project/issues/79240>`_).
+
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 - Implemented `P0847R7: Deducing this <https://wg21.link/P0847R7>`_. Some related core issues were also

clang/include/clang/Serialization/ASTReader.h

@@ -2452,6 +2452,10 @@ private:
   uint32_t CurrentBitsIndex = ~0;
 };
 
+inline bool isFromExplicitGMF(const Decl *D) {
+  return D->getOwningModule() && D->getOwningModule()->isExplicitGlobalModule();
+}
+
 } // namespace clang
 
 #endif // LLVM_CLANG_SERIALIZATION_ASTREADER_H

clang/lib/Serialization/ASTReader.cpp

@@ -9743,6 +9743,9 @@ void ASTReader::finishPendingActions() {
 
         if (!FD->isLateTemplateParsed() &&
             !NonConstDefn->isLateTemplateParsed() &&
+            // We only perform ODR checks for decls not in the explicit
+            // global module fragment.
+            !isFromExplicitGMF(FD) &&
             FD->getODRHash() != NonConstDefn->getODRHash()) {
           if (!isa<CXXMethodDecl>(FD)) {
             PendingFunctionOdrMergeFailures[FD].push_back(NonConstDefn);

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -804,8 +804,10 @@ void ASTDeclReader::VisitEnumDecl(EnumDecl *ED) {
   ED->setScopedUsingClassTag(EnumDeclBits.getNextBit());
   ED->setFixed(EnumDeclBits.getNextBit());
 
-  ED->setHasODRHash(true);
-  ED->ODRHash = Record.readInt();
+  if (!isFromExplicitGMF(ED)) {
+    ED->setHasODRHash(true);
+    ED->ODRHash = Record.readInt();
+  }
 
   // If this is a definition subject to the ODR, and we already have a
   // definition, merge this one into it.
@@ -827,7 +829,9 @@ void ASTDeclReader::VisitEnumDecl(EnumDecl *ED) {
       Reader.MergedDeclContexts.insert(std::make_pair(ED, OldDef));
       ED->demoteThisDefinitionToDeclaration();
       Reader.mergeDefinitionVisibility(OldDef, ED);
-      if (OldDef->getODRHash() != ED->getODRHash())
+      // We don't want to check the ODR hash value for declarations from global
+      // module fragment.
+      if (!isFromExplicitGMF(ED) && OldDef->getODRHash() != ED->getODRHash())
         Reader.PendingEnumOdrMergeFailures[OldDef].push_back(ED);
     } else {
       OldDef = ED;
@@ -866,6 +870,9 @@ ASTDeclReader::VisitRecordDeclImpl(RecordDecl *RD) {
 
 void ASTDeclReader::VisitRecordDecl(RecordDecl *RD) {
   VisitRecordDeclImpl(RD);
+  // We should only reach here if we're in C/Objective-C. There is no
+  // global module fragment.
+  assert(!isFromExplicitGMF(RD));
   RD->setODRHash(Record.readInt());
 
   // Maintain the invariant of a redeclaration chain containing only
@@ -1094,8 +1101,10 @@ void ASTDeclReader::VisitFunctionDecl(FunctionDecl *FD) {
   if (FD->isExplicitlyDefaulted())
     FD->setDefaultLoc(readSourceLocation());
 
-  FD->ODRHash = Record.readInt();
-  FD->setHasODRHash(true);
+  if (!isFromExplicitGMF(FD)) {
+    FD->ODRHash = Record.readInt();
+    FD->setHasODRHash(true);
+  }
 
   if (FD->isDefaulted()) {
     if (unsigned NumLookups = Record.readInt()) {
@@ -1971,9 +1980,12 @@ void ASTDeclReader::ReadCXXDefinitionData(
 #include "clang/AST/CXXRecordDeclDefinitionBits.def"
 #undef FIELD
 
-  // Note: the caller has deserialized the IsLambda bit already.
-  Data.ODRHash = Record.readInt();
-  Data.HasODRHash = true;
+  // We only perform ODR checks for decls not in GMF.
+  if (!isFromExplicitGMF(D)) {
+    // Note: the caller has deserialized the IsLambda bit already.
+    Data.ODRHash = Record.readInt();
+    Data.HasODRHash = true;
+  }
 
   if (Record.readInt()) {
     Reader.DefinitionSource[D] =
@@ -2134,6 +2146,10 @@ void ASTDeclReader::MergeDefinitionData(
     }
   }
 
+  // We don't want to check ODR for decls in the global module fragment.
+  if (isFromExplicitGMF(MergeDD.Definition))
+    return;
+
   if (D->getODRHash() != MergeDD.ODRHash) {
     DetectedOdrViolation = true;
   }
@@ -3498,10 +3514,13 @@ ASTDeclReader::FindExistingResult ASTDeclReader::findExisting(NamedDecl *D) {
   // If this declaration is from a merged context, make a note that we need to
   // check that the canonical definition of that context contains the decl.
   //
+  // Note that we don't perform ODR checks for decls from the global module
+  // fragment.
+  //
   // FIXME: We should do something similar if we merge two definitions of the
   // same template specialization into the same CXXRecordDecl.
   auto MergedDCIt = Reader.MergedDeclContexts.find(D->getLexicalDeclContext());
-  if (MergedDCIt != Reader.MergedDeclContexts.end() &&
+  if (MergedDCIt != Reader.MergedDeclContexts.end() && !isFromExplicitGMF(D) &&
       MergedDCIt->second == D->getDeclContext())
     Reader.PendingOdrMergeChecks.push_back(D);
 

clang/lib/Serialization/ASTWriter.cpp

@@ -6022,8 +6022,12 @@ void ASTRecordWriter::AddCXXDefinitionData(const CXXRecordDecl *D) {
 
   Record->push_back(DefinitionBits);
 
-  // getODRHash will compute the ODRHash if it has not been previously computed.
-  Record->push_back(D->getODRHash());
+  // We only perform ODR checks for decls not in GMF.
+  if (!isFromExplicitGMF(D)) {
+    // getODRHash will compute the ODRHash if it has not been previously
+    // computed.
+    Record->push_back(D->getODRHash());
+  }
 
   bool ModulesDebugInfo =
       Writer->Context->getLangOpts().ModulesDebugInfo && !D->isDependentType();

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -493,7 +493,9 @@ void ASTDeclWriter::VisitEnumDecl(EnumDecl *D) {
   EnumDeclBits.addBit(D->isFixed());
   Record.push_back(EnumDeclBits);
 
-  Record.push_back(D->getODRHash());
+  // We only perform ODR checks for decls not in GMF.
+  if (!isFromExplicitGMF(D))
+    Record.push_back(D->getODRHash());
 
   if (MemberSpecializationInfo *MemberInfo = D->getMemberSpecializationInfo()) {
     Record.AddDeclRef(MemberInfo->getInstantiatedFrom());
@@ -510,7 +512,7 @@ void ASTDeclWriter::VisitEnumDecl(EnumDecl *D) {
       !D->isTopLevelDeclInObjCContainer() &&
       !CXXRecordDecl::classofKind(D->getKind()) &&
       !D->getIntegerTypeSourceInfo() && !D->getMemberSpecializationInfo() &&
-      !needsAnonymousDeclarationNumber(D) &&
+      !needsAnonymousDeclarationNumber(D) && !isFromExplicitGMF(D) &&
       D->getDeclName().getNameKind() == DeclarationName::Identifier)
     AbbrevToUse = Writer.getDeclEnumAbbrev();
 
@@ -701,7 +703,9 @@ void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
   if (D->isExplicitlyDefaulted())
     Record.AddSourceLocation(D->getDefaultLoc());
 
-  Record.push_back(D->getODRHash());
+  // We only perform ODR checks for decls not in GMF.
+  if (!isFromExplicitGMF(D))
+    Record.push_back(D->getODRHash());
 
   if (D->isDefaulted()) {
     if (auto *FDI = D->getDefaultedFunctionInfo()) {
@@ -1506,7 +1510,8 @@ void ASTDeclWriter::VisitCXXMethodDecl(CXXMethodDecl *D) {
       D->getFirstDecl() == D->getMostRecentDecl() && !D->isInvalidDecl() &&
       !D->hasAttrs() && !D->isTopLevelDeclInObjCContainer() &&
       D->getDeclName().getNameKind() == DeclarationName::Identifier &&
-      !D->hasExtInfo() && !D->isExplicitlyDefaulted()) {
+      !isFromExplicitGMF(D) && !D->hasExtInfo() &&
+      !D->isExplicitlyDefaulted()) {
     if (D->getTemplatedKind() == FunctionDecl::TK_NonTemplate ||
         D->getTemplatedKind() == FunctionDecl::TK_FunctionTemplate ||
         D->getTemplatedKind() == FunctionDecl::TK_MemberSpecialization ||

clang/test/Modules/concept.cppm

@@ -70,13 +70,6 @@ module;
 export module B;
 import A;
 
-#ifdef DIFFERENT
-// expected-error@foo.h:41 {{'__fn::operator()' from module 'A.<global>' is not present in definition of '__fn' provided earlier}}
-// expected-note@* 1+{{declaration of 'operator()' does not match}}
-#else
-// expected-no-diagnostics
-#endif
-
 template <class T>
 struct U {
   auto operator+(U) { return 0; }
@@ -94,3 +87,10 @@ void foo() {
 
     __fn{}(U<int>(), U<int>());
 }
+
+#ifdef DIFFERENT
+// expected-error@B.cppm:* {{call to object of type '__fn' is ambiguous}}
+// expected-note@* 1+{{candidate function}}
+#else
+// expected-no-diagnostics
+#endif

clang/test/Modules/no-eager-load.cppm

@@ -9,19 +9,10 @@
 // RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %t/d.cpp \
 // RUN:     -fprebuilt-module-path=%t
 //
-// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/e.cppm -o %t/e.pcm
-// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/f.cppm -o %t/f.pcm
-// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %t/g.cpp \
-// RUN:     -fprebuilt-module-path=%t
-//
 // RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/h.cppm \
 // RUN:     -fprebuilt-module-path=%t -o %t/h.pcm
-// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/i.cppm \
-// RUN:     -fprebuilt-module-path=%t -o %t/i.pcm
 // RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %t/j.cpp \
 // RUN:     -fprebuilt-module-path=%t
-// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %t/k.cpp \
-// RUN:     -fprebuilt-module-path=%t
 
 //--- a.cppm
 export module a;
@@ -53,58 +44,14 @@ void use() {
            // expected-note@* {{but in 'a' found a different body}}
 }
 
-//--- foo.h
-void foo() {
-
-}
-
-//--- bar.h
-void bar();
-void foo() {
-    bar();
-}
-
-//--- e.cppm
-module;
-#include "foo.h"
-export module e;
-export using ::foo;
-
-//--- f.cppm
-module;
-#include "bar.h"
-export module f;
-export using ::foo;
-
-//--- g.cpp
-import e;
-import f;
-void use() {
-    foo(); // expected-error@* {{'foo' has different definitions in different modules;}}
-           // expected-note@* {{but in 'e.<global>' found a different body}}
-}
-
 //--- h.cppm
 export module h;
 export import a;
 export import b;
 
-//--- i.cppm
-export module i;
-export import e;
-export import f;
-
 //--- j.cpp
 import h;
 void use() {
     foo(); // expected-error@* {{'foo' has different definitions in different modules;}}
            // expected-note@* {{but in 'a' found a different body}}
 }
-
-//--- k.cpp
-import i;
-void use() {
-    foo(); // expected-error@* {{'foo' has different definitions in different modules;}}
-           // expected-note@* {{but in 'e.<global>' found a different body}}
-}
-

clang/test/Modules/polluted-operator.cppm

@@ -46,12 +46,10 @@ module;
 export module a;
 
 //--- b.cppm
+// This is actually an ODR violation. But given https://github.com/llvm/llvm-project/issues/79240,
+// we don't count it as an ODR violation any more.
+// expected-no-diagnostics
 module;
 #include "bar.h"
 export module b;
 import a;
-
-// expected-error@* {{has different definitions in different modules; first difference is defined here found data member '_S_copy_ctor' with an initializer}}
-// expected-note@* {{but in 'a.<global>' found data member '_S_copy_ctor' with a different initializer}}
-// expected-error@* {{from module 'a.<global>' is not present in definition of 'variant<_Types...>' provided earlier}}
-// expected-note@* {{declaration of 'swap' does not match}}

clang/test/Modules/pr76638.cppm

@@ -57,6 +57,9 @@ export module mod3;
 export using std::align_val_t;
 
 //--- mod4.cppm
+// This is actually an ODR violation. But given https://github.com/llvm/llvm-project/issues/79240,
+// we don't count it as an ODR violation now.
+// expected-no-diagnostics
 module;
 #include "signed_size_t.h"
 #include "csize_t"
@@ -64,6 +67,3 @@ module;
 export module mod4;
 import mod3;
 export using std::align_val_t;
-
-// expected-error@align.h:* {{'std::align_val_t' has different definitions in different modules; defined here first difference is enum with specified type 'size_t' (aka 'int')}}
-// expected-note@align.h:* {{but in 'mod3.<global>' found enum with specified type 'size_t' (aka 'unsigned int')}}