[C++20] [Modules] Don't diagnose duplicated implicit decl in multiple named modules (#102423)

Close https://github.com/llvm/llvm-project/issues/102360
Close https://github.com/llvm/llvm-project/issues/102349

http://eel.is/c++draft/basic.def.odr#15.3 makes it clear that the
duplicated deinition are not allowed to be attached to named modules.

But we need to filter the implicit declarations as user can do nothing
about it and the diagnostic message is annoying.

(cherry picked from commit e72d956b99e920b0fe2a7946eb3a51b9e889c73c)

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -3684,6 +3684,54 @@ static void inheritDefaultTemplateArguments(ASTContext &Context,
   }
 }
 
+// [basic.link]/p10:
+//    If two declarations of an entity are attached to different modules,
+//    the program is ill-formed;
+static void checkMultipleDefinitionInNamedModules(ASTReader &Reader, Decl *D,
+                                                  Decl *Previous) {
+  Module *M = Previous->getOwningModule();
+
+  // We only care about the case in named modules.
+  if (!M || !M->isNamedModule())
+    return;
+
+  // If it is previous implcitly introduced, it is not meaningful to
+  // diagnose it.
+  if (Previous->isImplicit())
+    return;
+
+  // FIXME: Get rid of the enumeration of decl types once we have an appropriate
+  // abstract for decls of an entity. e.g., the namespace decl and using decl
+  // doesn't introduce an entity.
+  if (!isa<VarDecl, FunctionDecl, TagDecl, RedeclarableTemplateDecl>(Previous))
+    return;
+
+  // Skip implicit instantiations since it may give false positive diagnostic
+  // messages.
+  // FIXME: Maybe this shows the implicit instantiations may have incorrect
+  // module owner ships. But given we've finished the compilation of a module,
+  // how can we add new entities to that module?
+  if (auto *VTSD = dyn_cast<VarTemplateSpecializationDecl>(Previous);
+      VTSD && !VTSD->isExplicitSpecialization())
+    return;
+  if (auto *CTSD = dyn_cast<ClassTemplateSpecializationDecl>(Previous);
+      CTSD && !CTSD->isExplicitSpecialization())
+    return;
+  if (auto *Func = dyn_cast<FunctionDecl>(Previous))
+    if (auto *FTSI = Func->getTemplateSpecializationInfo();
+        FTSI && !FTSI->isExplicitSpecialization())
+      return;
+
+  // It is fine if they are in the same module.
+  if (Reader.getContext().isInSameModule(M, D->getOwningModule()))
+    return;
+
+  Reader.Diag(Previous->getLocation(),
+              diag::err_multiple_decl_in_different_modules)
+      << cast<NamedDecl>(Previous) << M->Name;
+  Reader.Diag(D->getLocation(), diag::note_also_found);
+}
+
 void ASTDeclReader::attachPreviousDecl(ASTReader &Reader, Decl *D,
                                        Decl *Previous, Decl *Canon) {
   assert(D && Previous);
@@ -3697,22 +3745,7 @@ void ASTDeclReader::attachPreviousDecl(ASTReader &Reader, Decl *D,
 #include "clang/AST/DeclNodes.inc"
   }
 
-  // [basic.link]/p10:
-  //    If two declarations of an entity are attached to different modules,
-  //    the program is ill-formed;
-  //
-  // FIXME: Get rid of the enumeration of decl types once we have an appropriate
-  // abstract for decls of an entity. e.g., the namespace decl and using decl
-  // doesn't introduce an entity.
-  if (Module *M = Previous->getOwningModule();
-      M && M->isNamedModule() &&
-      isa<VarDecl, FunctionDecl, TagDecl, RedeclarableTemplateDecl>(Previous) &&
-      !Reader.getContext().isInSameModule(M, D->getOwningModule())) {
-    Reader.Diag(Previous->getLocation(),
-                diag::err_multiple_decl_in_different_modules)
-        << cast<NamedDecl>(Previous) << M->Name;
-    Reader.Diag(D->getLocation(), diag::note_also_found);
-  }
+  checkMultipleDefinitionInNamedModules(Reader, D, Previous);
 
   // If the declaration was visible in one module, a redeclaration of it in
   // another module remains visible even if it wouldn't be visible by itself.

clang/test/Modules/pr102349.cppm

@@ -0,0 +1,52 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/b.cppm -emit-module-interface -o %t/b.pcm \
+// RUN:   -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/c.cppm -emit-module-interface -o %t/c.pcm \
+// RUN:   -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/d.cpp -fsyntax-only -verify \
+// RUN:   -fprebuilt-module-path=%t
+
+//--- a.cppm
+export module a;
+
+export template<typename>
+struct a;
+
+template<typename>
+struct a {
+};
+
+export template<typename T>
+constexpr auto aa = a<T>();
+
+//--- b.cppm
+export module b;
+
+import a;
+
+static void b() {
+	static_cast<void>(a<void>());
+}
+
+//--- c.cppm
+export module c;
+
+import a;
+
+static void c() {
+	static_cast<void>(aa<void>);
+}
+
+//--- d.cpp
+// expected-no-diagnostics
+import a;
+import b;
+import c;
+
+static void d() {
+	static_cast<void>(a<void>());
+}

clang/test/Modules/pr102360.cppm

@@ -0,0 +1,53 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/b.cppm -emit-module-interface -o %t/b.pcm \
+// RUN:   -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/c.cppm -emit-module-interface -o %t/c.pcm \
+// RUN:   -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/d.cpp -fsyntax-only -verify \
+// RUN:   -fprebuilt-module-path=%t
+
+//--- a.cppm
+export module a;
+
+template<typename>
+constexpr auto impl = true;
+
+export template<typename T>
+void a() {
+}
+
+export template<typename T> requires impl<T>
+void a() {
+}
+
+//--- b.cppm
+export module b;
+
+import a;
+
+static void b() {
+	a<void>();
+}
+
+//--- c.cppm
+export module c;
+
+import a;
+
+static void c() {
+	a<void>();
+}
+
+//--- d.cpp
+// expected-no-diagnostics
+import a;
+import b;
+import c;
+
+static void d() {
+	a<void>();
+}