[C++20] [Modules] Don't diagnose redeclaration of implicit instantiations in different modules

See the attached example for motivation. Although it is not sure how
should we treat the modules ownership of the implicit instantiations,
it makes no sense to diagnose redeclaration of implicit instantiations
in different modules.

Author: ChuanqiXu9

clang/lib/Sema/SemaDecl.cpp

@@ -1653,6 +1653,17 @@ void Sema::FilterLookupForScope(LookupResult &R, DeclContext *Ctx, Scope *S,
   F.done();
 }
 
+static bool isImplicitInstantiation(NamedDecl *D) {
+  if (auto *VD = dyn_cast<VarDecl>(D))
+    return VD->getTemplateSpecializationKind() == TSK_ImplicitInstantiation;
+  if (auto *FD = dyn_cast<FunctionDecl>(D))
+    return FD->getTemplateSpecializationKind() == TSK_ImplicitInstantiation;
+  if (auto *RD = dyn_cast<CXXRecordDecl>(D))
+    return RD->getTemplateSpecializationKind() == TSK_ImplicitInstantiation;
+
+  return false;
+}
+
 bool Sema::CheckRedeclarationModuleOwnership(NamedDecl *New, NamedDecl *Old) {
   // [module.interface]p7:
   // A declaration is attached to a module as follows:
@@ -1668,6 +1679,14 @@ bool Sema::CheckRedeclarationModuleOwnership(NamedDecl *New, NamedDecl *Old) {
     return false;
   }
 
+  // Although we have questions for the module ownership of implicit
+  // instantiations, it should be sure that we shouldn't diagnose the
+  // redeclaration of incorrect module ownership for different implicit
+  // instantiations in different modules. We will diagnose the redeclaration of
+  // incorrect module ownership for the template itself.
+  if (isImplicitInstantiation(New) || isImplicitInstantiation(Old))
+    return false;
+
   Module *NewM = New->getOwningModule();
   Module *OldM = Old->getOwningModule();
 

clang/test/Modules/added-visible-decls.cppm

@@ -0,0 +1,57 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -emit-reduced-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 %t/b.cppm -emit-reduced-module-interface -o %t/b.pcm -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/c.cppm -emit-reduced-module-interface -o %t/c.pcm -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 %t/d.cpp -fprebuilt-module-path=%t -fsyntax-only -verify
+
+//--- a.h
+template <typename T>
+struct A {
+  static const T value0;
+  static const T value1;
+
+  constexpr T get0() {
+    return value0;
+  }
+
+  constexpr T get1() {
+    return value1;
+  }
+};
+
+template <typename T>
+const T A<T>::value0 = T(43);
+template <typename T>
+const T A<T>::value1 = T(44);
+
+//--- a.cppm
+module;
+#include "a.h"
+export module a;
+export using ::A;
+
+//--- b.cppm
+export module b;
+export import a;
+
+export constexpr int bar() {
+    return A<int>().get0();
+}
+
+//--- c.cppm
+export module c;
+export import b;
+
+export constexpr int foo() {
+    return A<int>().get1() + A<int>().get0();
+}
+
+//--- d.cpp
+// expected-no-diagnostics
+
+import c;
+
+static_assert(bar() + foo() == 130);
+