[C++20] [Modules] Fix issues with non-exported in-class friend declarations

Close #159424
Close #133720

For in-class friend declaration, it is hard for the serializer to decide
if they are visible to other modules. But luckily, Sema can handle it
perfectly enough. So it is fine to make all of the in-class friend
declaration as generally visible in ASTWriter and let the Sema to make
the final call. This is safe as long as the corresponding class's
visibility are correct.

Author: ChuanqiXu9

clang/lib/Serialization/ASTWriter.cpp

@@ -4297,10 +4297,18 @@ static bool isModuleLocalDecl(NamedDecl *D) {
     if (auto *CDGD = dyn_cast<CXXDeductionGuideDecl>(FTD->getTemplatedDecl()))
       return isModuleLocalDecl(CDGD->getDeducedTemplate());
 
-  if (D->getFormalLinkage() == Linkage::Module)
-    return true;
+  if (D->getFormalLinkage() != Linkage::Module)
+    return false;
 
-  return false;
+  // It is hard for the serializer to judge if the in-class friend declaration
+  // is visible or not, so we just transfer the task to Sema. It should be a
+  // safe decision since Sema is able to handle the lookup rules for in-class
+  // friend declarations good enough already.
+  if (D->getFriendObjectKind() &&
+      isa<CXXRecordDecl>(D->getLexicalDeclContext()))
+    return false;
+
+  return true;
 }
 
 static bool isTULocalInNamedModules(NamedDecl *D) {

clang/test/Modules/pr133720.cppm

@@ -0,0 +1,31 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %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 -fmodule-file=a=%t/a.pcm -fsyntax-only -verify
+
+//--- a.cppm
+export module a;
+
+struct Base {
+    template <class T>
+    friend constexpr auto f(T) { return 0; }
+};
+export struct A: Base {};
+
+//--- b.cppm
+export module b;
+
+import a;
+
+namespace n {
+
+struct B {};
+
+auto b() -> void {
+	f(A{});
+    f(B{}); // expected-error {{use of undeclared identifier 'f'}}
+}
+
+} // namespace n

clang/test/Modules/pr159424.cppm

@@ -0,0 +1,36 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %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 -fmodule-file=a=%t/a.pcm -fsyntax-only -verify
+
+//--- a.cppm
+export module a;
+
+namespace n {
+
+struct monostate {
+	friend auto operator==(monostate, monostate) -> bool = default;
+};
+
+export struct a {
+	friend auto operator==(a, a) -> bool = default;
+	monostate m;
+};
+
+} // namespace n
+
+//--- b.cppm
+// expected-no-diagnostics
+export module b;
+
+import a;
+
+namespace n {
+
+export auto b() -> bool {
+	return a() == a();
+}
+
+} // namespace n