[AST] [Modules] Introduce Decl::getNonTransparentDeclContext to handle exported friends

Closing https://github.com/llvm/llvm-project/issues/56826.

The root cause for pr56826 is: when we collect the template args for the
friend, we need to judge if the friend lives in file context. However,
if the friend lives in ExportDecl lexically, the judgement here is
invalid.

The solution is easy. We should judge the non transparent context and
the ExportDecl is transparent context. So the solution should be good.

A main concern may be the patch doesn't handle all the places of the
same defect. I think it might not be bad since the patch itself should
be innocent.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D131651

clang/include/clang/AST/DeclBase.h

@@ -447,6 +447,14 @@ public:
     return const_cast<Decl*>(this)->getDeclContext();
   }
 
+  /// Return the non transparent context.
+  /// See the comment of `DeclContext::isTransparentContext()` for the
+  /// definition of transparent context.
+  DeclContext *getNonTransparentDeclContext();
+  const DeclContext *getNonTransparentDeclContext() const {
+    return const_cast<Decl *>(this)->getNonTransparentDeclContext();
+  }
+
   /// Find the innermost non-closure ancestor of this declaration,
   /// walking up through blocks, lambdas, etc.  If that ancestor is
   /// not a code context (!isFunctionOrMethod()), returns null.
@@ -2009,7 +2017,7 @@ public:
   /// Here, E is a transparent context, so its enumerator (Val1) will
   /// appear (semantically) that it is in the same context of E.
   /// Examples of transparent contexts include: enumerations (except for
-  /// C++0x scoped enums), and C++ linkage specifications.
+  /// C++0x scoped enums), C++ linkage specifications and export declaration.
   bool isTransparentContext() const;
 
   /// Determines whether this context or some of its ancestors is a

clang/lib/AST/DeclBase.cpp

@@ -1032,6 +1032,11 @@ const FunctionType *Decl::getFunctionType(bool BlocksToo) const {
   return Ty->getAs<FunctionType>();
 }
 
+DeclContext *Decl::getNonTransparentDeclContext() {
+  assert(getDeclContext());
+  return getDeclContext()->getNonTransparentContext();
+}
+
 /// Starting at a given context (a Decl or DeclContext), look for a
 /// code context that is not a closure (a lambda, block, etc.).
 template <class T> static Decl *getNonClosureContext(T *D) {

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -168,7 +168,7 @@ MultiLevelTemplateArgumentList Sema::getTemplateInstantiationArgs(
       // instead of its semantic parent, unless of course the pattern we're
       // instantiating actually comes from the file's context!
       if (Function->getFriendObjectKind() &&
-          Function->getDeclContext()->isFileContext() &&
+          Function->getNonTransparentDeclContext()->isFileContext() &&
           (!Pattern || !Pattern->getLexicalDeclContext()->isFileContext())) {
         Ctx = Function->getLexicalDeclContext();
         RelativeToPrimary = false;

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -6147,6 +6147,8 @@ NamedDecl *Sema::FindInstantiatedDecl(SourceLocation Loc, NamedDecl *D,
 
       // Move to the outer template scope.
       if (FunctionDecl *FD = dyn_cast<FunctionDecl>(DC)) {
+        // FIXME: We should use `getNonTransparentDeclContext()` here instead
+        // of `getDeclContext()` once we find the invalid test case.
         if (FD->getFriendObjectKind() && FD->getDeclContext()->isFileContext()){
           DC = FD->getLexicalDeclContext();
           continue;

clang/test/Modules/pr56826.cppm

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+export module m3;
+export template <class> struct X {
+  template <class Self> friend void f(Self &&self) {
+    (Self&)self; // expected-warning {{expression result unused}}
+  }
+};
+void g() { f(X<void>{}); } // expected-note {{in instantiation of function template specialization}}

clang/unittests/AST/DeclTest.cpp

@@ -260,3 +260,22 @@ TEST(Decl, ModuleAndInternalLinkage) {
   EXPECT_EQ(b->getLinkageInternal(), ModuleLinkage);
   EXPECT_EQ(g->getLinkageInternal(), ModuleLinkage);
 }
+
+TEST(Decl, GetNonTransparentDeclContext) {
+  llvm::Annotations Code(R"(
+    export module m3;
+    export template <class> struct X {
+      template <class Self> friend void f(Self &&self) {
+        (Self&)self;
+      }
+    };)");
+
+  auto AST =
+      tooling::buildASTFromCodeWithArgs(Code.code(), /*Args=*/{"-std=c++20"});
+  ASTContext &Ctx = AST->getASTContext();
+
+  auto *f = selectFirst<FunctionDecl>(
+      "f", match(functionDecl(hasName("f")).bind("f"), Ctx));
+
+  EXPECT_TRUE(f->getNonTransparentDeclContext()->isFileContext());
+}