[clang][ASTImporter] Allow import of similar friend template with different depth (#115734)

This fix applies to a case that occurs when the AST contains a friend
template that is contained within another template and this (outer)
template has specialization. (See the added test code in the commit.)

Author: balazske

clang/lib/AST/ASTImporter.cpp

@@ -6093,8 +6093,7 @@ ExpectedDecl ASTNodeImporter::VisitClassTemplateDecl(ClassTemplateDecl *D) {
                                               Decl::IDNS_TagFriend))
         continue;
 
-      Decl *Found = FoundDecl;
-      auto *FoundTemplate = dyn_cast<ClassTemplateDecl>(Found);
+      auto *FoundTemplate = dyn_cast<ClassTemplateDecl>(FoundDecl);
       if (FoundTemplate) {
         if (!hasSameVisibilityContextAndLinkage(FoundTemplate, D))
           continue;
@@ -6118,6 +6117,19 @@ ExpectedDecl ASTNodeImporter::VisitClassTemplateDecl(ClassTemplateDecl *D) {
           // see ASTTests test ImportExistingFriendClassTemplateDef.
           continue;
         }
+        // When importing a friend, it is possible that multiple declarations
+        // with same name can co-exist in specific cases (if a template contains
+        // a friend template and has a specialization). For this case the
+        // declarations should match, except that the "template depth" is
+        // different. No linking of previous declaration is needed in this case.
+        // FIXME: This condition may need refinement.
+        if (D->getFriendObjectKind() != Decl::FOK_None &&
+            FoundTemplate->getFriendObjectKind() != Decl::FOK_None &&
+            D->getFriendObjectKind() != FoundTemplate->getFriendObjectKind() &&
+            IsStructuralMatch(D, FoundTemplate, /*Complain=*/false,
+                              /*IgnoreTemplateParmDepth=*/true))
+          continue;
+
         ConflictingDecls.push_back(FoundDecl);
       }
     }

clang/unittests/AST/ASTImporterTest.cpp

@@ -10181,6 +10181,111 @@ TEST_P(ImportTemplateParmDeclDefaultValue,
       FromD, FromDInherited);
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase,
+       ExistingUndeclaredImportDeclaredFriend) {
+  Decl *ToTU = getToTuDecl(
+      R"(
+        template <class A, A>
+        struct foo;
+
+        template <class A>
+        struct X {
+          template <class A1, A1>
+          friend struct foo;
+        };
+      )",
+      Lang_CXX11);
+  Decl *FromTU = getTuDecl(
+      R"(
+        template <class A, A>
+        struct foo;
+
+        template <class A>
+        struct X {
+          template <class A1, A1>
+          friend struct foo;
+        };
+
+        X<int> x;
+      )",
+      Lang_CXX11);
+
+  auto *ToFr1 = FirstDeclMatcher<FriendDecl>().match(ToTU, friendDecl());
+  auto *ToFrD1 = ToFr1->getFriendDecl();
+
+  auto *FromFr1 = FirstDeclMatcher<FriendDecl>().match(FromTU, friendDecl());
+  auto *FromFr2 = LastDeclMatcher<FriendDecl>().match(FromTU, friendDecl());
+
+  auto *FromFrD1 = FromFr1->getFriendDecl();
+  auto *FromFrD2 = FromFr2->getFriendDecl();
+
+  auto *Ctx1 = cast<Decl>(FromFrD1->getDeclContext());
+  auto *Ctx2 = cast<Decl>(FromFrD2->getDeclContext());
+
+  ASSERT_EQ(Ctx1, Ctx2);
+  ASSERT_EQ(ToFrD1->getTemplateDepth(), 1u);
+  ASSERT_EQ(FromFrD2->getTemplateDepth(), 0u);
+  ASSERT_EQ(ToFrD1->getFriendObjectKind(), Decl::FOK_Undeclared);
+  ASSERT_EQ(FromFrD2->getFriendObjectKind(), Decl::FOK_Declared);
+
+  auto *ToFr2Imp = Import(FromFr2, Lang_CXX11);
+
+  EXPECT_TRUE(ToFr2Imp);
+}
+
+TEST_P(ASTImporterOptionSpecificTestBase,
+       ExistingDeclaredImportUndeclaredFriend) {
+  Decl *ToTU = getToTuDecl(
+      R"(
+        template <class A, A>
+        struct foo;
+
+        template <class A>
+        struct X {
+          template <class A1, A1>
+          friend struct foo;
+        };
+
+        X<int> x;
+      )",
+      Lang_CXX11);
+  Decl *FromTU = getTuDecl(
+      R"(
+        template <class A, A>
+        struct foo;
+
+        template <class A>
+        struct X {
+          template <class A1, A1>
+          friend struct foo;
+        };
+      )",
+      Lang_CXX11);
+
+  auto *ToFr1 = FirstDeclMatcher<FriendDecl>().match(ToTU, friendDecl());
+  auto *ToFr2 = LastDeclMatcher<FriendDecl>().match(ToTU, friendDecl());
+
+  auto *ToFrD1 = ToFr1->getFriendDecl();
+  auto *ToFrD2 = ToFr2->getFriendDecl();
+
+  auto *FromFr1 = FirstDeclMatcher<FriendDecl>().match(FromTU, friendDecl());
+  auto *FromFrD1 = FromFr1->getFriendDecl();
+
+  auto *Ctx1 = cast<Decl>(ToFrD1->getDeclContext());
+  auto *Ctx2 = cast<Decl>(ToFrD2->getDeclContext());
+
+  ASSERT_EQ(Ctx1, Ctx2);
+  ASSERT_EQ(FromFrD1->getTemplateDepth(), 1u);
+  ASSERT_EQ(ToFrD2->getTemplateDepth(), 0u);
+  ASSERT_EQ(FromFrD1->getFriendObjectKind(), Decl::FOK_Undeclared);
+  ASSERT_EQ(ToFrD2->getFriendObjectKind(), Decl::FOK_Declared);
+
+  auto *ToFr1Imp = Import(FromFr1, Lang_CXX11);
+
+  EXPECT_TRUE(ToFr1Imp);
+  EXPECT_EQ(ToFr1Imp, ToFr1);
+}
+
 INSTANTIATE_TEST_SUITE_P(ParameterizedTests, ASTImporterLookupTableTest,
                          DefaultTestValuesForRunOptions);