[clang][ASTImporter] Remove extra FunctionTemplateDecl introduced by templated friend

An extranous FunctionTemplateDecl is introduced in the following testcase:

  template <typename T> struct A {
    template <typename U> friend void f();
  };

"To" Context:

  ClassTemplateDecl 0x55dae7116618 <input.cc:1:1, col:73> col:30 A
  |-TemplateTypeParmDecl 0x55dae7116490 <col:11, col:20> col:20 typename depth 0 index 0 T
  `-CXXRecordDecl 0x55dae7116550 <col:23, col:73> col:30 struct A definition
    |-DefinitionData empty aggregate standard_layout trivially_copyable pod trivial literal has_constexpr_non_copy_move_ctor can_const_default_init
    | |-DefaultConstructor exists trivial constexpr needs_implicit defaulted_is_constexpr
    | |-CopyConstructor simple trivial has_const_param needs_implicit implicit_has_const_param
    | |-MoveConstructor exists simple trivial needs_implicit
    | |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param
    | |-MoveAssignment exists simple trivial needs_implicit
    | `-Destructor simple irrelevant trivial needs_implicit
    |-FunctionTemplateDecl 0x55dae7116a38 parent 0x55dae6fa2b68 <col:35, col:71> col:69 f                            // extranous node
    | |-TemplateTypeParmDecl 0x55dae7116860 <col:45, col:54> col:54 typename depth 1 index 0 U
    | `-FunctionDecl 0x55dae7116968 parent 0x55dae6fa2b68 <col:57, col:71> col:69 f 'void ()'
    |-FriendDecl 0x55dae7116aa0 <col:35, col:71> col:69
    | `-FunctionTemplateDecl 0x55dae7116a38 parent 0x55dae6fa2b68 <col:35, col:71> col:69 f
    |   |-TemplateTypeParmDecl 0x55dae7116860 <col:45, col:54> col:54 typename depth 1 index 0 U
    |   `-FunctionDecl 0x55dae7116968 parent 0x55dae6fa2b68 <col:57, col:71> col:69 f 'void ()'
    `-CXXRecordDecl 0x55dae7116ae0 <col:23, col:30> col:30 implicit struct A

Reviewed By: balazske

Differential Revision: https://reviews.llvm.org/D157691

Author: danix800

clang/lib/AST/ASTImporter.cpp

@@ -6459,7 +6459,7 @@ ASTNodeImporter::VisitFunctionTemplateDecl(FunctionTemplateDecl *D) {
 
   ToFunc->setAccess(D->getAccess());
   ToFunc->setLexicalDeclContext(LexicalDC);
-  LexicalDC->addDeclInternal(ToFunc);
+  addDeclToContexts(D, ToFunc);
 
   ASTImporterLookupTable *LT = Importer.SharedState->getLookupTable();
   if (LT && !OldParamDC.empty()) {

clang/unittests/AST/ASTImporterTest.cpp

@@ -5634,6 +5634,43 @@ TEST_P(ImportFriendFunctionTemplates, LookupShouldFindPreviousFriend) {
   EXPECT_EQ(Imported->getPreviousDecl(), Friend);
 }
 
+TEST_P(ImportFriendFunctionTemplates, ImportFriendFunctionInsideClassTemplate) {
+  Decl *From, *To;
+  std::tie(From, To) = getImportedDecl(
+      R"(
+      template <typename T> struct X {
+        template <typename U> friend void f();
+      };
+      )",
+      Lang_CXX03, "", Lang_CXX03, "X");
+
+  auto *FromFriend = FirstDeclMatcher<FriendDecl>().match(From, friendDecl());
+  auto *ToFriend = FirstDeclMatcher<FriendDecl>().match(To, friendDecl());
+
+  EXPECT_TRUE(FromFriend ==
+              LastDeclMatcher<FriendDecl>().match(From, friendDecl()));
+  EXPECT_TRUE(ToFriend ==
+              LastDeclMatcher<FriendDecl>().match(To, friendDecl()));
+
+  auto *FromDecl = FromFriend->getFriendDecl();
+  auto *FromDC = FromFriend->getDeclContext();
+  auto *FromLexicalDC = FromFriend->getLexicalDeclContext();
+
+  EXPECT_TRUE(FromDC->containsDecl(FromFriend));
+  EXPECT_FALSE(FromDC->containsDecl(FromDecl));
+  EXPECT_TRUE(FromLexicalDC->containsDecl(FromFriend));
+  EXPECT_FALSE(FromLexicalDC->containsDecl(FromDecl));
+
+  auto *ToDecl = ToFriend->getFriendDecl();
+  auto *ToDC = ToFriend->getDeclContext();
+  auto *ToLexicalDC = ToFriend->getLexicalDeclContext();
+
+  EXPECT_TRUE(ToDC->containsDecl(ToFriend));
+  EXPECT_FALSE(ToDC->containsDecl(ToDecl));
+  EXPECT_TRUE(ToLexicalDC->containsDecl(ToFriend));
+  EXPECT_FALSE(ToLexicalDC->containsDecl(ToDecl));
+}
+
 struct ASTImporterWithFakeErrors : ASTImporter {
   using ASTImporter::ASTImporter;
   bool returnWithErrorInTest() override { return true; }