[ASTImporter] Import the default argument of TemplateTemplateParmDecl

[teemperor]

Summary

Same idea as in D92103 and D92106, but I realised after creating those reviews that there are
also TemplateTemplateParmDecls that can have default arguments, so here's hopefully the
last patch for default template arguments.

[martong]

Looks good, thanks!

clang/lib/AST/ASTImporter.cpp

ASTNodeImporter::VisitTemplateTemplateParmDecl(TemplateTemplateParmDecl *D) {
   if (!LocationOrErr)
     return LocationOrErr.takeError();
 
   // Import template parameters.
   auto TemplateParamsOrErr = import(D->getTemplateParameters());
   if (!TemplateParamsOrErr)
     return TemplateParamsOrErr.takeError();
 
-  // FIXME: Import default argument.
-
   TemplateTemplateParmDecl *ToD = nullptr;
-  (void)GetImportedOrCreateDecl(
+  if (GetImportedOrCreateDecl(
           ToD, D, Importer.getToContext(),
           Importer.getToContext().getTranslationUnitDecl(), *LocationOrErr,
           D->getDepth(), D->getPosition(), D->isParameterPack(),
-      (*NameOrErr).getAsIdentifierInfo(),
-      *TemplateParamsOrErr);
+          (*NameOrErr).getAsIdentifierInfo(), *TemplateParamsOrErr))
+    return ToD;
+
+  if (D->hasDefaultArgument()) {
+    Expected<TemplateArgumentLoc> ToDefaultArgOrErr =
+        import(D->getDefaultArgument());
+    if (!ToDefaultArgOrErr)
+      return ToDefaultArgOrErr.takeError();
+    ToD->setDefaultArgument(Importer.getToContext(), *ToDefaultArgOrErr);
+  }
+
   return ToD;
 }
 
 // Returns the definition for a (forward) declaration of a TemplateDecl, if
 // it has any definition in the redecl chain.
 template <typename T> static auto getTemplateDefinition(T *D) -> T * {
   assert(D->getTemplatedDecl() && "Should be called on templates only");
   auto *ToTemplatedDef = D->getTemplatedDecl()->getDefinition();

clang/unittests/AST/ASTImporterTest.cpp

TEST_P(ASTImporterOptionSpecificTestBase, ImportBeginLocOfDeclRefExpr) {
   ASSERT_TRUE(
       cast<CStyleCastExpr>(cast<CompoundStmt>(To->getBody())->body_front())
           ->getSubExpr()
           ->getBeginLoc()
           .isValid());
 }
 
 TEST_P(ASTImporterOptionSpecificTestBase,
+       TemplateTemplateParmDeclNoDefaultArg) {
+  Decl *FromTU = getTuDecl(R"(
+                           template<template<typename> typename TT> struct Y {};
+                           )",
+                           Lang_CXX17);
+  auto From = FirstDeclMatcher<TemplateTemplateParmDecl>().match(
+      FromTU, templateTemplateParmDecl(hasName("TT")));
+  TemplateTemplateParmDecl *To = Import(From, Lang_CXX17);
+  ASSERT_FALSE(To->hasDefaultArgument());
+}
+
+TEST_P(ASTImporterOptionSpecificTestBase, TemplateTemplateParmDeclDefaultArg) {
+  Decl *FromTU = getTuDecl(R"(
+                           template<typename T> struct X {};
+                           template<template<typename> typename TT = X> struct Y {};
+                           )",
+                           Lang_CXX17);
+  auto From = FirstDeclMatcher<TemplateTemplateParmDecl>().match(
+      FromTU, templateTemplateParmDecl(hasName("TT")));
+  TemplateTemplateParmDecl *To = Import(From, Lang_CXX17);
+  ASSERT_TRUE(To->hasDefaultArgument());
+  const TemplateArgument &ToDefaultArg = To->getDefaultArgument().getArgument();
+  ASSERT_TRUE(To->isTemplateDecl());
+  TemplateDecl *ToTemplate = ToDefaultArg.getAsTemplate().getAsTemplateDecl();
+
+  // Find the default argument template 'X' in the AST and compare it against
+  // the default argument we got.
+  auto ToExpectedDecl = FirstDeclMatcher<ClassTemplateDecl>().match(
+      To->getTranslationUnitDecl(), classTemplateDecl(hasName("X")));
+  ASSERT_EQ(ToTemplate, ToExpectedDecl);
+}
+
+TEST_P(ASTImporterOptionSpecificTestBase,
        ImportOfTemplatedDeclOfClassTemplateDecl) {
   Decl *FromTU = getTuDecl("template<class X> struct S{};", Lang_CXX03);
   auto From =
       FirstDeclMatcher<ClassTemplateDecl>().match(FromTU, classTemplateDecl());
   ASSERT_TRUE(From);
   auto To = cast<ClassTemplateDecl>(Import(From, Lang_CXX03));
   ASSERT_TRUE(To);
   Decl *ToTemplated = To->getTemplatedDecl();