[clang][ASTImporter] Fix a possible assertion failure `NeedsInjectedClassNameType(Decl)'.

The assertion can happen if ASTImporter imports a CXXRecordDecl in a template
and then imports another redeclaration of this declaration, while the first import is in progress.
The process of first import did not set the "described template" yet
and the second import finds the first declaration at setting the injected types.
Setting the injected type requires in the assertion that the described template is set.
The exact assertion was:
clang/lib/AST/ASTContext.cpp:4411:
clang::QualType clang::ASTContext::getInjectedClassNameType(clang::CXXRecordDecl*, clang::QualType) const:
Assertion `NeedsInjectedClassNameType(Decl)' failed.

Reviewed By: shafik

Differential Revision: https://reviews.llvm.org/D94067

Author: balazske

clang/lib/AST/ASTImporter.cpp

@@ -5393,16 +5393,16 @@ ExpectedDecl ASTNodeImporter::VisitClassTemplateDecl(ClassTemplateDecl *D) {
 
   CXXRecordDecl *FromTemplated = D->getTemplatedDecl();
 
+  auto TemplateParamsOrErr = import(D->getTemplateParameters());
+  if (!TemplateParamsOrErr)
+    return TemplateParamsOrErr.takeError();
+
   // Create the declaration that is being templated.
   CXXRecordDecl *ToTemplated;
   if (Error Err = importInto(ToTemplated, FromTemplated))
     return std::move(Err);
 
   // Create the class template declaration itself.
-  auto TemplateParamsOrErr = import(D->getTemplateParameters());
-  if (!TemplateParamsOrErr)
-    return TemplateParamsOrErr.takeError();
-
   ClassTemplateDecl *D2;
   if (GetImportedOrCreateDecl(D2, D, Importer.getToContext(), DC, Loc, Name,
                               *TemplateParamsOrErr, ToTemplated))

clang/unittests/AST/ASTImporterTest.cpp

@@ -6141,6 +6141,41 @@ TEST_P(ASTImporterOptionSpecificTestBase, TypedefWithAttribute) {
   EXPECT_EQ(ToAttr->getAnnotation(), "A");
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase,
+       ImportOfTemplatedDeclWhenPreviousDeclHasNoDescribedTemplateSet) {
+  Decl *FromTU = getTuDecl(
+      R"(
+
+      namespace std {
+        template<typename T>
+        class basic_stringbuf;
+      }
+      namespace std {
+        class char_traits;
+        template<typename T = char_traits>
+        class basic_stringbuf;
+      }
+      namespace std {
+        template<typename T>
+        class basic_stringbuf {};
+      }
+
+      )",
+      Lang_CXX11);
+
+  auto *From1 = FirstDeclMatcher<ClassTemplateDecl>().match(
+      FromTU,
+      classTemplateDecl(hasName("basic_stringbuf"), unless(isImplicit())));
+  auto *To1 = cast_or_null<ClassTemplateDecl>(Import(From1, Lang_CXX11));
+  EXPECT_TRUE(To1);
+
+  auto *From2 = LastDeclMatcher<ClassTemplateDecl>().match(
+      FromTU,
+      classTemplateDecl(hasName("basic_stringbuf"), unless(isImplicit())));
+  auto *To2 = cast_or_null<ClassTemplateDecl>(Import(From2, Lang_CXX11));
+  EXPECT_TRUE(To2);
+}
+
 INSTANTIATE_TEST_CASE_P(ParameterizedTests, ASTImporterLookupTableTest,
                         DefaultTestValuesForRunOptions, );