[Clang] Fix crash in isCXXDeclarationSpecifier when attempting to ann…

…otate template name

When attempting to decide if in C++17 a type template for class template
argument deduction and the code is ill-formed the condition to break is
checking the current token is an identifier when it should be checking
if the next token is not ::.

This fixes: #57495
#63052

Differential Revision: https://reviews.llvm.org/D134334

clang/docs/ReleaseNotes.rst

@@ -553,6 +553,10 @@ Bug Fixes in This Version
   (`#48512 <https://github.com/llvm/llvm-project/issues/48512>`_).
 - Fixed a failing assertion when parsing incomplete destructor.
   (`#63503 <https://github.com/llvm/llvm-project/issues/63503>`_)
+- Fix C++17 mode assert when parsing malformed code and the compiler is
+  attempting to see if it could be type template for class template argument
+  deduction. This fixes
+  (`Issue 57495 <https://github.com/llvm/llvm-project/issues/57495>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Parse/ParseTentative.cpp

@@ -1656,7 +1656,10 @@ Parser::isCXXDeclarationSpecifier(ImplicitTypenameContext AllowImplicitTypename,
             if (getLangOpts().CPlusPlus17) {
               if (TryAnnotateTypeOrScopeToken())
                 return TPResult::Error;
-              if (Tok.isNot(tok::identifier))
+              // If we annotated then the current token should not still be ::
+              // FIXME we may want to also check for tok::annot_typename but
+              // currently don't have a test case.
+              if (Tok.isNot(tok::annot_cxxscope))
                 break;
             }
 

clang/test/Parser/cxx1z-class-template-argument-deduction.cpp

@@ -247,3 +247,11 @@ struct A2 {
 };
 
 }
+
+namespace GH57495 {
+template <typename T> struct vector{};
+
+void f() {
+  GH57495::vector.d; // expected-error {{cannot use dot operator on a type}}
+}
+}