[Concepts] Fix isDeclarationSpecifier to detect type-constraints correctly

isDeclarationSpecifiers did not handle some cases of placeholder-type-specifiers with
type-constraints, causing parsing bugs in abbreviated constructor templates.

Add comprehensive handling of type-constraints to isDeclarationSpecifier.

Author: saarraz

clang/lib/Parse/ParseDecl.cpp

@@ -5061,6 +5061,8 @@ bool Parser::isDeclarationSpecifier(bool DisambiguatingWithExpression) {
     // recurse to handle whatever we get.
     if (TryAnnotateTypeOrScopeToken())
       return true;
+    if (TryAnnotateTypeConstraint())
+      return true;
     if (Tok.is(tok::identifier))
       return false;
 
@@ -5193,11 +5195,14 @@ bool Parser::isDeclarationSpecifier(bool DisambiguatingWithExpression) {
 
     // placeholder-type-specifier
   case tok::annot_template_id: {
-    TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
-    return TemplateId->Kind == TNK_Concept_template &&
+    return isTypeConstraintAnnotation() &&
         (NextToken().is(tok::kw_auto) || NextToken().is(tok::kw_decltype));
   }
-
+  case tok::annot_cxxscope:
+    if (NextToken().is(tok::identifier) && TryAnnotateTypeConstraint())
+      return true;
+    return isTypeConstraintAnnotation() &&
+        GetLookAheadToken(2).isOneOf(tok::kw_auto, tok::kw_decltype);
   case tok::kw___declspec:
   case tok::kw___cdecl:
   case tok::kw___stdcall:

clang/test/Parser/cxx2a-abbreviated-templates.cpp

@@ -9,11 +9,36 @@ namespace ns {
   concept D = true;
 }
 
-void foo(C auto a,
-         C<int> auto b,
-         ns::D auto c,
-         ns::D<int> auto d,
-         const C auto e,
-         const C<int> auto f,
-         const ns::D auto g,
-         const ns::D<int> auto h);
+void foo1(C auto a,
+          C<int> auto b,
+          ns::D auto c,
+          ns::D<int> auto d,
+          const C auto e,
+          const C<int> auto f,
+          const ns::D auto g,
+          const ns::D<int> auto h);
+void foo2(C auto a);
+void foo3(C<int> auto b);
+void foo4(ns::D auto c);
+void foo5(ns::D<int> auto d);
+void foo6(const C auto e);
+void foo7(const C<int> auto f);
+void foo8(const ns::D auto g);
+void foo9(const ns::D<int> auto h);
+
+struct S1 { S1(C auto a,
+               C<int> auto b,
+               ns::D auto c,
+               ns::D<int> auto d,
+               const C auto e,
+               const C<int> auto f,
+               const ns::D auto g,
+               const ns::D<int> auto h); };
+struct S2 { S2(C auto a); };
+struct S3 { S3(C<int> auto b); };
+struct S4 { S4(ns::D auto c); };
+struct S5 { S5(ns::D<int> auto d); };
+struct S6 { S6(const C auto e); };
+struct S7 { S7(const C<int> auto f); };
+struct S8 { S8(const ns::D auto g); };
+struct S9 { S9(const ns::D<int> auto h); };