[C++20][Concepts] Fix a failed assertion on an invalid typename requirement

[aaron.ballman]

Summary

The parsing code for a typename requirement currently asserts when given something which is not a valid type-requirement (http://eel.is/c++draft/expr.prim.req.type#nt:type-requirement). This removes the assertion to continue on to the proper diagnostic.

This resolves PR53057.

Note that in that PR, it is using _BitInt(N) as a dependent type name. This patch does not attempt to support that as it is not clear that is a valid type requirement (it does not match the grammar production for one). The workaround in the PR, however, is definitely valid and works as expected.

[aaron.ballman]

I've commit in fa596fb0779ae9029edbcff80ff95e9d1a816206.

clang/lib/Parse/ParseExprCXX.cpp

             PossibleRequiresExprInSimpleRequirement = true;
         } else if (Tok.is(tok::kw_typename)) {
           // This might be 'typename T::value_type;' (a type requirement) or
           // 'typename T::value_type{};' (a simple requirement).
           TentativeParsingAction TPA(*this);
 
           // We need to consume the typename to allow 'requires { typename a; }'
           SourceLocation TypenameKWLoc = ConsumeToken();
-          if (TryAnnotateCXXScopeToken()) {
+          if (TryAnnotateOptionalCXXScopeToken()) {
             TPA.Commit();
             SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
             break;
           }
           CXXScopeSpec SS;
           if (Tok.is(tok::annot_cxxscope)) {
             Actions.RestoreNestedNameSpecifierAnnotation(
                 Tok.getAnnotationValue(), Tok.getAnnotationRange(), SS);

clang/test/Parser/cxx2a-concepts-requires-expr.cpp

 bool r39 = requires { requires () { 1; }; };
 // expected-error@-1 {{requires expression in requirement body; did you intend to place it in a nested requirement? (add another 'requires' before the expression)}}
 
 bool r40 = requires { requires (int i) { i; }; };
 // expected-error@-1 {{requires expression in requirement body; did you intend to place it in a nested requirement? (add another 'requires' before the expression)}}
 
 bool r41 = requires { requires (); };
 // expected-error@-1 {{expected expression}}
+
+bool r42 = requires { typename long; }; // expected-error {{expected a qualified name after 'typename'}}
+
+template <int N>
+requires requires {
+ typename _BitInt(N); // expected-error {{expected a qualified name after 'typename'}}
+} using r43 = void;
+
+template <int N>
+using BitInt = _BitInt(N);
+
+template <int N>
+requires requires {
+ typename BitInt<N>; // ok
+} using r44 = void;