[Clang][Parser] Don't evaluate concept when its definition is invalid (#111179)

Since #103867, the nullness of the concept declaration has been turned
to represent a state in which the concept definition is being parsed and
used for self-reference checking.

However, PR missed a case where such a definition could be invalid, and
we shall inhibit making it into evaluation.

Fixes https://github.com/llvm/llvm-project/issues/109780

clang/lib/Parse/ParseTemplate.cpp

@@ -331,6 +331,8 @@ Parser::ParseConceptDefinition(const ParsedTemplateInfo &TemplateInfo,
   if (!TryConsumeToken(tok::equal)) {
     Diag(Tok.getLocation(), diag::err_expected) << tok::equal;
     SkipUntil(tok::semi);
+    if (D)
+      D->setInvalidDecl();
     return nullptr;
   }
 
@@ -338,6 +340,8 @@ Parser::ParseConceptDefinition(const ParsedTemplateInfo &TemplateInfo,
       Actions.CorrectDelayedTyposInExpr(ParseConstraintExpression());
   if (ConstraintExprResult.isInvalid()) {
     SkipUntil(tok::semi);
+    if (D)
+      D->setInvalidDecl();
     return nullptr;
   }
 

clang/test/SemaTemplate/concepts.cpp

@@ -1151,3 +1151,17 @@ int test() {
 }
 
 }
+
+namespace GH109780 {
+
+template <typename T>
+concept Concept; // expected-error {{expected '='}}
+
+bool val = Concept<int>;
+
+template <typename T>
+concept C = invalid; // expected-error {{use of undeclared identifier 'invalid'}}
+
+bool val2 = C<int>;
+
+} // namespace GH109780