[Clang][Parser] Don't evaluate concept when its definition is invalid

clang/include/clang/Sema/Sema.h

@@ -11995,7 +11995,7 @@ class Sema final : public SemaBase {
 
   ConceptDecl *ActOnStartConceptDefinition(
       Scope *S, MultiTemplateParamsArg TemplateParameterLists,
-      const IdentifierInfo *Name, SourceLocation NameLoc);
+      const IdentifierInfo *Name, SourceLocation NameLoc, bool &AddedToScope);
 
   ConceptDecl *ActOnFinishConceptDefinition(Scope *S, ConceptDecl *C,
                                             Expr *ConstraintExpr,

clang/lib/Parse/ParseTemplate.cpp

@@ -322,13 +322,19 @@ Parser::ParseConceptDefinition(const ParsedTemplateInfo &TemplateInfo,
 
   // [C++26][basic.scope.pdecl]/p13
   // The locus of a concept-definition is immediately after its concept-name.
+  bool AddedToScope = false;
   ConceptDecl *D = Actions.ActOnStartConceptDefinition(
-      getCurScope(), *TemplateInfo.TemplateParams, Id, IdLoc);
+      getCurScope(), *TemplateInfo.TemplateParams, Id, IdLoc, AddedToScope);
 
   ParsedAttributes Attrs(AttrFactory);
   MaybeParseAttributes(PAKM_GNU | PAKM_CXX11, Attrs);
 
   if (!TryConsumeToken(tok::equal)) {
+    // The expression is unset until ActOnFinishConceptDefinition(), so remove
+    // the invalid declaration from the future lookup such that the evaluation
+    // wouldn't have to handle empty expressions.
+    if (AddedToScope)
+      Actions.CurContext->removeDecl(D);
     Diag(Tok.getLocation(), diag::err_expected) << tok::equal;
     SkipUntil(tok::semi);
     return nullptr;
@@ -337,6 +343,8 @@ Parser::ParseConceptDefinition(const ParsedTemplateInfo &TemplateInfo,
   ExprResult ConstraintExprResult =
       Actions.CorrectDelayedTyposInExpr(ParseConstraintExpression());
   if (ConstraintExprResult.isInvalid()) {
+    if (AddedToScope)
+      Actions.CurContext->removeDecl(D);
     SkipUntil(tok::semi);
     return nullptr;
   }

clang/lib/Sema/SemaTemplate.cpp

@@ -8632,7 +8632,7 @@ Decl *Sema::ActOnTemplateDeclarator(Scope *S,
 
 ConceptDecl *Sema::ActOnStartConceptDefinition(
     Scope *S, MultiTemplateParamsArg TemplateParameterLists,
-    const IdentifierInfo *Name, SourceLocation NameLoc) {
+    const IdentifierInfo *Name, SourceLocation NameLoc, bool &AddedToScope) {
   DeclContext *DC = CurContext;
 
   if (!DC->getRedeclContext()->isFileContext()) {
@@ -8688,8 +8688,10 @@ ConceptDecl *Sema::ActOnStartConceptDefinition(
   // We cannot properly handle redeclarations until we parse the constraint
   // expression, so only inject the name if we are sure we are not redeclaring a
   // symbol
-  if (Previous.empty())
+  if (Previous.empty()) {
     PushOnScopeChains(NewDecl, S, true);
+    AddedToScope = true;
+  }
 
   return NewDecl;
 }

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>; // expected-error {{use of undeclared identifier 'Concept'}}
+
+template <typename T>
+concept C = invalid; // expected-error {{use of undeclared identifier 'invalid'}}
+
+bool val2 = C<int>; // expected-error {{use of undeclared identifier 'C'}}
+
+} // namespace GH109780