diff --git a/clang/lib/Parse/ParseTemplate.cpp b/clang/lib/Parse/ParseTemplate.cpp
index de29652abbfd95f..0953cfc3c017efe 100644
--- a/clang/lib/Parse/ParseTemplate.cpp
+++ b/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;
   }
 
diff --git a/clang/test/SemaTemplate/concepts.cpp b/clang/test/SemaTemplate/concepts.cpp
index a98ca3939222bda..312469313fc5358 100644
--- a/clang/test/SemaTemplate/concepts.cpp
+++ b/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