Add requires clause for lambda_wrapper

Author: filipsajdak

include/cpp2util.h

@@ -638,6 +638,9 @@ auto is( X const& x ) -> bool {
 }
 
 template <typename F, typename Capture = std::monostate>
+    requires (
+        requires { &F::operator(); }
+    )
 struct lambda_wrapper {
     F f;
     Capture capture;