https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93470
--- Comment #2 from frankhb1989 at gmail dot com --- Case: #include <functional> void foo() {} int main() { std::ref(foo)(); } Sorry, I missed to mention it only failed with `clang++ -std=c++2a` (using Clang++ 9.0.1). G++ with `-std=c++2a` still accepts the code. However, it seems that Clang++ is correct here.