[Bug libstdc++/121940] function_ref CTAD is not SFINAE friendly

[hewillk at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121940

--- Comment #1 from 康桓瑋 <hewillk at gmail dot com> ---
Other testcase:

#include <functional>

struct S {
  int x;
};

int main() {
  int y;
  std::function_ref fr(std::nontype<&S::x>, y);
}

https://godbolt.org/z/6sEM1bocK