[Bug libstdc++/118842] `std::result_of` and `std::result_of_t` should be removed in C++20

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118842

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It should be:

struct A {} [[deprecated]];

or:

__attribute__((deprecated)) struct A {};