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

[cmingyi01 at gmail dot com via Gcc-bugs]

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

--- Comment #3 from Mingyi Chen <cmingyi01 at gmail dot com> ---
Just found out that the code below does not trigger the expected warning.

```c++
struct [[deprecated]] A {};

int main() {
    (void)A{};
}
```

See https://compiler-explorer.com/z/4r8hYYPPa.

Is this a c++ front-end issue?