[Bug libstdc++/115846] std::optional> is constant expression even in C++20 mode

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115846

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note here is a more reduced and easier to understand testcase (use a custom
class ratehr than unique_ptr):
```
#include <optional>

struct f
{
  ~f(){}
};

// static_assert([] {
//   f p; // non-constant condition for static assertion
//   return true;
// }());

static_assert([] {
  std::optional<f> opt;
  opt.reset();
  return true;
}());
```