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

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

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

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> I think it is due to:
> include/std/optional:     _GLIBCXX20_CONSTEXPR ~_Storage() { }
> 
> Which was done in r12-4389-g476f305b6cf11d (for https://wg21.link/p2231 ).

Yes, P2231 was approved as a DR against C++20, so std::optional has a constexpr
destructor.

The example works because no std::unique_ptr object needs to be destroyed, but
it's not related to unique_ptr at all. It works for any type:

Reduced:

#include <optional>
struct S { ~S() { } };
static_assert( not std::optional<S>{} );