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

[hewillk at gmail dot com]

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

--- Comment #7 from 康桓瑋 <hewillk at gmail dot com> ---
(In reply to Jonathan Wakely from comment #6)
> (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>{} );

It seems like ~_Storage() doesn't call _M_value.~_Up(). 
I could be wrong.