[Bug libstdc++/80390] std::pair of aligned type gives bogus warning

["daniel.kruegler at googlemail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80390

--- Comment #6 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
(In reply to Jonathan Wakely from comment #2)
> Created attachment 41235 [details]
> avoid over-aligned new in is_constructible
> 
> [..]. We can also combine it
> with the delete check, because PR 51222 is fixed.

It seems that PR 51222 didn't cover private destructors. Using this test class

struct PS 
{
  PS(int);
private:
  ~PS() = default;
};

The attempt to evaluate the revised __is_direct_constructible_impl<PS, int>
produces a compiler error (so not SFINAE-friendly), but the current
implementation (using std::is_destructible) works nicely. 

Do you recommend to reopen 51222 or should I open a separate bug? I'm inclined
to create a new one at the moment.

Another question I have is the following: What is you main motivation to
introduce the idiom

struct __uneval_new_t;
inline void* operator new(std::size_t, std::__uneval_new_t*);
decltype(::delete ::new((__uneval_new_t*)0) _Tp(std::declval<_Arg>()))

compared to the seemingly simpler alternative

decltype(::delete ::new _Tp(std::declval<_Arg>()))

?