https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70472
--- Comment #7 from TC <rs2740 at gmail dot com> ---
(In reply to Jonathan Wakely from comment #6)
> (In reply to TC from comment #5)
> > In any event, it would be wrong to SFINAE on
> > std::is_copy_constructible<value_type>. The requirement is CopyInsertable,
> > not CopyConstructible. The allocator's construct() can mutilate the
> > constructor arguments to its heart's content before passing them on, and I
> > don't see a way to check this.
>
> <bits/alloc_traits.h> has this:
>
> // true if _Alloc::value_type is CopyInsertable into containers using
> _Alloc
> template<typename _Alloc>
> struct __is_copy_insertable
> : __is_copy_insertable_impl<_Alloc>::type
> { };
>
> But using it requires that std::vector::~vector() is defined as defaulted,
> which would not be a simple change.
>
> We used to use that for the unordered containers until r204790.
That also requires the allocator's `construct` be SFINAE-friendly. Most aren't.
