https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113060
--- Comment #8 from Jiang An <de34 at live dot cn> ---
(In reply to Giuseppe D'Angelo from comment #7)
> Hi,
>
> > Note that this example adds a mediate function template
> > (test_array_element_initializable) to "reduce" the non-constexpr-ness of
> > std::declval.
>
> That's very clever, thank you!
>
> Is it _supposed_ to work, though? I had imagined (possibly erroneusly) that
> once one places the call to `test_array_element_initializable` using
> `declval` as an argument, it would disqualify the whole thing from being
> usable in constant expressions.
>
> (It would help to have another compiler that implements P2280, so to do more
> tests...)
I think it's supposed to work. Enclosing std::declval calls don't matter
because only the constantness in the trailing return type would affect overload
resolution.
Ah, we don't even need to call the function template or function pointers -
it's sufficient to only detect the well-formedness of the function type.
The simplest "fix" I found is changing
`void_t<decltype(_Arr<_Ti>{{std::declval<_Tp>()}})>`
to
`void_t<auto (_Tp&& __t) -> decltype(_Arr<_Ti>{{std::forward<_Tp>(__t)}})>`
, which seemingly works (https://godbolt.org/z/8M85zre5P).
