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From: "de34 at live dot cn" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/113060] std::variant converting
 constructor/assignment is non-conforming after P2280?
Date: Tue, 20 Feb 2024 01:47:06 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113060

--- Comment #8 from Jiang An <de34 at live dot cn> ---
(In reply to Giuseppe D'Angelo from comment #7)
> Hi,
>=20
> > Note that this example adds a mediate function template (test_array_ele=
ment_initializable) to "reduce" the non-constexpr-ness of std::declval.
>=20
> That's very clever, thank you!=20
>=20
> Is it _supposed_ to work, though? I had imagined (possibly erroneusly) th=
at
> 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.
>=20
> (It would help to have another compiler that implements P2280, so to do m=
ore
> 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 over=
load
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).=