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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/100667] [11/12/13/14 Regression] std::tuple<A&&>
 cannot be constructed from A&&, if A not defined (only forward declared)
Date: Wed, 27 Mar 2024 14:02:03 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100667

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #6)
> (In reply to Piotr Nycz from comment #0)
> > It looks that std library code start requiring this to pass:
> > std::is_nothrow_constructible<A&&, A>...
>=20
> Indeed, that's what the standard requires (Clang and MSVC reject this the
> same way). The standard also says that using traits like is_constructible
> requires complete types.
>=20
> However, that's clearly silly for is_constructible<A&&, A> because we know
> that reference binding is valid for any A whether it's complete or not.
>=20
> This is the subject of:
> https://cplusplus.github.io/LWG/issue2939

We can fix this in std::tuple by adding && to the source objects in all
is_constructible and is_convertible conditions. Or we could fix it in the t=
ype
traits themselves.

But I think it would be best to fix it in the compiler, so that we always a=
llow
directly binding T&& or const T& to T, even if T is incomplete. Otherwise w=
e'll
be playing whackamole all over the library.=