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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/95349] Using std::launder(p) produces unexpected behavior
 where (p) produces expected behavior
Date: Wed, 27 May 2020 09:40:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95349

--- Comment #3 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 27 May 2020, redi at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95349
>=20
> --- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
> Using=20
>=20
>   auto t =3D new(p) std::uint64_t;
>   std::memcpy(t, std::launder(storage), sizeof(storage));
>   return t;
>=20
> also prevents GCC from propagating the dynamic type of p to t.

So the language lawyer question is whether the testcase is valid
or not and what std::launder makes a difference semantics wise
(the dynamic type is still transfered, just the compiler no
longer knows which one it is).=