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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++/109049] std::declval gives wrong result for cv void
Date: Tue, 07 Mar 2023 12:32:24 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109049

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The reason it behaves this way is that we define declval as:

  template<typename _Tp>
    auto declval() noexcept -> decltype(__declval<_Tp>(0));

and decltype(__declval<_Tp>(0)) drops the cv-qualifiers. This implementatio=
n is
much faster to compile, and so we're not going to stop using it to "fix" the
useless testcase in this bug. Nobody cares about decltype(std::declval<const
void>) but they do care about compile times.=