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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109680] [13/14 Regression] is_convertible<int() const,
 int(*)()> incorrectly true
Date: Tue, 02 May 2023 07:36:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109680

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Not sure about the from case, but what do I know about C++?
While To type is directly returned from a function, std::declval doesn't re=
turn
>From but
>From &&.
#include <type_traits>

using F1 =3D int (int);
using T1 =3D int (*) (int);
static_assert (std::is_convertible_v<F1, T1>);
using F2 =3D int (int) const;
using T2 =3D int (*) (int);
static_assert (!std::is_convertible_v<F2, T2>);
using F3 =3D int (*) (int);
using T3 =3D int (int);
static_assert (!std::is_convertible_v<F3, T3>);
using F4 =3D int (*) (int);
using T4 =3D int (int) const;
static_assert (!std::is_convertible_v<F4, T4>);

passes in both g++ 12 and clang++.
And, on

using F1 =3D int (int);
using T1 =3D int (*) (int);
static_assert (__is_convertible(F1, T1));
using F2 =3D int (int) const;
using T2 =3D int (*) (int);
static_assert (!__is_convertible(F2, T2));
using F3 =3D int (*) (int);
using T3 =3D int (int);
static_assert (!__is_convertible(F3, T3));
using F4 =3D int (*) (int);
using T4 =3D int (int) const;
static_assert (!__is_convertible(F4, T4));

clang++ emits:
/tmp/2a.C:6:34: error: non-member function of type 'F2' (aka 'int (int) con=
st')
cannot have 'const' qualifier
static_assert (!__is_convertible(F2, T2));
                                 ^
/tmp/2a.C:6:1: error: static_assert failed due to requirement
'!__is_convertible(int (int), int (*)(int))'
static_assert (!__is_convertible(F2, T2));
^              ~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/2a.C:12:38: error: non-member function of type 'T4' (aka 'int (int)
const') cannot have 'const' qualifier
static_assert (!__is_convertible(F4, T4));
                                     ^
while g++ trunk
/tmp/2a.C:6:16: error: static assertion failed
    6 | static_assert (!__is_convertible(F2, T2));
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~=