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From: "daniel.kruegler at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/94565] New: C++20: Comparing comparison category
 types against 0/nullptr is not noexcept
Date: Sat, 11 Apr 2020 21:27:31 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94565

            Bug ID: 94565
           Summary: C++20: Comparing comparison category types against
                    0/nullptr is not noexcept
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: daniel.kruegler at googlemail dot com
  Target Milestone: ---

The following program should be accepted, but is rejected due to a violatio=
n of
the static_assertion:

Compiler options:

-Wall -Wextra -pedantic "-std=3Dc++20"

//---------------------
#include <compare>

int main()
{
  static_assert(noexcept(std::partial_ordering::less =3D=3D 0));
}
//---------------------

prog.cc: In function 'int main()':
prog.cc:5:17: error: static assertion failed
    5 |   static_assert(noexcept(std::partial_ordering::less =3D=3D 0));
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The corresponding comparison functions for are all declared as noexcept
([cmp.partialord], [cmp.weakord], [cmp.strongord]), so the clear intention =
is
that such an comparison should be observable as non-throwing operations. The
reason why above test fails for all of the existing mixed comparison functi=
ons
of all the three comparison category types against 0/nullptr is caused by t=
he
fact that the implementation-internal type std::__cmp_cat::__unspec in head=
er
<compare> misses to declare its converting constructor as noexcept:

struct __unspec
    {
      constexpr __unspec(__unspec*) { }
    };=