[Bug c++/111831] New: friend with requires keyword compilation error

[Bug c++/111831] New: friend with requires keyword compilation error

[janezz55 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111831

            Bug ID: 111831
           Summary: friend with requires keyword compilation error
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: janezz55 at gmail dot com
  Target Milestone: ---

If I compile list.cpp with:

g++ -std=c++20 list.cpp -o l

friend auto operator==(list const& l, list const& r)
  noexcept(noexcept(
      std::equal(l.begin(), l.end(), r.begin(), r.end())
    )
  )
  requires(requires{std::equal(l.begin(), l.end(), r.begin(), r.end());})
{
  return std::equal(l.begin(), l.end(), r.begin(), r.end());
}

gcc-13.2.1 will respond with a compilation error:

error: no match for 'operator==' (operand types are 'xl::list<int>' and
'xl::list<int>')
   51 |   std::cout << a.size() << " " << b.size() << " " << (a == b) <<
std::endl;
      |                                                       ~ ^~ ~
      |                                                       |    |
      |                                                       |    list<[...]>
      |                                                       list<[...]>

bug clang-16.0.6 won't. I think the bug is related to the friend declaration. I
have prepared an example:

https://github.com/user1095108/xl/blob/master/list.cpp

The relevant requires inside list.hpp is commented out, since the requires
serves no purpose at all in this case. Please uncomment, if you decide to
investigate.

[Bug c++/111831] friend with requires keyword compilation error

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111831

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                URL|https://github.com/user1095 |
                   |108/xl/blob/master/list.cpp |

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://github.com/user1095108/xl/blob/master/list.cpp

[Bug c++/111831] friend with requires keyword compilation error

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111831

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 56121
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56121&action=edit
preprocessed source

[Bug c++/111831] friend with requires keyword compilation error

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111831

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |needs-bisection,
                   |                            |needs-reduction,
                   |                            |rejects-valid
               Host|Linux e5-2620v2             |
                   |6.5.7-arch1-1 #1 SMP        |
                   |PREEMPT_DYNAMIC Tue, 10 Oct |
                   |2023 21:10:21 +0000 x86_64  |
                   |GNU/Linux                   |

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Seems to work on the trunk ...

[Bug c++/111831] friend with requires keyword compilation error

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111831

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Looks like this is a dup of PR109751, which has been fixed for GCC 13.3 / 14.

A workaround for earlier GCC is to turn the problematic constrained hidden
friend into a template:

  template <int = 0>
  friend auto operator==(list const& l, list const& r)
  ...

This prevents GCC from overeagerly checking the function's constraints.

*** This bug has been marked as a duplicate of bug 109751 ***