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From: "jwjagersma at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107439] New: use of static member function in
 requires-expression depends on declaration order
Date: Thu, 27 Oct 2022 16:22:19 +0000
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X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: jwjagersma at gmail dot com
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107439

            Bug ID: 107439
           Summary: use of static member function in requires-expression
                    depends on declaration order
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jwjagersma at gmail dot com
  Target Milestone: ---

The following example fails to compile, as g++ complains that a declaration=
 of
'check' is not available.  Reordering it so that 'check' is declared before
its use in the requires-expression makes it work.  This seems inconsistent,=
 as
member functions are normally expected to be usable anywhere within the cla=
ss
definition.

  $ cat requires-memfn.cpp
  struct A
  {
    template<typename T> requires (check<T>())
    auto func(T) { }

    template<typename T>
    static consteval bool check() { return true; }
  };

  $ g++ -std=3Dc++20 requires-memfn.cpp
  requires-memfn.cpp:3:34: error: there are no arguments to =E2=80=98check=
=E2=80=99 that depend
on a template parameter, so a declaration of =E2=80=98check=E2=80=99
   must be available [-fpermissive]
      3 |   template<typename T> requires (check<T>())
        |                                  ^~~~~~~~
  requires-memfn.cpp:3:34: note: (if you use =E2=80=98-fpermissive=E2=80=99=
, G++ will accept
your code, but allowing the use of an undeclared name is deprecated)=