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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/94830] New: Some concepts diagnostic messages are
 nondeterministic
Date: Tue, 28 Apr 2020 19:16:30 +0000
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X-Bugzilla-Version: 10.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94830

            Bug ID: 94830
           Summary: Some concepts diagnostic messages are nondeterministic
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ppalka at gcc dot gnu.org
  Target Milestone: ---

$ cat testcase.C
template<typename T, typename R>
  concept c =3D __is_same(T, R);

template<typename T, typename R>
  requires c<T,R>
void foo() { }

void bar()
{
  foo<int, char>();
}

$ g++ -std=3Dc++2a testcase.C
testcase.C: In function =E2=80=98void bar()=E2=80=99:
testcase.C:10:18: error: use of function =E2=80=98void foo() [with T =3D in=
t; R =3D char]=E2=80=99
with unsatisfied constraints
   10 |   foo<int, char>();
      |                  ^
testcase.C:6:6: note: declared here
    6 | void foo() { }
      |      ^~~
testcase.C:6:6: note: constraints not satisfied
testcase.C: In instantiation of =E2=80=98void foo() [with T =3D int; R =3D =
char]=E2=80=99:
testcase.C:10:18:   required from here
testcase.C:2:11:   required for the satisfaction of =E2=80=98c<T, R>=E2=80=
=99 [with T =3D int; R
=3D char]
testcase.C:2:15: note:   =E2=80=98int=E2=80=99 is not the same as =E2=80=98=
char=E2=80=99
    2 |   concept c =3D __is_same(T, R);
      |               ^~~~~~~~~~~~~~~


The diagnostic pointing to line 2:11 will on some invocations say "[with T =
=3D
int; R =3D char]" and on other invocations say "[with R =3D char; T =3D int=
]"=