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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/97407] New: Expanding alias template in concept
 satisfaction error is undesirable
Date: Tue, 13 Oct 2020 19:04:13 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97407

            Bug ID: 97407
           Summary: Expanding alias template in concept satisfaction error
                    is undesirable
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Compiling this with -std=3Dc++20:

#include <ranges>

template<typename T> concept False =3D false;

template<typename R>
requires False<std::ranges::iterator_t<R>>
void f(R&)
{ }

int main()
{
  int a[2];
  f(a);
}

Produces the expected error:

alias.C: In function =E2=80=98int main()=E2=80=99:
alias.C:13:6: error: use of function =E2=80=98void f(R&) [with R =3D int [2=
]]=E2=80=99 with
unsatisfied constraints
alias.C:7:6: note: declared here
alias.C:7:6: note: constraints not satisfied
alias.C: In instantiation of =E2=80=98void f(R&) [with R =3D int [2]]=E2=80=
=99:
alias.C:13:6:   required from here
alias.C:3:30:   required for the satisfaction of =E2=80=98False<decltype
(std::__detail::__ranges_begin(declval<_Container&>()))>=E2=80=99 [with _Co=
ntainer =3D
int[2]]
alias.C:3:38: note: the expression =E2=80=98false=E2=80=99 evaluated to =E2=
=80=98false=E2=80=99

However the second to last line refers to

std::__detail::__ranges_begin(declval<_Container&>()))

This is meaningless to users, and "_Container" is not used in the definitio=
ns
of ranges::iterator_t or anything it uses. The compiler seems drunk and is
remembering the template argument of a different, unrelated use of the same
alias template. That's probably PR 95310.

I think it would be better to leave that alias unexpanded, as
std::ranges::iterator_t<R>.=