[Bug c++/97407] New: Expanding alias template in concept satisfaction error is undesirable

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97407

            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=c++20:

#include <ranges>

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

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 definitions
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>.