[Bug c++/108393] New: circular concept false-positive

["nikolasklauser at berlin dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108393

            Bug ID: 108393
           Summary: circular concept false-positive
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nikolasklauser at berlin dot de
  Target Milestone: ---

GCC errors on the reproducer below with

./equality_comparable.cpp: In substitution of 'template<class _Iter>  requires 
C<_Iter> constexpr bool operator==(unreachable_sentinel_t, const _Iter&) [with
_Iter = S<unreachable_sentinel_t>]':
./equality_comparable.cpp:6:41:   required by substitution of 'template<class
T>  requires requires(T __t, T __u) {__t == __u;} struct iterator_traits<T>
[with T = S<unreachable_sentinel_t>]'
./equality_comparable.cpp:11:33:   required from here
./equality_comparable.cpp:11:9:   required for the satisfaction of 'C<_Iter>'
[with _Iter = S<unreachable_sentinel_t>]
./equality_comparable.cpp:11:13:   in requirements  [with T =
S<unreachable_sentinel_t>]
./equality_comparable.cpp:11:13: error: satisfaction of atomic constraint
'requires{typename iterator_traits<T>::A;} [with T = T]' depends on itself
   11 | concept C = requires { typename iterator_traits<T>::A; };
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./equality_comparable.cpp: In substitution of 'template<class _Iter>  requires 
C<_Iter> constexpr bool operator==(unreachable_sentinel_t, const _Iter&) [with
_Iter = S<unreachable_sentinel_t>]':
./equality_comparable.cpp:6:41:   required by substitution of 'template<class
T>  requires requires(T __t, T __u) {__t == __u;} struct iterator_traits<T>
[with T = S<unreachable_sentinel_t>]'
./equality_comparable.cpp:11:33:   required from here
./equality_comparable.cpp:11:9:   required for the satisfaction of 'C<_Iter>'
[with _Iter = S<unreachable_sentinel_t>]
./equality_comparable.cpp:11:13:   in requirements  [with T =
S<unreachable_sentinel_t>]
./equality_comparable.cpp:11:13: error: satisfaction of atomic constraint
'requires{typename iterator_traits<T>::A;} [with T = T]' depends on itself

But AFAICT this code should compile just fine.
iterator_traits<unreachable_sentinel_t> should never get instantiated and I
don't see another way this would result in a self-reference.

reproducer (Godbolt: https://godbolt.org/z/cfP5fqcMc):

template<class>
struct iterator_traits
{};

template<class T>
  requires requires(T __t, T __u) { __t == __u; }
struct iterator_traits<T>
{};

template<class T>
concept C = requires { typename iterator_traits<T>::A; };

struct unreachable_sentinel_t
{
  template<C _Iter>
  friend constexpr bool operator==(unreachable_sentinel_t, const _Iter&)
noexcept;
};

template<class T>
struct S
{};

static_assert(!C<S<unreachable_sentinel_t>>);