[Bug c++/92894] New: "declared using local type 'test01()::X', is used but never defined" during concept satisfaction

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92894

            Bug ID: 92894
           Summary: "declared using local type 'test01()::X', is used but
                    never defined" during concept satisfaction
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
            Blocks: 67491
  Target Milestone: ---

The following should compile, because the undefined member function is never
odr-used, only its return type is needed.

#include <iterator>

template<typename T,
         std::indirect_unary_predicate<std::projected<T*, std::identity>> Pred>
  constexpr void
  all_of(T*, Pred)
  { }

void
test01()
{
  struct X { };
  X x;
  all_of(&x, [](X&) { return false; });
}

Compiled with -std=gnu++2a and current trunk:

In file included from
/home/jwakely/gcc/10/include/c++/10.0.0/bits/stl_iterator_base_types.h:71,
                 from /home/jwakely/gcc/10/include/c++/10.0.0/iterator:61,
                 from proj.cc:1:
/home/jwakely/gcc/10/include/c++/10.0.0/bits/iterator_concepts.h:669:40: error:
'std::indirect_result_t<_Proj&, _Iter> std::projected<_Iter,
_Proj>::operator*() const [with _Iter = test01()::X*; _Proj = std::identity;
std::indirect_result_t<_Proj&, _Iter> = test01()::X&]', declared using local
type 'test01()::X', is used but never defined [-fpermissive]
  669 |       indirect_result_t<_Proj&, _Iter> operator*() const; // not
defined
      |                                        ^~~~~~~~


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67491
[Bug 67491] [meta-bug] concepts issues