[Bug c++/106810] New: Unexpected constraint recursion

[Bug c++/106810] New: Unexpected constraint recursion

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106810

            Bug ID: 106810
           Summary: Unexpected constraint recursion
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

GCC will produce recursive constraints check for the following code, suggesting
that there may be a language bug (or maybe not a bug).

https://godbolt.org/z/vYd7a5f19

#include <iterator>

template <class I2>
struct I {
  using value_type = int;
  using difference_type = int;

  value_type& operator*() const;
  I& operator++();
  I operator++(int);

  template<std::sentinel_for<I2> S>
  friend bool operator==(I, S);
};

static_assert(std::sentinel_for<int*, I<int*>>);

[Bug c++/106810] Unexpected constraint recursion

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106810

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Reduced down just this:
```
template <class a, class b>
  concept t = requires(a aa, b bb) { aa == bb; };

template <class I2>
struct I {
  using value_type = int;
  using difference_type = int;

  value_type& operator*() const;
  I& operator++();
  I operator++(int);

  template<t<I2> S>
  friend bool operator==(I, S);
};

static_assert(t<int*, I<int*>>);
```

I think GCC is correct in saying this is recusive even.

[Bug c++/106810] Unexpected constraint recursion

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106810

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Even though this bug is older, PR 108393 has the full analysis of what is going
on and even talks about a fix for the testcase and talking about CWG2369 too.

*** This bug has been marked as a duplicate of bug 108393 ***