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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++/99599] [11/12/13 Regression] Concepts requirement falsely
 reporting cyclic dependency, breaks tag_invoke pattern
Date: Thu, 21 Mar 2024 21:40:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99599

--- Comment #21 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Here's a very small reproducer:

template<typename T> struct indirect {
  template<typename U> requires // (!is_indirect<U>) &&
    requires (const T& t, const U& u) { t =3D=3D u; }
  friend constexpr bool operator=3D=3D(const indirect&, const U&)
  { return false; }

  T* _M_ptr{};
};

indirect<int> i;
bool b =3D i =3D=3D 1;


It compiles with trunk, although my real case that this was reduced from
doesn't.

It works if you uncomment the extra constraint and define:

template<typename T> class indirect;
template<typename T> constexpr bool is_indirect =3D false;
template<typename T> constexpr bool is_indirect<indirect<T>> =3D true;=