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From: "natattak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/96533] New: ICE with -Wunused-parameter when using
 three-way comparison
Date: Sat, 08 Aug 2020 02:29:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96533

            Bug ID: 96533
           Summary: ICE with -Wunused-parameter when using three-way
                    comparison
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: natattak at gmail dot com
  Target Milestone: ---

Minimal example:

  #include <compare>
  #include <functional>

  struct S {
    // making it a member fixes the issue
    friend auto operator<=3D>(S, S) =3D default;
  };

  // removing 'rhs' fixes the issue
  template <typename Lhs, typename Rhs>
  auto operator<(Lhs&&, Rhs&& rhs) {
    return true;
  }

  int main() {
    // using operator< directly fixes the issue
    //return S{} < S{};
    return std::less{}(S{}, S{});
  }

Compiled with `g++ -std=3Dc++20 -Wunused-parameter`. Output:

  '
  Internal compiler error: Error reporting routines re-entered.
  Please submit a full bug report,
  with preprocessed source if appropriate.
  See <https://gcc.gnu.org/bugs/> for instructions.

First noticed on g++ 10.1, tested on compiler explorer with current trunk:
https://godbolt.org/z/85j8TW=