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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] ICE with -Wunused-parameter when using three-way
 comparison
Date: Sun, 09 Aug 2020 15:16:03 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96533

--- Comment #1 from Nathaniel Shead <natattak at gmail dot com> ---
On further investigation, I suspect this is more general than just
`-Wunused-parameter`; the error looks to be caused by any error occurring
within `operator<`. For example,

  #include <compare>
  #include <functional>

  struct S {};
  auto operator<=3D>(S, S) =3D default;

  template <typename Lhs, typename Rhs>
  auto operator<(Lhs&&, Rhs&& rhs) {
    return rhs.nonexistent;
  }

  int main() {
    return std::less{}(S{}, S{});
  }

produces the same issue.

I also discovered that the issue is only when the templated `operator<` has
deduced type.=