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From: "kretz at kde dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/99201] [8/9/10/11 Regression] ICE in tsubst_copy, at
 cp/pt.c:16581
Date: Mon, 22 Feb 2021 21:12:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99201

--- Comment #4 from Matthias Kretz (Vir) <kretz at kde dot org> ---
Manual reduction which fails with 8-11 and compiles ok with 7:

template <class V, class... F>
  void
  test_values_2arg(F&&... fun_pack)
  {
    (fun_pack(V(), V()), ...);
  }

template <typename TestF, typename RefF>
  auto
  make_tester(const TestF& testfun, const RefF& reffun)
  {
    return [=3D](auto... inputs) {
      auto totest =3D reffun(inputs...);
      using R =3D decltype(totest);
      auto&& expected =3D [&](const auto&... vs) {
        if constexpr (sizeof(R) > 0)
          return [&](auto i) { return reffun(vs[i]...); }(0);
        else
          return [&](auto i) { return reffun(vs[i]...); }(0);
      };
    };
  }
template <typename T> T foo(T, T) {
}
int main()
{
  test_values_2arg<double>(make_tester(
                             [](auto... xs) { return foo(xs...); },
                             [](auto... xs) { return foo(xs...); }));
  return 0;
}=