[Bug c++/99201] [8/9/10/11 Regression] ICE in tsubst_copy, at cp/pt.c:16581

["kretz at kde dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99201

--- 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 [=](auto... inputs) {
      auto totest = reffun(inputs...);
      using R = decltype(totest);
      auto&& expected = [&](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;
}