clang++ -std=c++20 Crash.cpp

Crash.cpp

template <bool v>
struct BC {
  static constexpr bool value = v;
};

template <typename B> struct __and_ : B {
  static constexpr bool value = true;
};

template <typename A>
using _Requires = __and_<A>::value;

template <typename _Tp, typename _Args>
struct __is_constructible_impl : BC<__is_constructible(_Tp, _Args)> {};

template <typename _Tp>
struct optional {
  template <typename _Up, _Requires<__is_constructible_impl<_Tp, _Up>> = true>
    optional(_Up) {}
};

struct MO {};
struct S : MO {};
struct TB {
  TB(optional<S>) {}
};

class Try : public TB, MO {
  using TB::TB;
};

template <typename T>
struct MQB;

template <>
struct MQB<Try> {
    static_assert(__is_constructible_impl<Try, Try>::value);
};

void foo() {
  MQB<Try> m;
}