[Bug c++/95066] [C++ 20] Incorrect successful compilation with a conditional explicit

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95066

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Reduced:

template <typename T>
struct Foo {
  template <typename U>
  explicit(static_cast<U>(true)) operator Foo<U>();
};

template <typename T>
template <typename U>
Foo<T>::operator Foo<U>() {
  return {};
}

int
main ()
{
  Foo<float> a;
  Foo<int> b = a;
}