[Bug c++/123955] ‘Reference to void’ error for C++20 partial specialization of class template with void

[ppalka at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123955

--- Comment #8 from Patrick Palka <ppalka at gcc dot gnu.org> ---
The workaround is to define the partial specialization of InvariantClass as:

  template <typename T, std::same_as<void> U>
    requires InvariantClassLike<T, U>
  class InvariantClass<T, U> {
  ...