[Bug c++/94057] [9/10 Regression] -std=gnu++20 causes failure naming nested templated class since r9-4536

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94057

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
The root cause isn't the C++20 feature it seems.  The following version with
explicit 'typename' is rejected, but compiles fine with clang/icc:

template <typename T> class A {
  template <typename U> class B {
    B(typename A<T>::B<U>&);
    void fn(typename A<T>::B<U> &);
  };
};