I noticed a different behavior between gcc 9.2.0 and clang++ 9.0.1. My code is as follows

//header.hh
     ...
template <typename T>
class Outer {
     ...
  public:
     template <typename S>
     class Inner;
     ... 
};

template <typename T>
template <typename S>
class Inner {
     ...
      Inner& func();
     ...
};

then, since the function func() is implemented in another file

//implementation.cc
template <typename T>
template <typename S>
Outer<T>::Inner<S>& Outer<T>::Inner<S>::func() {
    ...
};

Now, if I use g++ the compilation is OK. If I use clang++ I get

src/implementation.cc:6:1: error: missing 'typename' prior to dependent type template name 'Outer<T>::Inner'
Outer<T>::Inner<S>& Outer<T>::Inner<S>::func() {
^
1 error generated.

However if I follow its suggestion and use

typename Outer<T>::Inner<S>& Outer<T>::Inner<S>::func()

I got another error:

src/implementation.cc:6:21: error: use 'template' keyword to treat 'Inner' as
a dependent template name typename Outer<T>::Inner<S>& Outer<T>
::Inner<S>::func() {

And now its suggestion seems very weird.

QUESTIONS

1. Why the two compiler are behaving differently?
2. What is the correct syntax to use?