Accessing Private Nested class member of base class outside base class

Question

I have a base class and nested class with structure given below.

Base Hpp file

class Base{
private:
class Nested;
Nested* nested_ptr;
public:
Base();
Nested& GetNested();

Base Cpp file

Base::Base(){
nested_ptr = new Nested();
}
Base::Nested& Base::GetNested(){
return *nested_ptr;
}

The nested class is implemented in a separate file.

My compiler is apple clang 11.0 and cpp standard is 17.

When I try to access nested class via

Base base{}
Base::Nested& nested = base.GetNested();

I am getting compiler error. But when i try to access nested via

Base base{}
auto& nested = base.GetNested();

This compiles successfully. Is this feature portable or is it a bug in apple clang compiler.

Base base{};
base.GetNested().DoSomething();

This also compiles successfully.