Neither clang nor g++ compile the snippet below. Why?

Question

Neither clang nor g++ compile this snippet:

struct A{
protected:    
    struct B{};
};

struct D: A::B, A{};

According to [class.access]/7, we have:

Similarly, the use of A::B as a base-specifier is well-formed because D is derived from A, so checking of base-specifiers must be deferred until the entire base-specifier-list has been seen.

The example and the sentence above are part of [class.access]/7, at least since C++11.