Accessing property of parent class inside another template class

Question

I have the following code:

template<typename T>
struct O {
    struct A {
        int a;
    };
    struct B: A {
        auto f() {
            this->a += 1; // OK
            a += 1;       // ERROR: 'a' was not declared in this scope
        }
    };
};

If I try to access a property of the base class using this pointer, compilation works great, but otherwise compiler does produce error: 'a' was not declared in this scope.

Why does it happen?

P.S. Looks like it does work in MSVC, but does not work in GCC and Clang.