template<typename T>
struct A {
    using ARet = T;

    virtual ~A() = default;

    ARet foo();
    virtual ARet bar() = 0;
};

template<typename T>
T A<T>::foo() { // OK, return type matches that in the declaration (T = ARet)
    return 42;
}

template<typename T>
struct B : A<T> {
    using BaseType = A<T>;
    using BRet = typename BaseType::ARet;

    BRet bar() override;
};

template<typename T>
T B<T>::bar() { // Error, return type differs from that in the declaration ( but T = BRet = ARet ??? )
    return 42;
}

template<typename T>
typename B<T>::BRet B<T>::bar() { // OK
    return 42;
}

template<typename T>
auto B<T>::bar() -> BRet {
    return 42;
}

template<>
struct A<int> {
    using ARet = void;
    //...
};