Nested template class argument deduction differences between compilers

The below example compiles fine in both gcc and msvc:

template <typename>
struct Foo {
    template <typename T>
    struct Bar {
        Bar(T val): 
            val_(val) 
        {             
        }

        T val_;
    };
};

auto func() {
    return Foo<int>::Bar(3);
}

In clang there seems to be a need for an extra deduction guide, for it to compile:

template <typename>
struct Foo {
    template <typename T>
    struct Bar {
        Bar(T val): 
            val_(val) 
        {             
        }

        T val_;
    };

    template <typename T>
    Bar(T) -> Bar<T>; // deduction guide
};

auto func() {
    return Foo<int>::Bar(3);
}

Is there a rationale behind this extra requirement in clang? Or is it just a difference in the interpretation of something that's unclear in the standard to begin with?