constexpr reference to a variable of an anonymous struct

Given struct B, which inherits an anonymous union data member from struct A:

#include <cstddef>

struct A
{
    union
    {
        struct { int a, b, c; };
        int vals[3];
    };
};

struct B: A
{
    constexpr B(int x)
    :
        A({ x, x, x })
    {}

    constexpr int& operator[](size_t i)
    {
        return this->vals[i];
    }

    constexpr const int& operator[](size_t i) const
    {
        return this->vals[i];
    }
};

I declare a constexpr variable of type B, then calling its operator[] to assign the return value to a constexpr int:

int main()
{
    constexpr B b(7);
    constexpr int i = b[2];

    return 0;
}

However Clang 3.8 gives me the error message

constexpr variable 'i' must be initialized by a constant expression

The issue is related to the anonymous union, since when I simply use int vals[3] then everything works fine.

Is there a C++14 constexpr restriction I'm missing? Anyhow, a solution would be highly appreciated.