Use of Union with reference

At work I've been using linux and the GCC compiler for C++11 and C++14. In some of the code at work, I've used a union to store both a reference and a pointer, as so: (Simplified to just the important parts)

struct MyStruct
{
    //Stuff
    union { double& x; double* x_ptr; };
    MyStruct(double& value) : x(value) {}
    //More stuff
};

I believe this code is clear, readable, unambiguous, and provides a convenient way to store references which can be shifted to something else. It provides easily understandable syntactic sugar without costing performance while improving readability. When I attempted to use code like this in visual studio 15, however, the code failed to compile due to "an illegal union member of type double&".

1. Is this code illegal under the standard, or just under Visual Studio 2015?
2. Can I MAKE it compile in Visual Studio 2015, or submit a bug report/change request/something?
3. Is use of a union in that way bad practice?

Note: At my work, pretty much all code is written for Linux and compiled with GCC, and for my specific project, C++11 is guaranteed and GCC is the only compiler that's going to be used.