An union with a const and a nonconst member?

Question

This appears to be undefined behavior

union A {
  int const x;
  float y;
};

A a = { 0 };
a.y = 1;

The spec says

Cr

Answer 1

If it's any consolation - the Microsoft Xbox 360 compiler (which is based on Visual Studio's compiler) does error out. Which is funny, because that's usually the most lenient of the bunch.

error C2220: warning treated as error - no 'object' file generated
warning C4510: 'A' : default constructor could not be generated
    : see declaration of 'A'
warning C4610: union 'A' can never be instantiated - user defined constructor required

This error goes away if I take the const away. gcc-based compilers don't complain.

EDIT: The Microsoft Visual C++ compiler has the same warning.