Question: This appears to be undefined behavior

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

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

```

The spec says

> 
> Creating a new object at the storage location that a const object with static, thread, or automatic storage duration occupies or, at the storage location that such a const object used to occupy before its lifetime ended results in undefined behavior.
> 
> 
> 

But no compiler warns me while it's an easy to diagnose mistake. Am I misinterpreting the wording?

Answer: 
It doesn't really make sense to have a `const` member of a `union`, and I'm surprised that the standard allows it. The purpose of all of the many limitations on what can go into a `union` is to arrive at a point where bitwise assignment will be a valid assignment operator for all members, and you can't use bitwise assignment to assign to a `const int`. My guess is that it's just a case that no one had previously thought of (although it affects C as well as C++, so it's been around for awhile).
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.