Enum that can be 0 or null

[tsbockman via Digitalmars-d-learn]

On Monday, 6 June 2016 at 20:32:23 UTC, Alex Parrill wrote:
> It relies on a lot of assumptions about the ABI that make a raw 
> pointer work the same as a structure containing just one 
> pointer, which is why I did not give it much consideration.

At least some of those assumptions could be verified at compile 
time:

     // insert at the end of the VkHandle mixin template:
     static assert(typeof(this).sizeof == typeof(bits).sizeof);
     static assert(typeof(this).alignof == typeof(bits).alignof);

If you want to be extra careful, you can also use a string mixin 
to declare each extern(C) function with its official C parameter 
types, plus a D pragma(inline, true) wrapper that accepts 
VkDevice or whatever, instead.

Of course, it's probably a lot easier to just tel your users to 
replace VK_NULL_HANDLE with VK_NULL_DISPATCHABLE or 
VK_NULL_NONDISPATCHABLE like you suggested earlier.