CTFE and cast

[Don Clugston]

On 13/01/12 10:01, k2 wrote:
> When replace typedef to enum, it became impossible to compile a certain
> portion.
> 
> dmd v2.057 Windows
> --------------------------------------------
> enum HANDLE : void* {init = (void*).init}
> 
> pure HANDLE int_to_HANDLE(int x)
> {
>      return cast(HANDLE)x;
> }
> 
> void bar()
> {
>      HANDLE a = cast(HANDLE)1;// ok
>      HANDLE b = int_to_HANDLE(2);// ok
> }
> 
> HANDLE c = cast(HANDLE)3;// ok
> HANDLE d = int_to_HANDLE(4);// NG
> --------------------------------------------
> foo.d(17): Error: cannot implicitly convert expression (cast(void*)4u)
> of type void* to HANDLE

It's a problem. Casting integers to pointers is a very unsafe operation,
and is disallowed in CTFE. There is a special hack, specifically for
Windows HANDLES, which allows you to cast integers to pointers at
compile time, but only after they've left CTFE.