extern(C) and mangling type names

[Kagamin via Digitalmars-d]

On Wednesday, 15 February 2017 at 12:00:51 UTC, Mike Parker wrote:
> version(UseX11) {
>     struct Display;
>     alias EGLNativeWindowType = Display*;
> }
> else version(UseWayland) {
>     struct wl_display;
>     EGLNativeWindowType = wgl_display*;
> }

If you use void* for Display*, why do you need casts? What do you 
cast it to?