Cannot copy void[] to void[] in @safe code?

[ag0aep6g]

On Friday, 8 July 2022 at 10:58:24 UTC, wjoe wrote:
> My understanding is that a void[] doesn't have a distinct type 
> but since the length is bytes and not elements this makes me 
> believe that under the hood they are byte arrays - or, rather, 
> managed chunks of memory.
> How's copying memory without a distinct type different from 
> copying, say, an ubyte[] to an ubyte[] ?

You're allowed to copy from `ubyte[]` to `ubyte[]`. But you're 
not allowed to copy from `ubyte[]` to `int*[]`, because 
reinterpreting a bunch of bytes as pointers is not safe.

The thing about `void[]` is that it can point to memory that also 
has a typed alias. You can have a `void[]` and a `ubyte[]` 
pointing to the same memory. And you can have a `void[]` and an 
`int*[]` pointing to the same memory. So if you were allowed to 
copy from `void[]` to `void[]`, you'd practically be allowed to 
copy from `ubyte[]` to `int*[]`. But that's not safe.

In code:

```d
void main() @safe
{
     ubyte[] some_bytes = [1, 2, 3, 4, 5, 6, 7, 8];
     int*[] some_pointers = [new int];
     void[] v1 = some_bytes;
     void[] v2 = some_pointers;
     v2[] = v1[]; /* creating invalid pointers */
     int x = *some_pointers[0]; /* undefined behavior */
}
```