const?? When and why? This is ugly!

[Christopher Wright]

Burton Radons wrote:
>    int [] a = new int [1];
> 
>    a [0] = 1;
> 
>    auto b = cast (invariant (int) []) a;
> 
>    a [0] += b [0];
>    a [0] += b [0];
>    writef ("%s\n", a [0]);
>    // Normal result: 4.
>    // Optimiser which assumes invariant data can't change: 3
> 
> Yes, the code is an abuse of the const system. THAT'S EXACTLY MY POINT. Casting mutable data to invariant leads to situations like these. Only data which will never change can be made invariant. Putting "alias invariant (char) [] string" in object.d induces these situations and makes it seem like it's a good idea.

You're going into undefined territory and complain that it doesn't work 
as you expect. Perhaps that should issue a warning, but you're doing 
something wrong and bad: you're saying that the same array is both 
mutable and immutable.

Think of the other approach: once you cast an array to invariant, the 
compiler finds all aliases of that array and turns them invariant. You'd 
be even more upset in that case. It would have long-reaching effects 
that are hard to track down.

Or you could forbid the cast. But it's a useful cast, and you really 
can't get rid of it if you ever want to convert something that is 
mutable to something that is invariant without copying.