Possible D2 solution to the upcasting array problem, and a related problem with const and nested arrays

[Jason House]

Stewart Gordon wrote:

> I was just looking at this
> http://d.puremagic.com/issues/show_bug.cgi?id=2544
> which describes how it's possible to bypass const by doing this:
> 
>      const(int)[] answers = [42];
>      int[][] unconsted = [[]];
>      const(int)[][] unsafe = unconsted;
>      unsafe[0] = answers;
>      unconsted[0][0] = 43;
> 
> The problem is that converting from int[][] to const(int)[][] isn't
> safe, even though the language/compiler seems to think it is.
> 
> Really, it's another version of how you can use a DerivedClass[] as a
> BaseClass[] and thereby place in it an object that isn't of type
> DerivedClass.
> 
> There's actually a simple solution to this: specify that, where
> DerivedClass derives from BaseClass, DerivedClass[] cannot be implicitly
> converted to BaseClass[], but only to const(BaseClass)[].
> 
> Java has had something like this for a while, albeit not with arrays.
> That is, IIRC, you can assign a DataStructure<DerivedClass> to a
> variable of type DataStructure<? extends BaseClass> (or even
> DataStructure<? extends DerivedClass>) - this creates a read-only view
> of the data structure.  My proposal implements the same basic concept as
> this, but in a simpler way.  (Java also supports write-only 'views' with
> DataStructure<? super FurtherDerivedClass>, but I'm not sure we need
> anything like this in D at the moment.)
> 
> Now let's apply the same principle to the example in the bug report.
> Try defining that, in general, T[][] can be converted to const(T[])[]
> but not const(T)[][].  Then
> 
>      const(int)[] answers = [42];
>      int[][] unconsted = [[]];
>      const(int)[][] unsafe = unconsted;
> 
> would be illegal.  One would have to do
> 
>      const(int[])[] safe = unconsted;
> 
> and now
> 
>      safe[0] = answers;
> 
> is illegal.

What about this code?
const int[] answers = [42];
int[][] unconsted = [[]];
const(int[])[] safe = unconsted;
safe[0] = answers;
safe[0][0] = 43;