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

[Denis Koroskin]

On Mon, 05 Jan 2009 04:16:43 +0300, Jason House <jason.james.house at gmail.com> wrote:

> 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;

Err... "safe[0][0] = 43;" shouldn't compile (it's const)