[Issue 6289] New: Make slices of const/immutable arrays mutable (but keep the elements const/immutable)

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=6289

           Summary: Make slices of const/immutable arrays mutable (but
                    keep the elements const/immutable)
           Product: D
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: jmdavisProg at gmx.com


--- Comment #0 from Jonathan M Davis <jmdavisProg at gmx.com> 2011-07-11 17:12:35 PDT ---
Okay. If you have

immutable val = [1, 2, 3, 4];
void func(immutable(int)[] arg) {}

you can pass val to func even though val is fully immutable. The compiler
realizes that it's safe, because func cannot alter the original array. The
array in the function is a slice of the original and can't affect the original
(since the elements are immutable). However, even though this is true, the type
of a slice of an immutable int[], is still immutable int[], not
immutable(int)[] as with the function parameter. I'd like to see

immutable val = [1, 2, 3, 4];
assert(is(typeof(val) == immutable(int[])));
assert(is(typeof(val[]) == immutable(int)[]));

The slice is then mutable (though its elements are not) - which makes sense,
since that's exactly what the function described above does. The reason that I
want this to be the case is that you can then use immutable arrays with
range-based functions if you slice them (just as is the case with static
arrays). As it stands, both

find(val, 3);

and

find(val[], 3);

are illegal, because the compiler tries to instantiate find with
immutable(int[]). But if the type of a slice of val were immutable(int)[], then
the second find call would work. This would help solve the problem of immutable
arrays not working with range-based functions (see bug# 6148). Then, if you
slice static arrays and immutable/const arrays (just as you'd have to do with
container), they will work with range-based functions. As it stands, that works
with static arrays but not immutable or const arrays.

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