http://d.puremagic.com/issues/show_bug.cgi?id=10960

           Summary: Copying a const value type should yield unqual
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: hsteoh@quickfur.ath.cx


--- Comment #0 from hsteoh@quickfur.ath.cx 2013-09-03 16:09:25 PDT ---
This is from a forum post.

Code:
------
void main(){
    auto A = [0, 1, 2];
    const B = [10, -20, 30];
    schwartzSort!(i => B[i])(A); // NG
}
------
This fails because the return type of the lambda is deduced as const(int). This is silly since a copied const value type no longer needs to be const; it should be unqual.

Explicitly naming the unqual type for the delegate fixes the problem:

    schwartzSort!(delegate int (i) => B[i])(A); // OK

The user shouldn't be required to do this; returning a const value type by value should be stripped of its constness, since it is not the original value being returned, but a copy.

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--- Comment #1 from bearophile_hugs@eml.cc 2013-09-03 16:35:12 PDT ---
(In reply to comment #0)

> returning a const value type by value should be stripped of
> its constness, since it is not the original value
> being returned, but a copy.

The main point of my question in D.learn was: if the array A is mutable, and the array B contains constant references (like const references to class instances), is code like this acceptable?

schwartzSort!(i => B[i])(A);

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--- Comment #2 from hsteoh@quickfur.ath.cx 2013-09-03 16:56:05 PDT ---
Oh I see. I misunderstood your question, my bad. :)

Nevertheless, both issues are worth consideration. The unqual issue should be fixed, definitely.

About your actual question: I'm not so sure this is possible, although in theory, it *could* be made possible if B's elements are rebindable. So in theory, this should work:

    class C { ... }
    auto A = [0, 1, 2];
    const B = [new C(1), new C(2), new C(3)];
    schwartzSort!(i => B[i])(A);

The idea being that schwartzSort stores an array of rebindable references to the C instances, and uses that to sort A. This probably doesn't work right now because the language currently has no way to express tail-const in class references, though I'm guessing that if you use Rebindable!C, it should work.

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