dup method const or not?

[Steven Schveighoffer]

On Sun, 27 Nov 2011 16:52:56 -0500, Matthias Walter  
<xammy at xammy.homelinux.net> wrote:

> Hi,
>
> Recently, I realized that several "dup" methods in D2's phobos are
> declared like the one for BitArray:
>
> @property BitArray dup()
>
> My question is why it is declared without "const"? Is it a bug or is
> there a reason for it? I can think of a case where one only has
> implemented a shallow copy and wants to omit the const due to the
> involved transitivity, but this shouldn't be the case for BitArray (the
> underlying array is dup'ed). Is there a general rule/recommendation when
> to make a dup method const and when not? My problem arises in a copy
> constructor for a struct S which takes a const(S) instance. But as S has
> a BitArray member, duping is not allowed, so I need a way to duplicate a
> const(BitArray) object.

dup should be const.  There is no compiler limitation as to why this isn't  
const.  You should file a bug.

-Steve