constancy and template parameters in D2.009+

[Janice Caron]

On 12/13/07, Janice Caron <caron800 at googlemail.com> wrote:
> On 12/13/07, Christopher Wright <dhasenan at gmail.com> wrote:
> > Of course, if you say that the constness of parameters should always be
> > preserved, you'd get:
> >
> > template RemoveHeadConst(T) {
> >     static if (is (T : const(U), U)) {
> >        alias U RemoveHeadConst;
> >     } else {
> >        alias T RemoveHeadConst;
> >     }
> > }
>
> Assuming that works, yes. It doesn't in D2.008, and I don't believe it
> did in D2.007 (although you'd imagine it should).

No wait - I must correct myself - I posted without thinking it through.

That template will remove ALL const, not just head const. You need
something that transforms const(int[]) into const(int)[]. And it has
to work for invariant too. (And even for const pointer to invariant -
yes such things /are/ allowed to exist in D).

It's a much harder template to write than you might first assume.

...which is why I like D2.008's default, in every regard but the
syntax (and the fact that it has some bugs which mean it doesn't
actually work as advertised in all cases).