fix dup for const/immutable reference type elements

[aG0aep6G]

The second overload takes care of all cases where the elements can be
converted to mutable. So, the first overload has no business
Unconst-ing anything.

[MartinNowak]

Why do you need Unconst here? If T were const it should match the other overload.
It's requiring a global import of core.internal.traits that I'd like to avoid.

[aG0aep6G]

The other overload only matches when is(const(T) : T). That's not true for const(Object), for example, so it's handled by the first overload.

The difference in the overloads is not that one takes mutable and the other const. Both take all mutable/const/immutable. Convertability to mutable is where they differ.

To check against the convertability, we need to strip any const/immutable from T => Unconst.

[MartinNowak]

The other overload only matches when is(const(T) : T). That's not true for const(Object), for example, so it's handled by the first overload.

That's intended as you cannot convert constant values with references to mutable, because it would require deep copying.

It doesn't work with const(Object), though ...

Same here, needs deep copying.

[aG0aep6G]

It doesn't work with const(Object), though ...

Same here, needs deep copying.

That was about std.array.array. That isn't supposed to convert to mutable, too, is it? How do I just get a shallow copy of an array?

[MartinNowak]

That was about std.array.array. That isn't supposed to convert to mutable, too, is it? How do I just get a shallow copy of an array?

Ah right, that's a pretty annoying limitation. There is currently no way to declare a tail-const object in D Issue 5325.
You can work with Rebindable!(const Object)[] instead, that will also work with .dup.

[MartinNowak]

Also see const(FAQ) - D Programming Language.

[aG0aep6G]

I don't want tail-const. But we don't have to go through this here now. I made a thread on D.learn: http://forum.dlang.org/thread/lncozysqrhzmrmypkegz@forum.dlang.org#post-lncozysqrhzmrmypkegz:40forum.dlang.org.