Add is-expression or `__traits` for Dynamic Arrays

[LightBender]

@WalterBright As discussed at DConf, we need to add an is-expression or a __traits for dynamic arrays.

[nordlow]

Does this mean __traits(isDynamicArray, $TYPE)?

[Herringway]

It would be good to write down the details before this issue gets buried and everything is forgotten.

[LightBender]

@nordlow Yes. We have isStaticArray and isAssociativeArray so this is a natural fit.

[LightBender]

@WalterBright Please do not allow this trait to accept enums!

[ntrel]

Add is-expression or

We have is(T == E[], E), and often the E is useful. But I suppose having __traits(isDynamicArray, T) is clearer in a template constraint. And sadly declaring E with the is form does not (yet) declare it in the template body - see #20849.

[Herringway]

@WalterBright Please do not allow this trait to accept enums!

I disagree. This would make it inconsistent with the other traits, especially isStaticArray and isAssociativeArray.

[nordlow]

@WalterBright Please do not allow this trait to accept enums!

I disagree. This would make it inconsistent with the other traits, especially isStaticArray and isAssociativeArray.

Exactly. If we change this behavior we should modify all traits at once presumably in a new D version/revision or whatever we call it.