On Sunday, 19 June 2016 at 21:06:43 UTC, Joerg Joergonson wrote:
A!b is derived from A!a if b is derived from a, is it not? If
not, then I am wrong, if so then D casting has a bug.
You are wrong.
The array example given by Adam is actually a neat illustration
of precisely your question if you just think of `T[]` as
`Slice!T`. In other words, `Slice!b` cannot be derived from
`Slice!a` automatically, since then you could use the `Slice!a`
interface to push `a` instances into your `Slice!b`.
If that's a bit too far of a mental leap to make, you can
certainly find a more thorough illustration of the concepts at
play here by looking up covariance vs. contravariance in
containers/generics online.
— David
