[typing] Type-erasure re generics

[Philippe Sigaud]

Jonathan:
> It's just easier to cope with containers not being assignable to one another
> just because the type of the elements of one is derived from the type of the
> elements of the other.

Scala does this with + or - annotation on types:

List[+T]  : List is parameterized by type T, and is covariant in T. A
contravariant version would use [-T]

See http://www.scala-lang.org/node/129


In D, a List(T) struct could have an opAssign doing the conversion:

struct List(T)
{
...

void opAssign(U : T)(List!U rhs)
{
... walk the list, transform the U's into T's
}
}

You can then test any container to see if it's covariant or
contravariant: see if the type defines the correct opAssign.


Or maybe an opCast(LU)() if (is(LU lu == List!U, U) && is(U : T))

                     ^^^^^^^^^^
I'm not sure we can do that in template constraints. We may have to
express it with an external template:

  if( isA!(List, LU) && is(ExtractType!LU : T))



Philippe