Question

For instance, List[T] forSome { type T } is equivalent to List[_], but is this true for every possible usage of forSome or are there cases where forSome cannot be replaced by an equivalent of the second syntax?

Answer 1

No, not all usages can be thus converted. Something like this (thanks to retronym, below, who should be getting the upvotes on this one)

def foo(xs: Map[T, T] forSome { type T})

The point here is that I can use the existential in more than one place but it is the same existential. Obviously, if you used wildcards, you would be referring to different existentials