swift234

IRGen: Pass existentials as generic parameters by unwrapping the container.

When an existential's contained type is used as a generic parameter, unwrap the existential container and save its metadata and witnesses to be used as polymorphic arguments.

Our AST representation can't quite express the distinction between a type parameter being satisfied by the existential type itself from being satisfied by the existential's contained yet. I use a goofy heuristic where I assume a protocol type bound to a type variable with no requirements is satisfied by the protocol type itself; this covers all of the existing <T> (Slice<T>, T) cases that come up in the library, while enabling the <T:Foo> (T) cases. This hopefully addresses <rdar://problem/14470097> well enough to unblock library work until we get a solid AST representation of this difference.

Swift SVN r6352