Issue Description:

This fails to compile,

struct Composite<Foo> { }
protocol Chain {
  associatedtype Next: Chain
  associatedtype Foo = Composite<Next.Foo> // Associated type `Foo` references itself.
}

But it shouldn't need to, since,

1. conforming types can override `Foo`, and
2. It's not referring directly to `Foo`, but `Next.Foo`.

This is one work around (from @filip-sakel over the Swift's forum):

typealias CompositeWithoutFoo<Value> = Composite<Value.Foo> where Value: Chain

protocol Chain {
  associatedtype Next: Chain
  associatedtype Foo = CompositeWithoutFoo<Next> // ✅ 
}

So either the checking is too restrictive, or the second code block should be illegal. Either way, this looks like a bug.