About type aliases in protocol extensions: is this a bug?

[DevAndArtist (Adrian Meister)]

I'm certainly sure this is not a bug, but just something that we have never really diagnosed. There is a similar case in the qualified lookup thread.

@Douglas_Gregor

// Preventing generic type parameter shadowing like the following one:
struct Generic<Element> {
	typealias Element = Int
	var element: Element
}

This suggests a shadowing rule, which basically says you can’t redefine Element in a way that makes the generic parameter Element impossible to name. This sounds great to me.

In that sense, we should diagnose such cases better and ban them altogether.