Default associated type for a protocol under certain constraints

[DevAndArtist (Adrian Meister)]

This is only a guess, but the typealias in the protocol extension feels to me like some sort of type shadowing instead of a conditionally default for the associated type.

Could be related:

Reconsider the semantics of type aliases in protocol extensions

This pitch is the continuation of Source breaking ability to override associated type in protocol extension The ability to declare type aliases that share names with associated types in protocols and protocol extensions has been a workaround for same-type constraints in protocol declarations, introduced in Swift 4 (SE-0142). Here, the declaration of a typealias that shares its name with an associatedtype is tantamount to a same-type constraint that is syntactically and semantically inconsiste…