Open Radar

 
Summary:
In protocol extensions can use `Self` as a parameter type, return type, type parameter, and typealias value, but not as a parameter in an expression chain like Optional<Self>.Some(…)


Steps to Reproduce:
1. This code:

protocol P {}

extension P {
	func f() -> Optional<Self> {
		return Optional<Self>.Some(self)
	}
}


Expected Results:
should compile without error


Actual Results:
but totally doesn’t:

boom.swift:7:23: error: '>' is not a postfix unary operator
                return Optional<Self>.Some(self)
                                    ^


Regression:
You can make a typealias whose value is Optional<Self> and use that on the left-hand side.


Notes:
I was trying to use this in a method returning Either<A, B<Self>> via a static function `with` with a trailing closure. There was some ill-typed expression in there and Swift decided to tell me I needed to disambiguate the type:

return .with { … }

so I wrote:

return Either<A, B<Self>>.with { … }

and got the stupid error above instead. Then I tried this:

typealias Result = Either<A, B<Self>>
return Result.with { … }