Open Radar

 
Summary:
Swift requires that `Self` occur only in protocols or as the return type of a class function. Further, only final classes may implement protocols using Self outside of parameters/returns. However, it fails to check whether occurrences of types matched to `Self` in the protocol’s declaration are nested within the return type, e.g. in a tuple, leading to spurious errors.


Steps to Reproduce:
1. Compile this:
protocol P {
	func f() -> (Self, Int)
}

class C: P {
	func f() -> (C, Int) {
		return (self, 0)
	}
}



Expected Results:
I expected it to compile; we’re using C in the return type which (as far as I can see) should be valid.


Actual Results:
Error:
./non-final.swift:8:7: error: protocol 'P' requirement 'f()' cannot be satisfied by a non-final class ('C') because it uses 'Self' in a non-parameter, non-result type position


Regression:
This error does not occur if the function returns Self in both the protocol and the class (a la ObjC’s instancetype I take it).
This error does not occur if the class is final (obviously).
A similar error occurs if you change the method’s return type to (Self, Int):

./non-final.swift:8:15: error: 'Self' is only available in a protocol or as the result of a class method; did you mean 'C'?
        func f() -> (Self, Int) {

This seems like it should be fine too.


Notes:
N/A