(edited)

tarunon

protocol XorY {} struct X: XorY {} struct Y: XorY {} protocol A1 { func a1() -> String } protocol A2: A1 { func a2() -> String } protocol A3: A2 { func a3() -> String } struct C<R>: A1 { func a1() -> String { return "a1\(R.self)" } } extension C: A2 where R: XorY { func a2() -> String { return "a2\(R.self)" } } extension C: A3 where R == Y { func a3() -> String { return "a3\(R.self)" } } print(C<X>().a1()) print(C<X>().a2()) // print(C<X>().a3()) // compile error print(C<Y>().a1()) print(C<Y>().a2()) print(C<Y>().a3())

Version:

swift-4.1.1-RELEASE

Output:

a1X a2X a1Y a2Y a3Y

Error: