Swift version 4.1 (swift-4.1-RELEASE) Target: x86_64-unknown-linux-gnu /usercode/main.swift:8:1: error: type 'S<T>' does not conform to protocol 'B' extension S : C where T: A {} ^ /usercode/main.swift:3:10: note: type 'S<T>' does not conform to inherited protocol 'B' protocol C : B {} ^ /usercode/main.swift:12:17: warning: 'is' test is always true print(S<K>.self is C.Type) ^