Open Radar

 
Summary:
You can conform a type to Hashable when it also conforms to another protocol requiring Equatable, but not if you declare the Hashable conformance in an extension.


Steps to Reproduce:
1. This code:
protocol P: Equatable {}
struct S: P { var hashValue: Int { return 0 } }
func == (_: S, _: S) -> Bool { return true }
extension S: Hashable {}

Expected Results:
should compile

Actual Results:
but doesn’t:
boom.swift:4:14: error: ambiguous implied conformance of 'S' to protocol 'Equatable' (via 'Hashable') requires explicit conformance
extension S: Hashable {}
             ^
boom.swift:4:14: note: implied protocol conformance 'Equatable' here can be made explicit
extension S: Hashable {}
             ^
             Equatable, 
boom.swift:2:8: note: 'S' implicitly conforms to protocol 'Equatable' (via conformance to 'P') here
struct S: P { var hashValue: Int { return 0 } }
       ^

Regression:
Doesn’t happen if you declare S to be Hashable on the line defining it.
Doesn’t happen if you do what the fixit says and declare S to be Equatable on the extension as well.
Doesn’t happen if S does not conform to P.

Notes:
This isn’t ambiguous. It doesn’t have to resolve some typealias to two different types or something; it’s just got to match Self twice (unambiguously, to the same Self!).