public enum Logging {
}

public extension Logging {
    typealias Metadata = [String: MetadataValue]

    enum MetadataValue {
        case string(String)
        case array([Metadata.Value])
    }
}


extension Logging.Metadata.Value: Equatable {
    // we have to implement it, in reality there's many more cases in the enum
    public static func ==(lhs: Logging.Metadata.Value, rhs: Logging.Metadata.Value) -> Bool {
        switch (lhs, rhs) {
        case (.string(let lhs), .string(let rhs)):
            return lhs == rhs
        case (.array(let lhs), .array(let rhs)):
            return lhs == rhs
        default:
            return false
        }
    }
}

$ swiftenv local 4.2.1
$ swiftc repro.swift


$ swiftenv local 5.0-DEVELOPMENT-SNAPSHOT-2019-01-16-a
$ swiftc repro.swift
repro.swift:29:24: error: type 'Logging.Metadata.Value' (aka 'Logging.MetadataValue') does not conform to protocol 'Equatable'
            return lhs == rhs
                       ^
repro.swift:29:24: error: '<Self where Self : Equatable> (Self.Type) -> (Self, Self) -> Bool' requires that 'Logging.Metadata.Value' (aka 'Logging.MetadataValue') conform to 'Equatable'
            return lhs == rhs
                       ^
repro.swift:29:24: note: requirement specified as 'Logging.Metadata.Value' (aka 'Logging.MetadataValue') : 'Equatable'
            return lhs == rhs
                       ^
repro.swift:29:24: note: requirement from conditional conformance of '[Logging.Metadata.Value]' (aka 'Array<Logging.MetadataValue>') to 'Equatable'
            return lhs == rhs
                       ^

// NOPE: extension Logging.Metadata.Value: Equatable {
// yes:
extension Logging.MetadataValue: Equatable {