$ cat z.swift
enum E {
    case empty
    case nonEmpty

    public func intersection(_ rhs: E) -> E {
        switch (self, rhs) {
        case (.empty, _):
            return .empty
        case (_, .empty):
            return .empty
        case (.nonEmpty, .nonEmpty):
            return .empty
        }
    }
}

$ swift-dev --driver-mode=swiftc -c z.swift
z.swift:13:9: error: switch must be exhaustive, consider adding a default clause
        }
        ^