Declarations with some Protocol return types require the Swift 5.1 runtime in iOS 13, macOS 10.15, watchOS 6, or tvOS 13, but the Swift compiler doesn’t enforce this. Running an app that uses some return types on previous operating system versions might crash at runtime, either because of missing swiftgetOpaqueTypeMetadata symbols, or because of a failure to demangle a string that contains the substring “Qo”. (50731151)