Swift 5 Release Notes for Xcode 10.2

The types provided by the simd framework, like and , are now type aliases of the new standard library types. Most existing code using types will continue to work with the new generic SIMD types, but there are a few changes to note. Because the types are now generic rather than concrete, if you have defined your own protocols over the framework types, it may be necessary to refactor the conformances because a Swift generic type can’t have multiple conditional conformances to a protocol.

Source: developer.apple.com