Using @unknown default in swift 5 enum : How to suppress "Default will never be executed" warning?

The warning is probably somewhat misleading as the spec says (emphasis added):

A nonfrozen enumeration is a special kind of enumeration that may gain new enumeration cases in the future—even after you compile and ship an app. Switching over a nonfrozen enumeration requires extra consideration. When a library’s authors mark an enumeration as nonfrozen, they reserve the right to add new enumeration cases, and any code that interacts with that enumeration must be able to handle those future cases without being recompiled. Only the standard library, Swift overlays for Apple frameworks, and C and Objective-C code can declare nonfrozen enumerations. Enumerations you declare in Swift can’t be nonfrozen.

So it's not so much that the branch will never be executed but that the feature is completely unsupported for your SomeEnumCases user-defined Swift enum.

There seems to be no supported way of doing what you wish in Swift 5, and some indications that adding cases is seen as a breaking change as it could/would break binary compatibility, but Swift is an ever moving target...