Unknowndefault -

: Like a regular default , it matches any value not explicitly covered by other cases.

: If your code is recompiled and there are known enum cases that you haven't explicitly handled, the compiler will issue a warning . A standard default would stay silent. Usage and Rules unknowndefault

In Swift, every switch statement must be exhaustive. While a standard default case acts as a "catch-all" and satisfies this requirement, it can hide errors if you accidentally forget to handle a new, known case. solves this by: : Like a regular default , it matches