swift

03527d9e - Implement @_downgrade_exhaustivity_check

Implement @_downgrade_exhaustivity_check Dispatch requests the ability to add a new case, but to treat missing instances of that case in patterns as warnings instead of errors. It is still an error to make reference to the annotated case in at least one pattern then not cover the rest of the space, but it is not an error to omit the space of patterns referencing the case entirely. This attribute is private and uglified to intentionally discourage its use outside just this one use case.