case-of and subtyping

[zilinc]

type V = <A U8 | B Bool>

foo : V -> ()
foo v = v | A x -> v | A x -> ()
                     | B x -> ()
          | v' -> v' | A x -> ()
                     | B x -> ()

This program passes surface, but crashes core typechecker.

In general, every time we match a tag in a variant, we can (by promotion) turn the taken alternative to an untaken one, and perform the same match again (assuming the scrutinee is not linear).