unsound = import "unsound"

x = !0

C = Class()
C|<: ContextManager|.
    __enter__ self =
        unsound.perform do!:
            print! "C: enter"
            x.inc!()
        self
    __exit__ self, _, _, _ =
        unsound.perform do!:
            print! "C: exit"
            x.dec!()
        False

with! C.new(), c =>
    assert x == 1
    print! c
assert x == 0