To elaborate a bit more on this alternative, which I will call Pattern conversion only at trans time. The idea would be that we treat constants as opaque when checking for exhaustiveness and dead-code, which ameliorates the privacy-busting and encapsulation-busting cons. It also permits associated constants to appear, since we only need to do pattern expansion during trans, at which point we are fully monomorphized.
It does not, however, resolve the fact that we have two notions of equality, so there is still abstraction-busting that lets you distinguish “deeply structural equal” and “semantically equal” (even if you didn’t intend to do so).