Implementation of the check that recover needs, see https://github.com/nim-lang/RFCs/issues/244 for more details.
proc canAlias(arg, ret: PType): bool {....raises: [Exception, KeyError, ERecoverableError], tags: [RootEffect].}
proc checkIsolate(n: PNode): bool {....raises: [Exception, KeyError, ERecoverableError], tags: [RootEffect].}