It would make fish a safer shell if we made dereferencing an undefined variable an error. This would also be consistent with our failglob behavior. See issue #805 and this comment in the issue discussing whether we should do a major release. The example from that comment:

> set hippopotamus
> ls $typopotamus
fish: $typopotamus is not defined.
ls $typopotamus
   ^

Note that using an index that doesn't exist would still silently substitute nothing. That is, this would still be legal and work like it does today:

> set var a
> echo "var[2]=$var[2]:"
var[2]=: