Adding it open a can of worms. Your type-inference/checking must account for "similar but..." types, you need to follow hierarchies/graphs/trees,etc, you need a way to "re-import" code that "belongs to the super type" (and probably recheck it?), it not always mesh well with other features (or make it harder).
Aside: One of the big reasons to make a bit list of "NO" is to avoid the temptation of "add some sugar to make this easier" but without fully understanding the consequences until becomes later.
Adding it open a can of worms. Your type-inference/checking must account for "similar but..." types, you need to follow hierarchies/graphs/trees,etc, you need a way to "re-import" code that "belongs to the super type" (and probably recheck it?), it not always mesh well with other features (or make it harder).
Aside: One of the big reasons to make a bit list of "NO" is to avoid the temptation of "add some sugar to make this easier" but without fully understanding the consequences until becomes later.