[catnaroek]

I never said subtyping isn't necessary, and if you read my reply to btilly, you'd see that I actually suggested otherwise: subtyping is basic, natural and necessary, so languages should do it right.

Also, as I again previously said, nominal typing and even nominal subtyping are fine (well, I said “tolerable”, since they have downsides for modularity, but that's a topic for another day), but conflating inheritance with subtyping is a problem. To put it in Java terms, a subclass should only be considered a subtype if:

(0) The subclass doesn't override any methods that aren't abstract in the superclass. A subclass can do whatever its implementor wishes, but a subtype can't behave differently from a supertype.

(1) The subclass doesn't directly mutate any inherited fields from the superclass - this destroys inherited invariants. OTOH, reading inherited fields is just fine in a subtype.

In other words, a subclass is a subtype if and only if the type-checker has enough information to tell that the Liskov substitution principle actually holds.