[red-knot] simplify subtypes from unions

[carljm]

Add Type::is_subtype_of method, and simplify subtypes out of unions.

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[MichaReiser]

This change makes UnionBuilder::map O(N^2) but I don't think there's a way to avoid that

[carljm]

This change makes UnionBuilder::map O(N^2) but I don't think there's a way to avoid that

I assume you mean UnionBuilder::add? There is no UnionBuilder::map.

But yes, I agree; it's now O(n^2), and I don't think that's avoidable.

EDIT: oh, I'm guessing you actually meant UnionType::map. UnionBuilder::add is not really O(n^2); it is O(n*m) if you add a union to the builder.

[AlexWaygood]

If we switch to using a Vec for the elements of a union, that will make tests such as union.contains(Type::Any) O(n), right? Not sure how important that is to consider

[carljm]

If we switch to using a Vec for the elements of a union, that will make tests such as union.contains(Type::Any) O(n), right? Not sure how important that is to consider

I don't think in general this is an operation we will need often, for the same reason -- the question will usually be about subtyping or assignability (or equivalence), not simple type equality.

It's possible we will have the specific case of needing to know if Any/Unknown is in the union, but I think if that's an issue we could store an extra boolean flag on every union (and potentially even leave the actual Any/Unknown entry out of it) for less cost than the cost of the FxOrderSet.

[MichaReiser]

EDIT: oh, I'm guessing you actually meant UnionType::map. UnionBuilder::add is not really O(n^2); it is O(n*m) if you add a union to the builder.

Yes sorry. I still think it is because we loop over n elements and loops over all elements that have been added to this point. So it's probably n(n-1)/2

[carljm]

I still think it is because we loop over n elements and loops over all elements that have been added to this point. So it's probably n(n-1)/2

Yes, I agree that this is accurate for UnionType::map.

I was correcting myself about UnionBuilder::add, which is itself only O(n), or O(n*m) if we are adding another union (which won't ever be the case from UnionType::map.)