[ty] Use partial-order-friendly representation of typevar constraints#20306
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[ty] Use partial-order-friendly representation of typevar constraints#20306
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dcreager
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Diagnostic diff on typing conformance testsNo changes detected when running ty on typing conformance tests ✅ |
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Sep 8, 2025
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Looks reasonable!
I'm sure if I tell you that all this math seems well-suited to a nice unit test suite, it won't be the first you've thought of it ;)
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I'm tackling this in a separate PR, #20319 |
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The constraint representation that we added in #19997 was subtly wrong, in that it didn't correctly model that type assignability is a partial order — it's possible for two types to be incomparable, with neither a subtype of the other. That means the negation of a constraint like
T ≤ t(typevarTmust be a subtype oft) is nott < T, but rathert < T ∨ T ≁ t(using ≁ to mean "not comparable to").That means we need to update our constraint representation to be an enum, so that we can track both range constraints (upper/lower bound on the typevar), and these new incomparable constraints.
Since we need an enum now, that also lets us simplify how we were modeling range constraints. Before, we let the lower/upper bounds be either open (<) or closed (≤). Now, range constraints are always closed, and we add a third kind of constraint for not equivalent (≠). We can translate an open upper bound
T < tintoT ≤ t ∧ T ≠ t.We already had the logic for doing adding clauses to a set by doing a pairwise simplification. We copy that over to where we add constraints to a clause. To calculate the intersection or union of two constraints, the new enum representation makes it easy to break down all of the possibilities into a small number of cases: intersect range with range, intersect range with not-equivalent, etc. I've done the math here to show that the simplifications for each of these cases is correct.