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[ty] Update "constraint implication" relation to work on constraints between two typevars (#21068)

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It's possible for a constraint to mention two typevars. For instance, in
the body of

```py
def f[S: int, T: S](): ...
```

the baseline constraint set would be `(T ≤ S) ∧ (S ≤ int)`. That is, `S`
must specialize to some subtype of `int`, and `T` must specialize to a
subtype of the type that `S` specializes to.

This PR updates the new "constraint implication" relationship from
#21010 to work on these kinds of constraint sets. For instance, in the
example above, we should be able to see that `T ≤ int` must always hold:

```py
def f[S, T]():
    constraints = ConstraintSet.range(Never, S, int) & ConstraintSet.range(Never, T, S)
    static_assert(constraints.implies_subtype_of(T, int))  # now succeeds!
```

This did not require major changes to the implementation of
`implies_subtype_of`. That method already relies on how our `simplify`
and `domain` methods expand a constraint set to include the transitive
closure of the constraints that it mentions, and to mark certain
combinations of constraints as impossible. Previously, that transitive
closure logic only looked at pairs of constraints that constrain the
same typevar. (For instance, to notice that `(T ≤ bool) ∧ ¬(T ≤ int)` is
impossible.)

Now we also look at pairs of constraints that constraint different
typevars, if one of the constraints is bound by the other — that is,
pairs of the form `T ≤ S` and `S ≤ something`, or `S ≤ T` and `something
≤ S`. In those cases, transitivity lets us add a new derived constraint
that `T ≤ something` or `something ≤ T`, respectively. Having done that,
our existing `implies_subtype_of` logic finds and takes into account
that derived constraint.
This commit is contained in:
Douglas Creager 2025-10-30 16:11:04 -04:00 • committed by GitHub
parent 1c7ea690a8
commit c0b04d4b7c
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2 changed files with 89 additions and 28 deletions
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4
crates/ty_python_semantic/resources/mdtest/type_properties/implies_subtype_of.md
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@ -180,16 +180,12 @@ This might require propagating constraints from other typevars.
def mutually_constrained[T, U]():
# If [T = U ∧ U ≤ int], then [T ≤ int] must be true as well.
given_int = ConstraintSet.range(U, T, U) & ConstraintSet.range(Never, U, int)
# TODO: no static-assert-error
# error: [static-assert-error]
static_assert(given_int.implies_subtype_of(T, int))
static_assert(not given_int.implies_subtype_of(T, bool))
static_assert(not given_int.implies_subtype_of(T, str))
# If [T ≤ U ∧ U ≤ int], then [T ≤ int] must be true as well.
given_int = ConstraintSet.range(Never, T, U) & ConstraintSet.range(Never, U, int)
# TODO: no static-assert-error
# error: [static-assert-error]
static_assert(given_int.implies_subtype_of(T, int))
static_assert(not given_int.implies_subtype_of(T, bool))
static_assert(not given_int.implies_subtype_of(T, str))