Dex Devlon e15f88ff21 [ty] Fix contravariant type variable bound checking in specialization inference (#22488) ...

## Summary

Correctly handle upper bounds for contravariant type variables during
specialization inference. Previously, the type checker incorrectly
applied covariant subtyping rules, requiring the actual type to directly
satisfy the bound rather than checking for a valid intersection.

In contravariant positions, subtyping relationships are inverted. The
bug caused valid code like `f(x: Contra[str])` where `f` expects
`Contra[T: int]` to be incorrectly rejected, when it should solve `T` to
`Never` (the intersection of `int` and `str`).

Closes https://github.com/astral-sh/ty/issues/2427

## Details

- Added `is_contravariant()` helper to `TypeVarVariance` in
`variance.rs`
- Updated `SpecializationBuilder::infer_map_impl` in `generics.rs` to
treat bounds and constraints differently based on variance:
  * Skip immediate `ty <: bound` check for contravariant upper bounds
* Flip constraint check to `constraint <: ty` for contravariant
positions
- Added test case for bounded contravariant type variables in
`variance.md`
- All 308 mdtest cases pass & 150 ty_python_semantic unit tests pass

---------

Co-authored-by: Douglas Creager <dcreager@dcreager.net>

2026-01-12 05:00:35 -05:00

..

legacy

[ty] Fix contravariant type variable bound checking in specialization inference (#22488)

2026-01-12 05:00:35 -05:00

pep695

[ty] Fix contravariant type variable bound checking in specialization inference (#22488)

2026-01-12 05:00:35 -05:00

builtins.md

[ty] Support using legacy typing aliases for generic classes in type annotations (#18404)

2025-06-03 12:09:51 +01:00

scoping.md

[ty] Enable even more goto-definition on inlay hints (#21950)

2025-12-12 12:57:38 -05:00

specialize_constrained.md

[ty] Infer typevar specializations for Callable types (#21551)

2025-12-16 09:16:49 -08:00