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### Summary This PR includes two changes, both of which are necessary to resolve https://github.com/astral-sh/ty/issues/1196: * For a generic class `C[T]`, we previously used `C[Unknown]` as the upper bound of the `Self` type variable. There were two problems with this. For one, when `Self` appeared in contravariant position, we would materialize its upper bound to `Bottom[C[Unknown]]` (which might simplify to `C[Never]` if `C` is covariant in `T`) when accessing methods on `Top[C[Unknown]]`. This would result in `invalid-argument` errors on the `self` parameter. Also, using an upper bound of `C[Unknown]` would mean that inside methods, references to `T` would be treated as `Unknown`. This could lead to false negatives. To fix this, we now use `C[T]` (with a "nested" typevar) as the upper bound for `Self` on `C[T]`. * In order to make this work, we needed to allow assignability/subtyping of inferable typevars to other types, since we now check assignability of e.g. `C[int]` to `C[T]` (when checking assignability to the upper bound of `Self`) when calling an instance-method on `C[int]` whose `self` parameter is annotated as `self: Self` (or implicitly `Self`, following https://github.com/astral-sh/ruff/pull/18007). closes https://github.com/astral-sh/ty/issues/1196 closes https://github.com/astral-sh/ty/issues/1208 ### Test Plan Regression tests for both issues. |
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