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[ty] Use C[T] instead of C[Unknown] for the upper bound of Self (#20479)

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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.
This commit is contained in:
David Peter
2025-09-23 14:02:25 +02:00
committed by GitHub
parent fd5c48c539
commit 742f8a4ee6
7 changed files with 213 additions and 15 deletions
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14
crates/ty_python_semantic/resources/mdtest/narrow/isinstance.md
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@@ -321,8 +321,11 @@ a covariant generic, this is equivalent to using the upper bound of the type par
`object`):
```py
from typing import Self
class Covariant[T]:
def get(self) -> T:
# TODO: remove the explicit `Self` annotation, once we support the implicit type of `self`
def get(self: Self) -> T:
raise NotImplementedError
def _(x: object):
@@ -335,7 +338,8 @@ Similarly, contravariant type parameters use their lower bound of `Never`:
```py
class Contravariant[T]:
def push(self, x: T) -> None: ...
# TODO: remove the explicit `Self` annotation, once we support the implicit type of `self`
def push(self: Self, x: T) -> None: ...
def _(x: object):
if isinstance(x, Contravariant):
@@ -350,8 +354,10 @@ the type system, so we represent it with the internal `Top[]` special form.
```py
class Invariant[T]:
def push(self, x: T) -> None: ...
def get(self) -> T:
# TODO: remove the explicit `Self` annotation, once we support the implicit type of `self`
def push(self: Self, x: T) -> None: ...
# TODO: remove the explicit `Self` annotation, once we support the implicit type of `self`
def get(self: Self) -> T:
raise NotImplementedError
def _(x: object):