[PEP 695] Allow Self return types with contravariance #17786
+104
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Fix variance inference in this fragment from a typing conformance test:
```
class ClassA[T1, T2, T3](list[T1]):
def method1(self, a: T2) -> None:
...
def method2(self) -> T3:
...
```
Previously T2 was incorrectly inferred as invariant due to `list` having
methods that return `Self`. Be more flexible with return types to allow
inferring contravariance for type variables even if there are `Self` return
types, in particular.
We could probably make this even more lenient, but after thinking about
this for a while, I wasn't sure what the most general rule would be, so
I decided to just make a tweak to support the likely most common use case
(which is probably actually not that common either).
Link to conformance test:
https://github.com/python/typing/blob/main/conformance/tests/generics_variance_inference.py#L15C1-L20C12
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The build failure looks like a flake. |
|
According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
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Fix variance inference in this fragment from a typing conformance test:
Previously T2 was incorrectly inferred as invariant due to
listhaving methods that returnSelf. Be more flexible with return types to allow inferring contravariance for type variables even if there areSelfreturn types, in particular.We could probably make this even more lenient, but after thinking about this for a while, I wasn't sure what the most general rule would be, so I decided to just make a tweak to support the likely most common use case (which is probably actually not that common either).
Link to conformance test:
https://github.com/python/typing/blob/main/conformance/tests/generics_variance_inference.py#L15C1-L20C12