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Reject bound covariant type variable in List argument #9269

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Reject bound covariant type variable in List argument #9269

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LiuYuHui
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@LiuYuHui LiuYuHui commented Aug 5, 2020

Fixes #734

@LiuYuHui LiuYuHui force-pushed the reject_list_covariant branch from c964566 to b58e8ee Compare August 5, 2020 13:32
@TH3CHARLie TH3CHARLie requested a review from JukkaL August 5, 2020 13:52
@LiuYuHui LiuYuHui force-pushed the reject_list_covariant branch from ecc51e4 to 85d5ba9 Compare August 6, 2020 00:46
sobolevn
sobolevn reviewed Aug 16, 2020
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mypy/checker.py
@@ -947,6 +947,14 @@ def check_func_def(self, defn: FuncItem, typ: CallableType, name: Optional[str])
if ctx.line < 0:
ctx = typ
self.fail(message_registry.FUNCTION_PARAMETER_CANNOT_BE_COVARIANT, ctx)
elif is_named_instance(arg_type, 'builtins.list'):
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What about Dict and (possibly) other types?

from typing import Dict, TypeVar

T_co = TypeVar('T_co', covariant=True)
T_contra = TypeVar('T_contra', contravariant=True)

def cov(a: Dict[T_co, T_co]) -> Dict[T_co, T_co]:
    ...

def contra(a: Dict[T_contra, T_contra]) -> Dict[T_contra, T_contra]:
    ...

reveal_type(cov({'a': 1}))
reveal_type(contra({'a': 1}))

Outputs:

ex.py:12: error: Cannot infer type argument 1 of "cov"
ex.py:12: note: Revealed type is 'builtins.dict[Any, Any]'
ex.py:13: error: Cannot infer type argument 1 of "contra"
ex.py:13: note: Revealed type is 'builtins.dict[Any, Any]'

@msullivan
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I don't think we want to take a PR that does this only for List, and not other things. Happy to discuss how to generalize it.

(I'm closing the PR as part of trying to tidy up, but I'm happy to work with you on moving forward if you're still interested)

@msullivan msullivan closed this Oct 18, 2020
This was referenced Nov 6, 2020
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@JukkaL JukkaL Awaiting requested review from JukkaL

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Reject bound covariant type variable in an argument type
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@LiuYuHui @msullivan @sobolevn