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mypy doesn't take value restrictions into account when solving type variables #11880
Description
This came up in python/typeshed#6762
from typing import Any, TypeVar, Union
T = TypeVar("T")
V = TypeVar("V", str, bytes)
def check_t(x: T | list[T]) -> T: ...
def check_v(x: V | list[V]) -> V: ...
x_str: str
x_list_any: list[Any]
x_str_list_any: str | list[Any]
# mypy does the correct thing here
reveal_type(check_t(x_str)) # str
reveal_type(check_t(x_list_any)) # list[Any] | Any
reveal_type(check_t(x_str_list_any)) # str | list[Any] | Any
reveal_type(check_v(x_str)) # str
# but maybe not here
# E: Value of type variable "V" of "check_v" cannot be "Union[List[Any], Any]"
# N: Revealed type is "Union[builtins.list[Any], Any]"
reveal_type(check_v(x_list_any))
# E: Value of type variable "V" of "check_v" cannot be "Union[str, List[Any], Any]"
# N: Revealed type is "Union[builtins.str, builtins.list[Any], Any]"
reveal_type(check_v(x_str_list_any))Looking at the output, my guess is that mypy isn't taking value restrictions into account when solving constraints and only using value restriction as a check later. That is, list[Any] cannot match the T part of T | list[T] because it would violate the value restriction.
Reacted by Martin Fischer, SpaghettDev, Alex Waygood and Arseniy Terekhin
Activity
hauntsaninja commentedon Jan 1, 2022
Playing around with the following as a potential fix, but I think there are some other issues with how mypy handles this kind of thing.
JelleZijlstra commentedon Jan 9, 2022
#3644 is a similar issue with
T | Cls[T].