Description
Bug Report
mypy seems to not like an argument of type Union[T, Iterable[T]] where T = TypeVar("T"), instead considering it to be Iterable[Never].
To Reproduce
https://mypy-play.net/?mypy=latest&python=3.12&gist=1471ed0391c1e251edc0ecd85a28d684
from typing import List, Iterable, TypeVar, Union
T = TypeVar("T")
def coerce_list(x: Union[T, Iterable[T]]) -> List[T]:
reveal_type(x)
raise NotImplementedError()
def test() -> None:
coerce_list(1)
coerce_list([1, 2])Expected Behavior
I expected no errors. I expected [1, 2] to match Iterable[T] with T = int.
Actual Behavior
example.py:6: note: Revealed type is "Union[T`-1, typing.Iterable[T`-1]]"
example.py:11: error: Argument 1 to "coerce_list" has incompatible type "list[int]"; expected "Iterable[Never]" [arg-type]
Found 1 error in 1 file (checked 1 source file)
I'm confused why the revealed type of x looks to be correct (although I don't understand the -1 part), but the expected type of the parameter is Iterable[Never].
Your Environment
- Mypy version used:
mypy 1.8.0 (compiled: yes) - Mypy command-line flags: (none)
- Mypy configuration options from
mypy.ini(and other config files): (none) - Python version used: 3.11.2 (Debian 12)