Union subtyping doesn't work with covariant type constructors Tuple and Callable
Open
Nobody has claimed this yet.
bug
false-positive
priority-0-high
topic-union-types
- Dominant language
- Python
- Stars
- 20.6k
- Forks
- 3.3k
- PR merge metrics
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Description
Both calls in this code fail, but I think they should be allowed:
from typing import Callable, Union, Tuple
def f(x: Union[Callable[..., int], Callable[..., str]]) -> None: ...
x: Callable[..., Union[int, str]]
f(x)
def g(y: Union[Tuple[int], Tuple[str]]) -> None: ...
y: Tuple[Union[int, str]]
g(y)
Moreover, I would say types in each pair are actually equivalent.
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Research direction
Start by reproducing both failing calls from the issue in mypy. The payload names no source file or test entry point, so locating the union-subtyping and covariant-constructor logic will require exploring the type-checker implementation. Done means both calls are accepted with the stated types without weakening unrelated type checks.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- compilers, devtools
- Issue type
- Bug
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Clearly specified
- Newbie friendliness
- 35/100