no error when `TypeVar` with explicit variance is used in the wrong position when it's part of a union
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bug
- Dominant language
- Python
- Stars
- 20.6k
- Forks
- 3.3k
- PR merge metrics
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Description
from typing import Generic, TypeVar
T = TypeVar("T", covariant=True)
class Foo(Generic[T]):
def b(self, value: T | int) -> None: ... # no error
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the Python reproducer in the issue and run it in the linked mypy-play playground. Trace how explicit TypeVar variance is checked when the variable appears inside a union; done when this example reports an error for covariant T used in the parameter position.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 48/100