(🐞) contravariant generic met to become `Never`, expected intersection
Open
Nobody has claimed this yet.
bug
- Dominant language
- Python
- Stars
- 20.6k
- Forks
- 3.3k
- PR merge metrics
- PR metrics pending
Description
from typing import TypeVar, Generic
T_in = TypeVar("T_in", contravariant=True)
T = TypeVar("T")
class A(Generic[T_in]): pass
class B: pass
class C: pass
def meet(a1: A[T], a2: A[T]) -> T: pass
reveal_type(meet(A[B](), A[C]())) # "<nothing>", expected "B & C"
This is related to, but not the same as, https://github.com/python/mypy/labels/topic-join-v-union
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
No source file or test is named. Start by running the minimal Python reproduction and tracing mypy's generic type inference for the contravariant A[T] arguments; done means reveal_type reports the expected B & C intersection rather than Never.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100