Flow typing doesn't infer generic arguments
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Description
Bug Report
I think the following should type check, but mypy errors on the last line. It's unable to infer that matching on the type of x.inner means we know that we have a Foo[str] and not a Foo[int].
#!/usr/bin/env python
from typing import *
T = TypeVar('T')
class Foo(Generic[T]):
def __init__(self, inner: T) -> None:
self.inner = inner
def foo(x: Foo[int] | Foo[str]) -> Foo[str]:
match x.inner:
case int():
raise Exception("Unsupported")
case str():
return x # error: Incompatible return value type (got "Foo[int] | Foo[str]", expected "Foo[str]") [return-value]
To Reproduce
Gist URL: https://gist.github.com/mypy-play/2be996e76f0d979d32be113f891803ed
Playground URL: https://mypy-play.net/?mypy=latest&python=3.12&gist=2be996e76f0d979d32be113f891803ed
Expected Behavior
It should type check :)
Actual Behavior
error: Incompatible return value type (got "Foo[int] | Foo[str]", expected "Foo[str]") [return-value]
Your Environment
- Mypy version used: mypy 1.10.0 (compiled: yes)
- Mypy command-line flags: `--strict``
- Mypy configuration options from
mypy.ini(and other config files): N/A - Python version used: 3.12
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 by running the supplied Python 3.12 reproducer with mypy 1.10.0 and --strict, using the linked gist or playground. Trace the flow-typing behavior around the match on x.inner and verify that the example returns Foo[str] without the incompatible-return error.
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
- 42/100