Closed as not planned
Description
Bug Report
When a value whose type is a TypeVar is tested via isinstance, mypy so aggressively narrows types that it refuses to let them be broadened back to the original TypeVar, even when the value is untouched.
To Reproduce
https://mypy-play.net/?mypy=latest&python=3.10&gist=ac798d8bf4d5a39d1446d86bd5c30cf3
from typing import TypeVar
T = TypeVar('T')
def foo(t: T) -> T:
if isinstance(t, str):
return t + "."
elif isinstance(t, dict):
return t
elif isinstance(t, list):
return t
else:
return t
Expected Behavior
no type errors
Actual Behavior
main.py:8: error: Incompatible return value type (got "str", expected "T")
main.py:10: error: Incompatible return value type (got "Dict[Any, Any]", expected "T")
main.py:12: error: Incompatible return value type (got "List[Any]", expected "T")
Found 3 errors in 1 file (checked 1 source file)
Your Environment
- Mypy version used: 0.971 (reproduces in all versions I've tried in the playground)
- Mypy command-line flags: N/A
- Python version used: 3.7 (reproduces in all versions I've tried in the playground)