False positive when type checking overloads with generics #9023
Description
Are you reporting a bug, or opening a feature request? Bug
MyPy has some strange behavior when type-checking overloaded functions with multiple generic parameters.
Example
from typing import Callable, TypeVar, Union, overload
T = TypeVar('T')
U = TypeVar('U')
@overload
def call(*, f: Callable[[], T]) -> T: ...
@overload
def call(*, u: U) -> U: ...
def call(*, f: Callable[[], T] = None, u: U = None) -> Union[T, U, None]:
return f() if f else u
Actual behavior
$ mypy example.py
example.py:12: error: Overloaded function implementation does not accept all possible arguments of signature 2
example.py:12: error: Overloaded function implementation cannot produce return type of signature 2
Found 2 errors in 1 file (checked 1 source file)
If I swap the order of the parameters on line 12 then mypy complains about signature 1 instead of signature 2.
def call(*, u: U = None, f: Callable[[], T] = None) -> Union[T, U, None]:
I can get mypy to accept the module by adding a dummy parameter in signature 2:
def call(*, f: None = None, u: U) -> U: ...
Expected behavior
$ mypy example.py
Success: no issues found in 1 source file
Versions
$ mypy --version
mypy 0.780
$ python3 --version
Python 3.8.1
Do you see the same issue after installing mypy from Git master? Yes