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Overloads with Generics "does not accept all possible arguments" #19053
Description
I have a function whose return type is determined by the type of an input parameter (with a default value). I have attempted to type-hint it with generics. I get "error: Overloaded function implementation does not accept all possible arguments of signature".
To Reproduce
In the toy example below, the function has two return types (original-type or list) depending on two possible types of the force_list input (Force or NoForce). For completeness I have overloaded all 4 permutations of the input and output types.
Playground: https://mypy-play.net/?mypy=latest&python=3.12&gist=283290e33e2618dfcdf447513d0f4543
from typing import TypeVar, assert_type, overload
class Force: ...
ForceInstance = Force()
class NoForce: ...
NoForceInstance = NoForce()
T = TypeVar("T")
@overload
def list_or_orig(x: list[T], force_list: Force = ...) -> list[T]: ...
@overload
def list_or_orig(x: list[T], force_list: NoForce = ...) -> list[T]: ...
@overload
def list_or_orig(x: T, force_list: Force = ...) -> list[T]: ...
@overload
def list_or_orig(x: T, force_list: NoForce = ...) -> T: ...
def list_or_orig(x: list[T] | T, force_list: Force | NoForce = ForceInstance) -> list[T] | T:
"""Return a list or scalar depending on force_list flag."""
if isinstance(x, list):
return x
elif isinstance(force_list, Force):
return [x]
else:
return x
a_list = list_or_orig(1) # Since default is Force, this will return list[int]
assert_type(a_list, list[int])
b_list = list_or_orig(1, ForceInstance)
assert_type(b_list, list[int])
a_int = list_or_orig(1, NoForceInstance) # This will return int
assert_type(a_int, int)Expected Behavior
No errors when running through mypy.
Actual Behavior
main.py:29: error: Overloaded function implementation does not accept all possible arguments of signature 1 [misc]
main.py:29: error: Overloaded function implementation does not accept all possible arguments of signature 2 [misc]
Found 2 errors in 1 file (checked 1 source file)
Notes
If I replace the generic type T with a specific type, say, int then it's fine.
I appreciate there is an ambiguity between list[T] and T but have included the less-general list[T] first in the overloads.
The same code passes in the pyright playground.
Activity
sterliakov commentedon May 9, 2025
This is definitely a bug. As a workaround, you can simplify the implementation signature: https://mypy-play.net/?mypy=latest&python=3.12&gist=9b83a8f13349f84b0fa64914e7440065
(this doesn't affect external callers and the body still passes)
New signature is no less specific that OP original def, so it certainly doesn't accept any arguments not accepted by that one, confirming this as a bug.