functools.partial always assumes the first signature of an @overloaded function

[finite-state-machine]

Bug Report

When functools.partial() is applied to an overloaded function, the return type of the first @overload signature is always assumed.

To Reproduce

import functools
from typing import (
 Literal,
 overload,
 Union,
 )

@overload
def function(a: Literal[True]) -> int: ...
@overload
def function(a: Literal[False] = False) -> str: ...

def function(a: bool = False) -> Union[int, str]:
 if a:
 return 42
 else:
 return 'hello'

p0 = functools.partial(function)
reveal_type(p0)
 # actual: Revealed type is "functools.partial[builtins.int*]"
 # expected: Revealed type is "functools.partial[Union[int, str]]"
 # (because 'p0()' would accept any value for 'a')

v0 = p0()
reveal_type(v0)
 # actual: Revealed type is "builtins.int*"
 # expected: either "str" or "Union[int, str]"
 # (the latter is less precise, but at least it's not wrong)

p1 = functools.partial(function, a=False)
reveal_type(p1)
 # actual: Revealed type is "functools.partial[builtins.int*]"
 # expected: Revealed type is "functools.partial[Union[int, str]]"
 # (because 'p1()' can still accept any value for 'a' if given as a kwarg;
 # 'functools.partial()' defaults can be overridden)

p2 = functools.partial(function, a=True)
reveal_type(p2)
 # actual: Revealed type is "functools.partial[builtins.int*]"
 # expected: Revealed type is "functools.partial[Union[int, str]]"
 # (by similar logic)

Expected Behavior

If mypy cannot intelligently handle the interaction of @overload and functools.partial() (i.e., the partial object has overloaded behaviour), it should at least assume the return type of the partial is that of the @overloaded function's implementation (e.g. Union[int, str]). Assuming the return type of the first @overloaded signature is unsound.

Actual Behavior

mypy seems to assume the partial object has the return type of the first @overloaded signature, regardless of the arguments. Other signatures and the implementation signature are ignored.

Your Environment

• Mypy version used: 0.942
• Mypy command-line flags: (none)
• Mypy configuration options from mypy.ini (and other config files): (none)
• Python version used: 3.8
• Operating system and version: macOS 10.14

[AlexWaygood]

Looks like this has already been brought up in the big functools.partial tracking issue here #1484 (comment) -- maybe it might be better to leave a comment on that issue, to keep all the functools.partial discussion in one place?

[finite-state-machine]

Looks like this has already been brought up in the big functools.partial tracking issue here #1484 (comment) -- maybe it might be better to leave a comment on that issue, to keep all the functools.partial discussion in one place?

My apologies, I didn't search comments on existing issues before filing. If, in your best judgment, this issue should be closed as duplicate, I have no objection.

[AlexWaygood]

No worries! Yes, I think it's best to keep all the functools.partial discussion in one place :)