partial plugin does not handle overloads

[asottile-sentry]

Bug Report

it appears the new partial plugin added in mypy 1.11 doesn't properly handle overloads (and just picks the first one)

To Reproduce

import functools
from typing import assert_type, overload

@overload
def identity(x: int, *, p: str) -> int: ...
@overload
def identity(x: str, *, p: str) -> str: ...
def identity(x: int | str, *, p: str) -> int | str: return x


p = functools.partial(identity, p='hi')
assert_type(p(1), int)
assert_type(p('hi'), str)

Expected Behavior

(no errors)

Actual Behavior

$ mypy t.py 
t.py:13: error: Expression is of type "int", not "str"  [assert-type]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.11.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.12.2

[asottile-sentry]

an alternative spelling with TypeVars seems to not work either:

import functools
from typing import assert_type, overload, TypeVar

T = TypeVar("T", int, str)

def identity(x: T, *, p: str) -> T: return x


p = functools.partial(identity, p='hi')
assert_type(p(1), int)
assert_type(p('hi'), str)

$ ./venv/bin/mypy t.py 
t.py:10: error: Expression is of type "Any", not "int"  [assert-type]
t.py:11: error: Expression is of type "Any", not "str"  [assert-type]
Found 2 errors in 1 file (checked 1 source file)

[hauntsaninja]

Yes, the behaviour with overloads is unchanged from mypy 1.10. Shouldn't be too bad to add support for it.

mypy/mypy/plugins/functools.py

Lines 131 to 132 in d1d3c78

	# TODO: handle overloads, just fall back to whatever the non-plugin code does
	return ctx.default_return_type

The constrained type var example you have is a duplicate of #17411. Note that unconstrained type vars or bounded type vars should work correctly. In general, mypy's handling of constrained type vars is a little weird.