functools.partial always assumes the first signature of an @overloaded function #12675

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)