Union type narrowing behaves differently depending on argument position

Bug Report

In the following example the type narrowing of the union types works as expected if it appears in the first or second argument position, but breaks if it appears in position three (or later).

This issue probably falls into the topic-join-v-union.

To Reproduce

Example on mypy playground

from typing import Callable, TypeVar

T = TypeVar("T")

def func(
    a: Callable[[], T],
    b: Callable[[], T],
    c: Callable[[], T],
) -> T:
    raise NotImplementedError()
        

class Foo:
    ...
    
class Bar:
    ...

def make_foo() -> Foo:
    raise NotImplementedError()
    
def make_bar() -> Bar:
    raise NotImplementedError()
    
def make_foo_or_bar() -> Foo | Bar:
    raise NotImplementedError()

x1: Foo | Bar = func(a=make_foo_or_bar, b=make_foo, c=make_bar)
x2: Foo | Bar = func(a=make_foo, b=make_foo_or_bar, c=make_bar)
x3: Foo | Bar = func(a=make_foo, b=make_bar, c=make_foo_or_bar)

Expected Behavior

The example should type check properly.

Actual Behavior

Inferring the return type works properly for x1 and x2, but it fails for x3 with:

Incompatible types in assignment (expression has type "object", variable has type "Foo | Bar")  [assignment]

Considering that the code is fully valid and symmetric, there should not be a difference in the behavior.

Side note: The issue cannot be work-around at call site by just swapping the order of a and c in the func(...) call. Apparently it has to do with the order in which they appear in the function signature.

Your Environment

• Mypy version used: 1.6.1
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10, 3.11

Issue の再現コードから始め、mypy 1.6.1 または現行バージョンに対して実行して、位置による違いを確認します。func の callable 引数の型推論を追跡し、3 番目のパラメーターが object になる理由を調査します。x1、x2、x3 のすべてで一貫して型チェックが通れば完了です。