[stubtest] False positive involving overloaded function with default argument for parameter with type variable

Bug Report

Given the runtime function:

def foo(x=0, ret=1):
    return ret

stubtest is happy with these stubs:

from typing import TypeVar, overload

_T = TypeVar("_T")

@overload
def foo(x: int = 0) -> int: ...
@overload
def foo(x: int, ret: _T) -> _T: ...

but complains about these stubs (with a third overload added):

from typing import TypeVar, overload

_T = TypeVar("_T")

@overload
def foo(x: int = 0) -> int: ...
@overload
def foo(x: int, ret: _T) -> _T: ...
@overload
def foo(x: int = 0, *, ret: _T) -> _T: ...  # added

with the error:

error: main.foo is inconsistent, runtime parameter "ret" has a default value of type Literal[1], which is incompatible with stub parameter type _T | _T. This is often caused by overloads failing to account for explicitly passing in the default value.
Stub: in file main.pyi:5
Overload(def (x: int =) -> int, def [_T] (x: int, ret: _T) -> _T, def [_T] (x: int =, *, ret: _T) -> _T)
Inferred signature: def (x: int = ..., ret: _T | _T = ...)
Runtime: in file main.py:1
def (x=0, ret=1)

Found 1 error (checked 1 module)

Your Environment

• Mypy version used: master, mypy 2.2.0+dev.e15a6d58d
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.13.13

Beginne damit, die gezeigten Kombinationen aus foo-Laufzeit und Stubs mit stubtest zu reproduzieren, und verfolge dann die Konsistenzprüfung für Overloads und Standardargumente. Als erledigt gilt die Aufgabe, wenn der Fall mit dem dritten Overload keinen False Positive mehr meldet, während tatsächlich inkompatible Laufzeit-Standardwerte weiterhin erkannt werden.