[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

Inizia riproducendo con stubtest le combinazioni mostrate di runtime e stub di foo, quindi segui il controllo di coerenza tra overload e argomenti predefiniti. Il lavoro è completato quando il caso del terzo overload non segnala più un falso positivo, mentre i valori predefiniti del runtime realmente incompatibili continuano a essere rilevati.