[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

Empieza reproduciendo con stubtest las combinaciones mostradas de runtime y stubs de foo, y después sigue la comprobación de coherencia entre overloads y argumentos predeterminados. Se considera completado cuando el caso del tercer overload ya no informa de un falso positivo, mientras que los valores predeterminados del runtime que realmente sean incompatibles siguen detectándose.