functools.singledispatch registration prioritizes the types of normal arguments before positional-only ones

Bug report

Bug description:

Decorator-based functools.singledispatch registration prioritizes the types of normal arguments before positional-only ones when getting "the type of the first argument".

This means that a function of def f(pos: str, /, other: int) will be registered to be dispatched for other's type (int) instead of pos (str).

To reproduce:

import functools

@functools.singledispatch
def f(arg, /, extra):
    return f"{arg}: undispatched type"

@f.register
def f_int(arg: int, /, extra: str):
    return f"{arg} is a {type(arg)} (dispatched by int)"

@f.register
def f_str(arg: str, /, extra: int):
    return f"{arg} is a {type(arg)} (dispatched by str)"

print(f(None, "extra"))
print(f(1, "extra"))
print(f("s", "extra"))

Result:

None: undispatched type
1 is a <class 'int'> (dispatched by str)
s is a <class 'str'> (dispatched by int)

Expected:

None: undispatched type
1 is a <class 'int'> (dispatched by int)
s is a <class 'str'> (dispatched by str)

CPython versions tested on:

3.9, 3.10, 3.11, 3.12, 3.13, 3.14, CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-143888
• gh-143465

Parte de la ruta de registro de functools.singledispatch y reproduce el ejemplo con argumentos solo posicionales y argumentos normales. La tarea está terminada cuando el registro selecciona el tipo del primer argumento solo posicional, los resultados de dispatch mostrados coinciden con la salida esperada, se ha añadido cobertura de regresión y las pruebas pertinentes pasan.