functools.partialmethod loses return type and callable signature information #16499

For code of this shape:

from functools import partialmethod

def pprint(arg: object, /, *, options: dict) -> str:
    return "dummy"

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

Bar.__repr__ = partialmethod(pprint, options={})

it is difficult for type checkers to verify that partialmethod is compatible with the target method since it is currently typed with only a return-type parameter:

class partialmethod(Generic[_T]):
      ...
      def __get__(self, obj: Any, cls: type[Any] | None = None) -> Callable[..., _T]: ...

The current stub does not preserve the wrapped callable’s parameters or receiver type, so type checkers cannot model these class and instance access forms even though this ebhaviour is valid at runtime, and partialmethod is a descriptor:

Bar.__repr__      # T: (self: Bar) -> str
Bar().__repr__    # T: () -> str

This forces Pyrefly and Pyright to reject this valid code. Mypy on the other hand, looks at stub’s descriptor fallback (Any) and infers partialmethod[Any] even when the wrapped callable returns str.

We should preserve descriptor binding and as much callable information as possible, while accepting that a fully precise residual signature needs typing-spec support.

Born from Pyrefly issue, facebook/pyrefly#5138