"is not assignable to upper bound" when using Protocol

[MarcoGorelli]

Describe the Bug

The following example type checks fine with mypy/pyright, but raises

ERROR `MySeries` is not assignable to upper bound `NativeSeries` of type variable `T` [bad-specialization]
  --> t.py:15:17
   |
15 | def to_foo() -> Foo[MySeries]:
   |                 ^^^^^^^^^^^^^
   |
  `MySeries.filter` has type `BoundMethod[MySeries, (self: MySeries, _predicate: Iterable[bool]) -> MySeries]`, which is not assignable to `BoundMethod[MySeries, (self: MySeries, *args: Any, **kwargs: Any) -> Any]`, the type of `NativeSeries.filter`

for pyrefly

from typing import Generic, Sized, Iterable, Any, TypeVar, Protocol, Self

class NativeSeries(Protocol):
    def filter(self, *args: Any, **kwargs: Any) -> Any: ...

class MySeries:
    def filter(self, _predicate: Iterable[bool]) -> Self:
        return self

T = TypeVar('T', bound=NativeSeries)

class Foo(Generic[T]):
    ...

def to_foo() -> Foo[MySeries]:
    ...

spotted in narwhals

Sandbox Link

https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeS4ATrgLYAEALqcROgOZ0Q3G6UN0BxGOhiUIAYwA0dAMoQAXjEzSAkg1GpssaQEF0paQBVmMAGqpK0gArUGucbijSZMKGAA66T%2BKio4cOgA5VAYIADcYFzF4AAobXDsHKABKRE86DLpMGDA6SCh1Shi4VzBpACoLNjhEOj0DOnLygGsAdyqauv1kugBaAD4u0lrCUc9vX386AFlSKIh4NPRMrJy86ELi0ukAfWJKJQkQmFq1DS0YAG08RwBdHoHZUqWVlYOGAFdKZZK3cfRDHQALx0YzEMwWGIAckMUOkeA%2BWCBwVCEXm8GS-x8fgCADFcLgYkIRGJxJdDPcXplRoR-tlcnYdmACTEHoN8bhLrN0XBblSMjT-iBJCAPqEoHASORECAAMR0ACq4ogTDyiPEoVw6DgdLWzMoNBCO3QHxo2FEMXwtVYDDZdDgDEo-Lo7y%2Byw8IECpvNTrowHwAF93CBPMKQGQDmAoKRCHYaFAKPKrKRI9H7RgcAQ6A50JA2F8QhAtbT0PKXDA6AALBgMYg1AD09YjOWjhF4bHrwnrmHscHrObzBc16HreV4dFQYVQ0E0sGzWsHlELWrouGIw8lnjIDErWt6EUocCLyxBwYAzIQAIwAJmDoYDItQGvCMFx0BgFDQWDwRDIIADQA

(Only applicable for extension issues) IDE Information

No response

[kinto0]

looks like a function with concrete arguments is not necessarily a subtype of one with a *args, *kwargs in pyrefly. see this even simpler example

naively, I don't think Pyrefly is incorrect here, since a function that accepts a NativeSeries might end up passing arbitrary/multiple arguments. those arbitrary arguments would cause a runtime type error in this case where the implementation only has one parameter.

[randolf-scholz]

@kinto0 As per the typing spec, *args: Any, **kwargs:Any is a gradual type: https://typing.python.org/en/latest/spec/callables.html#meaning-of-in-callable

Essentially, this Protocol is equivalent to saying filter is Callable[..., Any], which is ok to override withMySeries.filter.
You would be correct if it were something like *args: Never, **kwargs: Never instead.