Fix an issubclass failure for protocols with overloaded methods

[BvB93]

Description

Mypy currently rejects issubclass checks for protocols containing non-method members.
What I noticed is that this includes overloaded functions as well, the latter being incorrectly
identified as a "non-method" due to a missing isinstance(typ, mypy.types.Overloaded) check.

This PR attempts to remedy aforementioned issue.

Examples

The issubclass behavior prior to this PR:

from typing import Protocol, overload, Sequence, runtime_checkable

@runtime_checkable
class A(Protocol):
    @overload
    def get_item(self, a: int) -> int: ...
    @overload
    def get_item(self, a: slice) -> Sequence[int]: ...

# test.py:10:1: error: Only protocols that don't have non-method members can be used with issubclass()  [misc]
# test.py:10:1: note: Protocol "A" has non-method member(s): get_item
# Found 1 error in 1 file (checked 1 source file)
issubclass(int, A)

Test Plan

A new issubclass test has been added for overload-containing protocols.

[JukkaL]

Thanks for the fix!

Left an (optional) request to update the test case, otherwise looks good.

[JukkaL]

Could you also add a concrete class (e.g., COverload) that has a suitable overloaded method, and test that issubclass can be used to narrow down from Type[Union[COverload, E]], similar to above around line 2218?

[BvB93]

So while trying the add the test you proposed I seem to have stumbled across another bug.

The if block successfully narrows down the Union to Type[COverload] but the else block
on the other hand fails to do the same with E, revealing the type as the original Union.

Would you happen to know where in mypy the type inference of if / else statements is handled?
I imagine that change similar to non_method_protocol_members() will have to be made there,
but I'm not quite sure where to look.

Examples

class COverload:
    x: str
    @overload
    def meth(self, a: int) -> float: ...
    @overload
    def meth(self, a: str) -> Sequence[float]: ...
    def meth(self, a):
        pass

cls_overload: Type[Union[COverload, E]]
if issubclass(cls_overload, POverload):
    reveal_type(cls_overload)  # N: Revealed type is 'Type[__main__.COverload]'
else:
    reveal_type(cls_overload)  # N: Revealed type is 'Type[__main__.E]'

The test output:

Expected:
  ...
  main:45: note: Revealed type is 'Type[__main__.COverload]'
  main:47: note: Revealed type is 'Type[__main__.E]' (diff)
Actual:
  ...
  main:45: note: Revealed type is 'Type[__main__.COverload]'
  main:47: note: Revealed type is 'Union[Type[__main__.COverload], Type[__main__.E]]' (diff)

[97littleleaf11]

Could you please open a new issue about it?

[BvB93]

Could you please open a new issue about it?

I just created an issue at #12228.

[97littleleaf11]

Passes all the tests now. Shall we merge this one? @hauntsaninja

[JelleZijlstra]

Jukka already approved it, so let's land it.