Type objects that conform to Protocol definitions are not recognized.

[mostrows2]

A class with a classmethod that matches a Protocol definition is not recognized as such.

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request?

Bug

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

See end of report.

• What is the actual behavior/output?

% mypy /tmp/test2.py                                                                                                                                         
/tmp/test2.py:22: error: Incompatible types in assignment (expression has type "Type[Class2]", variable has type "Proto")
Found 1 error in 1 file (checked 1 source file)

• What is the behavior/output you expect?

No errors

• What are the versions of mypy and Python you are using?

0.770, 3..8.4

from typing import Protocol


class Proto(Protocol):
    def func(self) -> int:
        ...

class Class1:
    def func(self) -> int:
        return 0

class Class2:
    @classmethod
    def func(cls) -> int:
        return 1


a: Proto = Class1()
b: Proto = Class2()
c: Proto = Class2

aa: int = a.func()
bb: int = b.func()
cc: int = c.func()

[dhilst]

I was experimenting with this, it seems that @classmethod make Class2.func a kind of bound method. A callable that receives no argument so that first argument type is lose in the process.

classmethod.py:18: note: Revealed type is 'def () -> classmethod.Class1'
classmethod.py:19: note: Revealed type is 'def (self: classmethod.Class1) -> builtins.int'
classmethod.py:20: note: Revealed type is 'def () -> classmethod.Class2'
classmethod.py:21: note: Revealed type is 'def () -> builtins.int'
classmethod.py:24: error: Incompatible types in assignment (expression has type "Type[Class2]", variable has type "Proto")
Found 1 error in 1 file (checked 1 source file)

I put these lines on the sample

reveal_type(Class1)
reveal_type(Class1.func)
reveal_type(Class2)
reveal_type(Class2.func)
a: Proto = Class1()
b: Proto = Class2()
c: Proto = Class2

[JukkaL]

Closing as duplicate of #4536.