Nested class in Protocol causes mypy exception

[microblag]

• 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.

from typing_extensions import Protocol

class SomeProto(Protocol):
    var: int

    class Meta:
        abstract = True
        
class SomeImpl:
    def __init__(self) -> None:
        var = 5

def a_func(a: SomeProto) -> None:
    pass

a_func(SomeImpl())

• What are the versions of mypy and Python you are using?
  0.670

• Do you see the same issue after installing mypy from Git master?
  Yes

• What are the mypy flags you are using? (For example --strict-optional)
  Doesn't seem to matter what flags are used, Was using defaults on https://mypy-play.net/ to produce the sample code pasted above

• If mypy crashed with a traceback, please paste
  the full traceback below.

Traceback (most recent call last):
  File "<myvenv>/bin/mypy", line 10, in <module>
    sys.exit(console_entry())
  File "mypy/checkexpr.py", line 3188, in accept
  File "mypy/nodes.py", line 1446, in accept
  File "mypy/checkexpr.py", line 235, in visit_call_expr__ExpressionVisitor_glue
  File "mypy/checkexpr.py", line 244, in visit_call_expr
  File "mypy/checkexpr.py", line 314, in visit_call_expr_inner
  File "mypy/checkexpr.py", line 655, in check_call_expr_with_callee_type
  File "mypy/checkexpr.py", line 693, in check_call
  File "mypy/checkexpr.py", line 776, in check_callable_call
  File "mypy/checkexpr.py", line 989, in infer_function_type_arguments
  File "mypy/infer.py", line 30, in infer_function_type_arguments
  File "mypy/constraints.py", line 66, in infer_constraints_for_callable
  File "mypy/constraints.py", line 143, in infer_constraints
  File "mypy/types.py", line 620, in accept
  File "mypy/constraints.py", line 279, in visit_instance__TypeVisitor_glue
  File "mypy/constraints.py", line 328, in visit_instance
  File "mypy/constraints.py", line 143, in infer_constraints
  File "mypy/types.py", line 620, in accept
  File "mypy/constraints.py", line 279, in visit_instance__TypeVisitor_glue
  File "mypy/constraints.py", line 344, in visit_instance
  File "mypy/subtypes.py", line 472, in is_protocol_implementation
AssertionError:

[ilevkivskyi]

Hm, actually nested classes in protocols are not supported (also one will be able to mimic it with a protocol field when #4536 will be fixed). Anyway, we definitely should not crash.

[microblag]

Thanks, the nested class was in there as a vestige of some refactoring. It took a little while to figure out what was going on because the error doesn't present when running the Protocol subclass through mypy only when you process something with a call to a function using it. So I thought it was worth fixing as other than the mention of "is_protocol_implementation" in the traceback it's not at all clear where to look for the problem.

[ilevkivskyi]

An even simpler repro of this problem is in #8397

[hauntsaninja]

Just checked, this still reproes, along with the reproducer in #8397

from typing import TypeVar, Protocol

T = TypeVar("T", covariant=True)


class MyProtocol(Protocol[T]):
    class Nested:
        pass