Not resolving nested protocol when combined through multiple inheritance

[willtalmadge]

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

This may be a bug if it is not intended behavior.

• 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.

I've constructed the following minimal example that demonstrates the issue. I've placed a comment before the line where mypy is having a problem.

from dataclasses import dataclass
from typing import List

from typing_extensions import Protocol


class Name(Protocol):
    first: str
    last: str


class HasWand(Protocol):
    has_wand: bool


class NameProp(Protocol):
    name: Name


class Wizard(NameProp, HasWand, Protocol): ...


def wand_report(arg: Wizard) -> None:
    print(arg.name.first)
    print(arg.name.last)
    if arg.has_wand:
        print('This wizard has a wand.')
    else:
        print('This wizard needs to visit Ollivanders.')


@dataclass
class NameExtended:
    first: str
    last: str
    middle_names: List[str]


@dataclass
class DarkWizard:
    name: NameExtended
    at_large: bool
    power_level: int
    has_wand: bool = True


carl = DarkWizard(
    name=NameExtended(
        first='Carl',
        last='Fizzlbuz',
        middle_names=['Modfife']
    ),
    power_level=2,
    at_large=True,
)

# Pycharm is fine with the following line, as I'd expect, since 'NameExtended'
# structurally implements 'Name'. mypy complains about this line.
wand_report(carl)

• What is the actual behavior/output?

nest_protocols.py:59: error: Argument 1 to "wand_report" has incompatible type "DarkWizard"; expected "Wizard"
nest_protocols.py:59: note: Following member(s) of "DarkWizard" have conflicts:
nest_protocols.py:59: note:     name: expected "Name", got "NameExtended"

• What is the behavior/output you expect?
  I would expect no error to be raised for the file above with the --strict flag.
• What are the versions of mypy and Python you are using?
  mypy 0.641
  Python 3.7.1
• 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)
  --strict

[ilevkivskyi]

Your code is actually unsafe, see for example https://mypy.readthedocs.io/en/latest/common_issues.html#covariant-subtyping-of-mutable-protocol-members-is-rejected. To make it safe you need to make name read-only, for example a @property in the NameProp protocol.

[ilevkivskyi]

(no need to make it read-only in the implementations)

[willtalmadge]

Thanks for helping me understand this. This type of issue makes me glad mypy exists. This is a bug that would be hard to spot in a larger code base without the help of the type checker.