• 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 import Protocol


class Person(Protocol):
    first_name: str

    def greeting(self, message: str) -> str:
        ...


class FrenchPerson(Person):
    prenom: str

    def __init__(self, name: str):
        self.prenom = name

    def salutation(self, message: str) -> str:
        return f"{message} {self.prenom}"


def print_person(person: Person) -> str:
    return person.greeting(message="Je m'appelle")


def main():
    fp = FrenchPerson(name='Henri')
    response = print_person(fp)
    print(response)

• What is the actual behavior/output?

PEP 544 says you can state that an implementation supports a protocol by
subclassing the protocol.

It also says: "If one omits Protocol in the base class list, this would
be a regular (non-protocol) class that must implement Sized."

• What is the behavior/output you expect?

I would hope mypy would, either at declaration time or in the print_person function, spot that something claimed to be a Person but had neither the protocol variable name nor the method greeting.

It's also a shame that FrenchPerson got the greeting method added to it from the subclassing.

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

mypy 0.770 and Python 3.8.1

• What are the mypy flags you are using? (For example --strict-optional)

None.