False positive Argument 2 to "get" of "Message" has incompatible type "str"; expected "bool"

Note: I wasn't sure what a proper title was for this issue, so I chose one that is specific to the issue at hand. I checked typeshed, it looks like the issue is about mypy, not typeshed. I wouldn't be surprised if an issue already existed for this false positive.

Minimal repro:

assert isinstance(email, EmailMessage)

# Fine
r1 = email.get("List-Id", "")
# Fine
r2 = email.get("List-Id", "") or email.get("List-Unsubscribe", "")
# Fine
r3 = bool(email.get("List-Id", "") or email.get("List-Unsubscribe", ""))
# FAILS: Argument 2 to "get" of "Message" has incompatible type "str"; expected "bool"
r4 = bool(
    email.get("Auto-Submitted", "") in ("no", "")
    or email.get("List-Unsubscribe", "")
)
# FAILS: Argument 2 to "get" of "Message" has incompatible type "str"; expected "bool"
r5 = email.get("Auto-Submitted", "") in ("no", "") or email.get(
    "List-Unsubscribe", ""
)

Both are false positives.

Typeshed:

_T = TypeVar('_T')

class Message:
    ...
    def get(self, name: str, failobj: _T = ...) -> Union[_HeaderType, _T]: ...

• What is the actual behavior/output? Argument 2 to "get" of "Message" has incompatible type "str"; expected "bool"

• What is the behavior/output you expect? No error.

• What are the versions of mypy and Python you are using? mypy 0.711, Python 3.6.7
  Do you see the same issue after installing mypy from Git master? haven't tried it but since 0.711 is fairly recent...

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

[mypy]
ignore_missing_imports = True
follow_imports = skip
check_untyped_defs = False
strict_equality = True
disallow_incomplete_defs = True

Empieza con la reproducción mínima en Python y la firma mostrada de Message.get en typeshed, usando mypy 0.711, Python 3.6.7 y los flags de mypy indicados. Compara los dos falsos positivos reportados con el comportamiento esperado sin errores y, a continuación, sigue la ruta de comprobación de tipos que gestiona la llamada a get y la expresión booleana.