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

Beginne mit dem minimalen Python-Repro und der gezeigten typeshed-Signatur von Message.get unter Verwendung von mypy 0.711, Python 3.6.7 und den aufgeführten mypy-Flags. Vergleiche die beiden gemeldeten False Positives mit dem erwarteten Verhalten ohne Fehler und verfolge anschließend den Pfad der Typprüfung, der den get-Aufruf und den booleschen Ausdruck behandelt.