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

从最小的 Python 复现代码和所示的 typeshed Message.get 签名开始，使用 mypy 0.711、Python 3.6.7 以及列出的 mypy 标志。将报告的两个误报与预期的无错误行为进行比较，然后跟踪处理 get 调用和布尔表达式的类型检查路径。

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