FOO and next(iter(FOO))

Bug Report

Python code

FOO = frozenset("abc")
BAR = FOO and next(iter(FOO))

Mypy reports and error

wtf.py:2: error: Argument 1 to "iter" has incompatible type "FrozenSet[str]"; expected "Iterable[FrozenSet[str]]"

Expected Behavior

Ideally, mypy should infer that type of BAR is "falsey or string".
If not that, then "empty set or string".
if not that, then "set or string".

Actual Behavior

mypy seems to be tripped by X op func(X), gets confused 😖
mypy complains about the argument to iter 🤷🏿

Note that expression like BAR = next(iter(FOO)) is understood correctly.

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.9.5
• Operating system and version: macos, universal2

まず、mypy 0.910 を使って最小限の Python 例を再現し、型チェッカーが iter(FOO) 周辺のブール値 and 式をどのように処理するかを調べます。例で互換性のない引数エラーが報告されなくなり、BAR の推論された型が期待される選択肢のいずれかと一致すれば完了です。