FOO and next(iter(FOO))

[dimaqq]

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