inferred type of "(cond and a) or b"

[rwbarton]

If cond: Bool, a: A, and b: B, then mypy thinks (cond and a) or b has type Union[bool, A, B]. But in fact the type should just be Union[A, B]. The only way (cond and a) or b can be a bool is for it to be (cond and a), and for that to be truth-y; but in that case (cond and a) would have type A, and never type bool (from cond).

[rwbarton]

See also discussion at #1698.

With False and True types we could type check this as follows: with

• cond: bool, a: A, b: B, we first have
• cond and a: Union[falsey_type(bool), A] = Union[False, A], and then
• (cond and a) or b: Union[truthy_type(Union[False, A]), B] = Union[A, B].

[dmoisset]

Just checked that my PR #1989 happens to fix this too

[ddfisher]

Fixed by #1989.