(🎁) final class without a __bool__ or __len__ method can never test false

[KotlinIsland]

This clashes with truthy-bool error code, which assumes that subtypes won't implement these methods.

from typing import final

@final
class A:
    b: int

if A():
    print("hi")  # no unreachable error

print(A() and "hi")

Any instance of A will never have a __bool__ or __len__, method so these condition will always test true.

More real life scenario:

from typing import final

@final
class A:
    b: int
    
a: A | None
r1: int | None = a.b if a else None
r2: int | None = a and a.b  # error: Incompatible types in assignment (expression has type "Union[A, None, int]", variable has type "Optional[int]")

[sobolevn]

Related:

• Check expr that are implicitly true for lack dunder bool/len #10666
• Fail when conditioning on implicit bool #10557

[KotlinIsland]

• Resolved in @final class without __bool__ cannot have falsey instances #16566