bool(obj) does not infer from obj.__bool__()

Bug Report

(Although there are other issues that allude to this problem, I haven't been able to locate an issue which specifically addresses truthiness where __bool__() is defined and @final. The search term "truthy bool" may be helpful for finding related issues.)

Implicit truthiness does not (always?) take an object's __bool__() into account, even when that method is @final.

To Reproduce

This example is posted to mypy-play.net.

from typing import *

@final
class SomeClass:
    def __bool__(self) -> Literal[True]:
        return True

instance: SomeClass

reveal_type(instance.__bool__())    # 'Literal[True]' - correct
reveal_type(bool(instance))         # 'bool' - incorrect (should be 'Literal[True]')

if instance:
    reveal_type(instance)           # 'SomeClass' - correct
else:
    assert_never(instance)          # ...'SomeClass' - incorrect (should be silent)

(Whether the class or method is decorated @final makes no difference.)

Expected Behavior

if instance: ... and bool(instnace) should take into account that SomeClass has a @final __bool__() with return type Literal[True], and consistently treat instances of SomeClass as definitely truthy.

In general, implicit truthiness for an object should take into account the truthiness of the return type of any @final __bool__() the object's class defines.

Comments inline in the above code highlight differences from expected behaviour.

Actual Behavior

main.py:10: note: Revealed type is "Literal[True]"
main.py:11: note: Revealed type is "builtins.bool"
main.py:14: note: Revealed type is "__main__.SomeClass"
main.py:16: error: Argument 1 to "assert_never" has incompatible type "SomeClass"; expected "NoReturn"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.4.0, master (2023-06-26)
• Mypy command-line flags: (none)
• Mypy configuration options from mypy.ini (and other config files): (none)
• Python version used: 3.11

issue の main.py 再現コードから始め、instance.bool()、bool(instance)、および if 文での truthiness narrowing の処理を比較します。関連する型推論と narrowing のエントリーポイントを追跡し、その後 Literal[True] を返す final bool() のカバレッジを追加します。bool(instance) が Literal[True] を明らかにし、到達不能な else 分岐で assert_never エラーが発生しなくなれば完了です。