Truthyness inference incorrect for some `Enum` types

[Daverball]

Bug Report

It looks like the enum plugin incorrectly assumes that __bool__ on enum literals will always return Literal[True], while this is the default behavior, it is not correct when someone explicitly overrides __bool__. Moreover this special casing is only applied to enum literals, but not the base enum type, which should behave the same as individual members.

To Reproduce

https://mypy-play.net/?mypy=master&python=3.12&gist=dc60c5c82ba6589e5f1d03654439b05e

import enum
from typing import Any, Literal

class Foo(enum.Enum):
    X = 0

class Bar(enum.Enum):
    X = 0
    
    def __bool__(self) -> Literal[False]:
        return False

def a(x: Any | Literal[Foo.X]):
    reveal_type(x)
    if x:
        reveal_type(x)
    else:
        reveal_type(x)

def b(x: Any | Foo):
    reveal_type(x)
    if x:
        reveal_type(x)
    else:
        reveal_type(x)  # wrong, doesn't match literal behavior

def c(x: Any | Literal[Bar.X]):
    reveal_type(x)
    if x:
        reveal_type(x)  # wrong, it's backwards
    else:
        reveal_type(x)  # wrong, it's backwards

def d(x: Any | Bar):
    reveal_type(x)
    if x:
        reveal_type(x)
    else:
        reveal_type(x)

Your Environment

• Mypy version used: Playground with master branch
• Mypy command-line flags: Default
• Mypy configuration options from mypy.ini (and other config files): Default
• Python version used: 3.12

[Daverball]

Looks like the culprit is this part:

mypy/mypy/types.py

Lines 2751 to 2755 in 6682563

	def can_be_false_default(self) -> bool:
	return not self.value
	def can_be_true_default(self) -> bool:
	return bool(self.value)

Since Enum literals have a string value and are disambiguated from actual string literals, based on whether or not the fallback type is an Enum, they will always implicitly be True, since the Enum members can't have an empty name.

This part should be relatively easy to fix, but it will mean that a will now have the same incorrect behavior as b. I assume this is because the metaclass's __bool__ method is not considered for truthyness:
https://mypy-play.net/?mypy=latest&python=3.12&flags=warn-unreachable&gist=ab7d1143bb3993a5b63d0ca827aea6cf

I'd consider this a separate issue, not sure if it has already been reported, but I imagine it shouldn't be that difficult to fix either.

I'll see if I can work on a PR for this.