__bool__ check should look at Never return

[ndmitchell]

Describe the Bug

from typing import Never

class Foo:
   def __bool__(self) -> Never:
     raise TypeError

if Foo():
    print("Hello World")

This works fine, but if I change it to __bool__ : int then it gives an error that Foo cannot be used as a boolean. It should raise an error even above. The above formulation causes the expected error in Pyright, and recommended in https://discuss.python.org/t/how-to-get-a-type-check-error-on-bool/74879/7

The code in question is at

pyrefly/pyrefly/lib/alt/attr.rs

Lines 1695 to 1709 in 54a4038

	if let Some(ty) = cond_bool_ty
	&& !matches!(ty, Type::Union(_) | Type::Never(_))
	&& self.as_call_target(ty.clone()).is_none()
	{
	self.error(
	errors,
	range,
	ErrorInfo::Kind(ErrorKind::InvalidArgument),
	format!(
	"`{}.__bool__` has type `{}`, which is not callable",
	self.for_display(condition_type.clone()),
	self.for_display(ty.clone()),
	),
	);
	}

. We check if the type itself is union or never, then get the call target. We should probably be looking at the return type of the call target.

Sandbox Link

https://pyrefly.org/sandbox/?code=GYJw9gtgBALgngBwJYDsDmUkQWEMoByApgG5EgBQFAxgDYCGAzo1AGJhgBcFUvAJkWBQA%2BsIBGHWqIAUjIrWABKKAFoAfIVLluvXSHpI5UACqIiAURDhKFJEPZhpinboQhUMaQCIAEvNpgUADquLR8XopAA&version=3.12

(Only applicable for extension issues) IDE Information

No response

[yangdanny97]

To clarify the Never case specifically:

• if __bool__ is Never, we do not give an error (this is as-designed)
• if __bool__ is a callable that returns Never, this issue says we should give an error (we currently do not)

I'm not sure if the linked discussion shows any consensus on this. We would be aligning with Pyright over Mypy by doing that, tho that may not be a bad thing.