New rule to only allow boolean statements in logical expressions

[oefirouz]

Disallow if (expr) unless expr is a boolean.

So no if (obj)... rather if (obj != null)

[ashwinr]

the "unless expr is a boolean" might be troublesome since we don't use the typing annotations from the compiler yet. we'll need to investigate that.

[adidahiya]

I think the new TS APIs have support for type information about nodes. This should be doable.

[adidahiya]

blocked on #680

[jkillian]

See microsoft/TypeScript#9702 as well, although as far as I can tell they haven't decided to implement this yet. This shouldn't be too hard to do now on our end!

[Sammons]

I would really like this to help with not coercing promise-like types to booleans, but I think it would be really nice if we could explicitly enable certain types (strings, numbers) for coercion.

[radykov]

I think the new TS APIs have support for type information about nodes. This should be doable.

Would it be able to pick up things such as x = y || z;?
I'm unfamiliar with TS APIs and what was meant by nodes

[adidahiya]

@radykov

Would it be able to pick up things such as x = y || z;?

yes, that check should probably be incorporated into this lint rule.

[OliverJAsh]

This would be really handy and catch so many bugs.

[adidahiya]

fixed via #1820