New rule: LHS of nullish coalescing operator (??) should be nullable

[bondo]

This should give a linting error

const x = 42 ?? 60;

and this

async function foo(): Promise<number|null> {
  return null;
}
async function bar(): Promise<number|null> {
  return foo() ?? 42;
}

[bondo]

I'm think this could realistically become an issue when changing foo from function foo(): number|null to async function foo(): Promise<number|null>

[kastermester]

I think this should simply be a check that ?? is only applied to something that is nullable. I think that would be of greater general use. Maybe there's something out there already that does this?

[bondo]

After a little bit of searching I can't find this anywhere, either for TSLint or ESLint

[kastermester]

Alright, then we should probably make this ourselves. Can you fix the issue title, etc? :)