[prefer-nullish-coalescing] Doesn't work if typescript is not in project mode

[entropitor]

Repro

{
  "rules": {
    "@typescript-eslint/prefer-nullish-coalescing": ["error"]
  }
}

Expected Result
It validates using the rule

Actual Result
It errors because the tsconfig doesn't use project mode

Versions

package	version
@typescript-eslint/eslint-plugin	2.9.0
@typescript-eslint/parser	2.9.0
TypeScript	3.7.2
ESLint	6.6.0
node	13.1.0

[bradzacher]

What do you mean? Could you provide more information?

Are you saying you don't think the rule should need parserOptions.project?

[bradzacher]

Looking at this again - I'm assuming you want a rule that doesn't require type information.
For nullish coalescing, this is impossible. We need to know the type, or else we cannot tell in a || b whether a is boolean (i.e. a correct logical expression), or something | null (i.e. safe for nullish-coalescing).

If my assumption was incorrect, please comment and we can reopen.

[entropitor]

@bradzacher I understand it needs type information to do this check. What I don't get is that it seems to suggest we need typescript in project mode for this? I might be misinterpreting the error message though?

I've tried adopting this for my monorepo but I wasn't able to make it work so currently I'm still in a mode similar to typescript <3.x. This lint rule seems to fail on that

[bradzacher]

The error message is unrelated to any tsc settings.

You need to provide your tsconfig to us via parserOptions.project.
See the readme for the parser for more info

[entropitor]

Ah okay, my bad!