[no-unused-vars] Don’t mark declare class expressions as unused

[j-f1]

Repro

{
  "rules": {
    "@typescript-eslint/no-unused-vars": ["error", { "args": "none" }]
  }
}

declare class ResizeObserver {
  public constructor(cb: (entries: ReadonlyArray<IResizeObserverEntry>) => void)

  public disconnect(): void
  public observe(e: HTMLElement): void
}

Expected Result

No error

Actual Result

.../desktop/app/src/lib/globals.d.ts
  220:15  error  'ResizeObserver' is defined but never used  @typescript-eslint/no-unused-vars

Versions

package	version
@typescript-eslint/eslint-plugin	1.0.0
@typescript-eslint/parser	1.0.0
ESLint	5.8.0
node	8.12.0
npm	???

[armano2]

we have a lot of issues with declarations, i solved some of them in #88

[j-f1]

Trying to fix this. The code in #88 is marking Foo as used, but the rule still reports it as unused.

[armano2]

i think we should move this to parser as @mysticatea proposed in some old PR in eslint/typescript-eslint-parser#558

with that we could control it better, we can store information that we in "augmentation" like we do with types and override __define to mark is as used when is set

otherwise we should stop overriding eslint rule and write a little more complex logic by ourself (they have something similar for exports)

we have also false positives from #54

[armano2]

i just realized that #88 is leading to some false positives

var baz = 2
declare namespace Foo {
  var foo: bar.baz
}

baz is going to be marked as used

[j-f1]

Not sure how moving the check to the parser would help as the rule is still ignoring markVariableAsUsed.

[armano2]

i was thinking a little to far ahead, and didn't explain it (i tend to do so)

• ResizeObserver is defined in 2 scopes class and module
• markVariableAsUsed is setting eslintUsed for variable in one scope, in this case for class scope
• scope of variable is determined by https://github.com/eslint/eslint/blob/master/lib/linter.js#L522
  const currentScope = getScope(scopeManager, currentNode);
  and we already entered class scope because Identifier is a child of ClassDeclaration
• eslint checks variable in scope module and its not marked as used
  and there is an error