[no-unused-vars] global never used

[Tansito]

Repro

Updating @typescript-eslint/eslint-plugin and @typescript-eslint/parser from 3.10.0 to 4.1.0 the linter is showing the next message:

warning  'global' is defined but never used  @typescript-eslint/no-unused-vars

Our code is the next:

import { Tab } from './Tab';

customElements.define('custom-tab', Tab);

declare global {
  interface HTMLElementTagNameMap {
    'custom-tab': Tab;
  }
}

And we solved the problem thanks to the next rule:

"rules": {
  "@typescript-eslint/no-unused-vars": [
    "error",
    {
      "varsIgnorePattern": "^global$"
    }
  ]
}

Our current rules for the project: https://github.com/IBMResearch/eslint-config-ibmresearch

Expected Result

How in this case the global reserved word references to the global scope, this message should not be shown as a warning.

Actual Result

The plugin is showing global as an unused variable.

Versions

package	version
@typescript-eslint/eslint-plugin	4.1.0
@typescript-eslint/parser	4.1.0
TypeScript	4.0.2
ESLint	7.9.0
node	14.10.1

[bradzacher]

#2523 et al.

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