Before You File a Bug Report Please Confirm You Have Done The Following...

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have searched for related issues and found none that matched my issue.
• I have read the FAQ and my problem is not listed.

Playground Link

(BRANCH PLAYGROUND - NOT ON MAIN YET)

Repro Code

function join<T extends string | number>(els: T[]) {
  return els.map(el => '' + el).join(',');
}

ESLint Config

module.exports = {
  "rules": {
    "@typescript-eslint/no-unnecessary-type-parameters": "warn"
  }
}

tsconfig

{
  "compilerOptions": {
    "strictNullChecks": true
  }
}

Expected Result

function join(els: (string | number)[]) {
  return els.map(el => '' + el).join(',');
}

Actual Result

function join(els: string | number[]) {
  return els.map(el => '' + el).join(',');
}

Additional Info

This is a followup to #9536 (comment). note that, while this specific report case isn't merged yet,

• I'm pretty sure it will be merged imminently
• I'm pretty sure we could figure out a repro where the parenthesization is wrong on a case the rule already reports on, but I haven't bothered thinking too hard about it