Type information lost when passed as object key

TypeScript Version: 4.2.0-dev.20201222

Search Terms: type information lost

Code

js:

/** @typedef {{ [K in string] : (data: any) => void }} Event */

/**
 * @template {Event} E
 */
function createEventEmitter () {
  /** @type {Set<{ [T in keyof E]: E[T] }>} */
  const events = new Set()

  /**
   * @template {keyof E} T
   *
   * @param {T} event event register to
   * @param {E[T]} listener listener to register
   */
  function on (event, listener) {
    events.add({ [event]: listener })
  }

  return { on }
}

module.exports = { createEventEmitter }

ts:

type Event = { [K in string] : (data: any) => void }

export function createEventEmitter<E extends Event> () {
  const events: Set<{ [T in keyof E]: E[T] }> = new Set()

  function on<T extends keyof E> (event: T, listener: E[T]) {
    events.add({ [event]: listener })
  }

  return { on }
}

Expected behavior:
No errors

Actual behavior:
In function on variable event has type T extends keyof E, but add operation shows an error

Related Issues:
https://github.com/microsoft/TypeScript/issues/42060

Start by compiling the TypeScript and JavaScript examples in the issue and inspect the error on events.add({ [event]: listener }). Trace the checker handling of generic computed object keys and mapped types. Done means both examples compile without errors and a regression test covers this case.