Type is lost when result of type guard is assigned and strictNullChecks is on #39657

// @ts-check

// NOTE: Problem only happens when strictNullChecks is on. 
// repro: https://www.typescriptlang.org/play?strictNullChecks=true&ts=3.9.3&useJavaScript=true#

/**
 *
 * @typedef Thing
 * @property {number} id
 */

/**
 * @param {Thing|null} variable
 * @return {variable is Thing}
 */
const isNotNull = variable => variable !== null;

/**
 * @param {Thing|null} thing
 */
const fn = thing => {
  thing; // as expected, type is Thing | null

  if (isNotNull(thing)) {
    thing; // as expected, type is Thing
    thing.id; // no error
  }

  const isAvailable = isNotNull(thing);
  if (isAvailable) {
    thing; // unexpected: type is Thing | null
    thing.id; // error: Object is possibly null
  }
};