Example:

fun someCalc(x: Int) = x * 42

fun foo(x: Int): Int {
  val a = someCalc(x)
  return when (a) {
    1 -> a
    2 -> 2 * a
    else -> 24
  }
}

After the quick-fix is applied:

fun foo(x: Int): Int {
  return when (val a = someCalc(x)) {
    1 -> a
    2 -> 2 * a
    else -> 24
  }
}