A question about relation between Kotlin's Unit and other types

UnitReturn.kt

fun foo(): Unit = run<Unit> {
  1 + 1
}

fun add(x: Int, y: Int): Int = run {
  x + y
}

fun main(args: Array<String>) {
  /*
    val x: Unit = add(1, 2)
    error: type mismatch: inferred type is kotlin.Int but kotlin.Unit was expected
    val x: Unit = add(1, 2)
              ^
   */
  /*
    It seems that implicit conversion from T to Unit is not defined.
    Then, why does foo() function compiles ?
   */
}