Support union types: https://youtrack.jetbrains.com/issue/KT-13108

[chachako]

Temporary solving https://youtrack.jetbrains.com/issue/KT-13108#focus=Comments-27-5474923.0-0

fun save(data: Union4<String, Int, Long, List<String>>): UnionType<Boolean, String> {
  var result = Result(false)

  when {
    data is String -> { ... }
    data is Int -> { ... }
    data is Long -> { ... }
    data is List<String> -> { ... }
    data is Boolean -> { ... } // ERROR: Never this type
  }

- return result // ERROR: Can only return 'Boolean' or 'String'
+ return result.value // OK: 'Boolean'
+ return result.value.toString() // OK: 'String'
}

fun main() {
  save("Username") // OK: 'String'
  save(50) // OK: 'Int'
  save(1000000L) // OK: 'Long'
  save(listOf("A", "B")) // OK: 'List<String>'
  save(listOf("A", "B", 50)) // ERROR: Only 'List<String>' type are allowed

  val any: Any = getAnyData()
  save(any) // ERROR: 'Any' type not allowed
}

Diagnosis:

TypeChecker
CallChecker
`is` & `as` Checker