Introduce plusElement and minusElement to disambiguate situations like List<List<T>> + List<T>

#KT-9992 Fixed

libraries/stdlib/test/collections/ArraysTest.kt

@@ -430,6 +430,20 @@ class ArraysTest {
         assertTrue(intArrayOf(1).isNotEmpty())
     }
 
+    @test fun plusInference() {
+        val arrayOfArrays: Array<Array<out Any>> = arrayOf(arrayOf<Any>("s") as Array<out Any>)
+        val elementArray = arrayOf<Any>("a") as Array<out Any>
+        val arrayPlusElement: Array<Array<out Any>> = arrayOfArrays.plusElement(elementArray)
+        assertEquals("a", arrayPlusElement[1][0])
+        // ambiguity
+        // val arrayPlusArray: Array<Array<out Any>> = arrayOfArrays + arrayOfArrays
+
+        val arrayOfStringArrays = arrayOf(arrayOf("s"))
+        val arrayPlusArray = arrayOfStringArrays + arrayOfStringArrays
+        assertEquals("s", arrayPlusArray[1][0])
+    }
+
+
     @test fun plus() {
         assertEquals(listOf("1", "2", "3", "4"), listOf("1", "2") + arrayOf("3", "4"))
         assertArrayNotSameButEquals(arrayOf("1", "2", "3"), arrayOf("1", "2") + "3")

libraries/stdlib/test/collections/CollectionTest.kt

@@ -247,6 +247,18 @@ class CollectionTest {
         assertEquals(listOf("foo", "bar", "cheese", "wine"), list)
     }
 
+    @test fun plusCollectionInference() {
+        val listOfLists = listOf(listOf("s"))
+        val elementList = listOf("a")
+        val result: List<List<String>> = listOfLists.plusElement(elementList)
+        assertEquals(listOf(listOf("s"), listOf("a")), result, "should be list + element")
+
+        val listOfAny = listOf<Any>("a") + listOf<Any>("b")
+        assertEquals(listOf("a", "b"), listOfAny,  "should be list + list")
+
+        val listOfAnyAndList = listOf<Any>("a") + listOf<Any>("b") as Any
+        assertEquals(listOf("a", listOf("b")), listOfAnyAndList, "should be list + Any")
+    }
 
     @test fun plusAssign() {
         // lets use a mutable variable of readonly list