Fix KT-50777

The problem is that the last valid start-index of a substring is the
length of the string, not the last valid index of the string. This is
only a problem, when searching for the empty string. The fix is to start
any search for a last occurence of a substring from index=length instead
of index=lastIndex.

libraries/stdlib/src/kotlin/text/Strings.kt

diff --git a/libraries/stdlib/src/kotlin/text/Strings.kt b/libraries/stdlib/src/kotlin/text/Strings.kt
index a7e49c0..cf283ce 100644
--- a/libraries/stdlib/src/kotlin/text/Strings.kt
+++ b/libraries/stdlib/src/kotlin/text/Strings.kt
@@ -996,7 +996,7 @@
         return if (index < 0) null else index to string
     }
 
-    val indices = if (!last) startIndex.coerceAtLeast(0)..length else startIndex.coerceAtMost(lastIndex) downTo 0
+    val indices = if (!last) startIndex.coerceAtLeast(0)..length else startIndex.coerceAtMost(length) downTo 0
 
     if (this is String) {
         for (index in indices) {
@@ -1042,7 +1042,7 @@
  * the end toward the beginning of this string, and finds at each position the first element in [strings]
  * that matches this string at that position.
  */
-public fun CharSequence.findLastAnyOf(strings: Collection<String>, startIndex: Int = lastIndex, ignoreCase: Boolean = false): Pair<Int, String>? =
+public fun CharSequence.findLastAnyOf(strings: Collection<String>, startIndex: Int = length, ignoreCase: Boolean = false): Pair<Int, String>? =
     findAnyOf(strings, startIndex, ignoreCase, last = true)
 
 /**
@@ -1071,7 +1071,7 @@
  * the end toward the beginning of this string, and finds at each position the first element in [strings]
  * that matches this string at that position.
  */
-public fun CharSequence.lastIndexOfAny(strings: Collection<String>, startIndex: Int = lastIndex, ignoreCase: Boolean = false): Int =
+public fun CharSequence.lastIndexOfAny(strings: Collection<String>, startIndex: Int = length, ignoreCase: Boolean = false): Int =
     findAnyOf(strings, startIndex, ignoreCase, last = true)?.first ?: -1
 
 
@@ -1128,7 +1128,7 @@
  * @param ignoreCase `true` to ignore character case when matching a string. By default `false`.
  * @return An index of the last occurrence of [string] or -1 if none is found.
  */
-public fun CharSequence.lastIndexOf(string: String, startIndex: Int = lastIndex, ignoreCase: Boolean = false): Int {
+public fun CharSequence.lastIndexOf(string: String, startIndex: Int = length, ignoreCase: Boolean = false): Int {
     return if (ignoreCase || this !is String)
         indexOf(string, startIndex, 0, ignoreCase, last = true)
     else

libraries/stdlib/test/text/StringTest.kt

diff --git a/libraries/stdlib/test/text/StringTest.kt b/libraries/stdlib/test/text/StringTest.kt
index 7d4e491..6aa60c1 100644
--- a/libraries/stdlib/test/text/StringTest.kt
+++ b/libraries/stdlib/test/text/StringTest.kt
@@ -750,6 +750,24 @@
         assertEquals(-1, string.lastIndexOfAny(substrings, startIndex = 1, ignoreCase = true))
     }
 
+    @Test fun lastIndexOfEmptyString() {
+        assertEquals(0, "".lastIndexOf(""))
+        assertEquals(1, " ".lastIndexOf(""))
+        assertEquals(11, "abracadabra".lastIndexOf(""))
+    }
+
+    @Test fun lastIndexOfAnyWithEmpty() {
+        assertEquals(0, "".lastIndexOfAny(listOf("")))
+        assertEquals(0, "".lastIndexOfAny(listOf("", "a")))
+        assertEquals(0, "".lastIndexOfAny(listOf("a", "")))
+        assertEquals(1, "a".lastIndexOfAny(listOf("")))
+        assertEquals(1, "a".lastIndexOfAny(listOf("", "a")))
+        assertEquals(1, "a".lastIndexOfAny(listOf("a", "")))
+        assertEquals(11, "abracadabra".lastIndexOfAny(listOf("")))
+        assertEquals(11, "abracadabra".lastIndexOfAny(listOf("", "a")))
+        assertEquals(11, "abracadabra".lastIndexOfAny(listOf("a", "")))
+    }
+
     @Test fun findAnyOfStrings() = withOneCharSequenceArg("abracadabra") { string ->
         val substrings = listOf("rac", "ra")
         assertEquals(2 to "rac", string.findAnyOf(substrings))