Locale-agnostic case conversions by default #KT-43023

(cherry picked from commit 1b565500b935239484580005a31ae2630c61b25d)

kotlin-native/runtime/src/main/kotlin/kotlin/text/Char.kt

@@ -64,17 +64,75 @@ external public fun Char.isUpperCase(): Boolean
 external public fun Char.isLowerCase(): Boolean
 
 /**
- * Converts this character to uppercase.
+ * Converts this character to upper case using Unicode mapping rules of the invariant locale.
  */
 @SymbolName("Kotlin_Char_toUpperCase")
 external public actual fun Char.toUpperCase(): Char
 
 /**
- * Converts this character to lowercase.
+ * Converts this character to upper case using Unicode mapping rules of the invariant locale.
+ *
+ * This function performs one-to-one character mapping.
+ * To support one-to-many character mapping use the [uppercase] function.
+ * If this character has no mapping equivalent, the character itself is returned.
+ *
+ * @sample samples.text.Chars.uppercase
+ */
+@SinceKotlin("1.4")
+@ExperimentalStdlibApi
+public actual fun Char.uppercaseChar(): Char = toUpperCase()
+
+/**
+ * Converts this character to upper case using Unicode mapping rules of the invariant locale.
+ *
+ * This function supports one-to-many character mapping, thus the length of the returned string can be greater than one.
+ * For example, `'\uFB00'.uppercase()` returns `"\u0046\u0046"`,
+ * where `'\uFB00'` is the LATIN SMALL LIGATURE FF character (`ff`).
+ * If this character has no upper case mapping, the result of `toString()` of this char is returned.
+ *
+ * @sample samples.text.Chars.uppercase
+ */
+@SinceKotlin("1.4")
+@ExperimentalStdlibApi
+public actual fun Char.uppercase(): String {
+    return uppercaseChar().toString()
+}
+
+/**
+ * Converts this character to lower case using Unicode mapping rules of the invariant locale.
  */
 @SymbolName("Kotlin_Char_toLowerCase")
 external public actual fun Char.toLowerCase(): Char
 
+/**
+ * Converts this character to lower case using Unicode mapping rules of the invariant locale.
+ *
+ * This function performs one-to-one character mapping.
+ * To support one-to-many character mapping use the [lowercase] function.
+ * If this character has no mapping equivalent, the character itself is returned.
+ *
+ * @sample samples.text.Chars.lowercase
+ */
+@SinceKotlin("1.4")
+@ExperimentalStdlibApi
+public actual fun Char.lowercaseChar(): Char = toLowerCase()
+
+/**
+ * Converts this character to lower case using Unicode mapping rules of the invariant locale.
+ *
+ * This function supports one-to-many character mapping, thus the length of the returned string can be greater than one.
+ * For example, `'\u0130'.lowercase()` returns `"\u0069\u0307"`,
+ * where `'\u0130'` is the LATIN CAPITAL LETTER I WITH DOT ABOVE character (`İ`).
+ * If this character has no lower case mapping, the result of `toString()` of this char is returned.
+ *
+ * @sample samples.text.Chars.lowercase
+ */
+@SinceKotlin("1.4")
+@ExperimentalStdlibApi
+public actual fun Char.lowercase(): String {
+    return lowercaseChar().toString()
+}
+
 /**
  * Returns `true` if this character is a Unicode high-surrogate code unit (also known as leading-surrogate code unit).
  */

kotlin-native/runtime/src/main/kotlin/kotlin/text/Strings.kt

@@ -158,13 +158,36 @@ public external fun String.regionMatches(
 @SymbolName("Kotlin_String_toUpperCase")
 public actual external fun String.toUpperCase(): String
 
+/**
+ * Returns a copy of this string converted to upper case using Unicode mapping rules of the invariant locale.
+ *
+ * This function supports one-to-many and many-to-one character mapping,
+ * thus the length of the returned string can be different from the length of the original string.
+ *
+ * @sample samples.text.Strings.uppercase
+ */
+@SinceKotlin("1.4")
+@ExperimentalStdlibApi
+public actual fun String.uppercase(): String = toUpperCase()
+
 /**
  * Returns a copy of this string converted to lower case using the rules of the default locale.
  */
 @SymbolName("Kotlin_String_toLowerCase")
-@Suppress("NOTHING_TO_INLINE")
 public actual external fun String.toLowerCase(): String
 
+/**
+ * Returns a copy of this string converted to lower case using Unicode mapping rules of the invariant locale.
+ *
+ * This function supports one-to-many and many-to-one character mapping,
+ * thus the length of the returned string can be different from the length of the original string.
+ *
+ * @sample samples.text.Strings.lowercase
+ */
+@SinceKotlin("1.4")
+@ExperimentalStdlibApi
+public actual fun String.lowercase(): String = toLowerCase()
+
 /**
  * Returns a [CharArray] containing characters of this string.
  */