Implement stringByFoldingWithOptions

ChangeLog

@@ -30,7 +30,16 @@
 	exceptions or null pointer crashes in code which expects the
 	-UTF8String method to work for all strings.
 	Remove a few old testcases and add a load of new ones.
-
+
+2026-04-22 Hendrik Huebner <hendrik.huebner@algoriddim.com>
+
+	* Headers/Foundation/NSString.h:
+	Declare -stringByFoldingWithOptions:locale:.
+	* Source/NSString.m:
+	Implement -stringByFoldingWithOptions:locale: using ICU transliteration.
+	* Tests/base/NSString/locale.m:
+	Add folding tests for case, diacritic, width and combined behavior.
+
 2026-04-20  Wolfgang Lux  <wolfgang.lux@gmail.com>
 
 	* Headers/Foundation/NSFileManager.h:

Headers/Foundation/NSString.h

@@ -657,6 +657,14 @@ GS_EXPORT_CLASS
 - (NSString *) precomposedStringWithCanonicalMapping;
 #endif
 
+#if OS_API_VERSION(MAC_OS_X_VERSION_10_5,GS_API_LATEST)
+/** Returns a copy of the receiver suitable for comparison with the supplied
+ * options and locale.
+ */
+- (NSString *) stringByFoldingWithOptions: (NSStringCompareOptions)options
+				    locale: (NSLocale *)locale;
+#endif
+
 // Converting String Contents into a Property List
 - (id) propertyList;
 - (NSDictionary*) propertyListFromStringsFileFormat;