[Truffle] - String#downcase! raises a RuntimeError when self is frozen

Author: lucasallan

spec/truffle/tags/core/string/downcase_tags.txt

@@ -1,4 +1,3 @@
 fails:String#downcase is locale insensitive (only replaces A-Z)
 fails:String#downcase taints result when self is tainted
 fails:String#downcase returns a subclass instance for subclasses
-fails:String#downcase! raises a RuntimeError when self is frozen

truffle/src/main/java/org/jruby/truffle/nodes/core/StringNodes.java

@@ -778,6 +778,8 @@ public RubyBasicObject downcase(RubyString string) {
 
             if (newByteList.equal(string.getBytes())) {
                 return getContext().getCoreLibrary().getNilObject();
+            } else if (string.isFrozen()) {
+                throw new RaiseException(getContext().getCoreLibrary().runtimeError("can't modify frozen string", this));
             } else {
                 string.set(newByteList);
                 return string;