Fix and specs for splatted index in optional element assignment.

Author: aardvark179

spec/ruby/language/optional_assignments_spec.rb

@@ -300,6 +300,10 @@ def []=(k, v)
         (@b[:k] ||= 12).should == 12
       end
 
+      it 'correctly handles a splatted argument for the index' do
+        (@b[*[:k]] ||= 12).should == 12
+      end
+
       it "evaluates the index precisely once" do
         ary = [:x, :y]
         @a[:x] = 15

src/main/java/org/truffleruby/parser/BodyTranslator.java

@@ -2541,9 +2541,15 @@ private ParseNode operation(
             ArrayList<ValueFromNode> argValues,
             String op,
             ParseNode right) {
-        final ArrayParseNode readArguments = new ArrayParseNode(node.getPosition());
-        for (var arg : argValues) {
-            readArguments.add(arg.get(node.getPosition()));
+        ParseNode readArguments;
+        if (node.getArgsNode() instanceof ArrayParseNode) {
+            final ArrayParseNode readArgsArray = new ArrayParseNode(node.getPosition());
+            for (var arg : argValues) {
+                readArgsArray.add(arg.get(node.getPosition()));
+            }
+            readArguments = readArgsArray;
+        } else {
+            readArguments = argValues.get(0).get(node.getPosition());
         }
 
         final ParseNode read = new CallParseNode(