fix destructuring assignment to single element

Author: timfel

graalpython/com.oracle.graal.python.test/src/com/oracle/graal/python/test/grammar/TestParserTranslator.java

@@ -330,6 +330,7 @@ public void parseAssignments() {
         parseAs = parseAs("a = 1,2", WriteNameNode.class);
         assert parseAs.getRhs() instanceof TupleLiteralNode;
 
+        parseAs("a, = 1,", DestructuringAssignmentNode.class);
         parseAs("a,b = 1,2", DestructuringAssignmentNode.class);
         parseAs("a,*b,c = 1,2", DestructuringAssignmentNode.class);
         parseAs("[[a],*b],c = 1,2", DestructuringAssignmentNode.class);

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/parser/AssignmentTranslator.java

@@ -173,6 +173,7 @@ private PNode makeAugmentedAssignment(ExpressionNode lhs, String text, Expressio
     }
 
     private PNode visitTargetlist(ParserRuleContext ctx, int starSize) {
+        boolean endsWithComma = false;
         if (starSize > 0) {
             if (starSize > 1) {
                 throw errors.raise(SyntaxError, "%d starred expressions in assigment", starSize);
@@ -181,7 +182,11 @@ private PNode visitTargetlist(ParserRuleContext ctx, int starSize) {
         List<ExpressionNode> targets = new ArrayList<>();
         for (int i = 0; i < ctx.getChildCount(); i++) {
             ParseTree child = ctx.getChild(i);
+            endsWithComma = false;
             if (child instanceof TerminalNode) {
+                if (child.getText().equals(",")) {
+                    endsWithComma = true;
+                }
                 continue;
             } else if (child instanceof Python3Parser.TestContext ||
                             child instanceof Python3Parser.Star_exprContext ||
@@ -191,7 +196,7 @@ private PNode visitTargetlist(ParserRuleContext ctx, int starSize) {
                 assert false;
             }
         }
-        if (targets.size() == 1) {
+        if (targets.size() == 1 && !endsWithComma) {
             PNode pNode = targets.get(0);
             if (pNode instanceof ReadNode) {
                 return pNode;