--- a/nashorn/src/jdk/nashorn/internal/codegen/Attr.java	Thu Nov 07 17:26:46 2013 +0530
+++ b/nashorn/src/jdk/nashorn/internal/codegen/Attr.java	Mon Nov 11 14:25:01 2013 +0100
@@ -1265,12 +1265,17 @@
 
     @Override
     public Node leaveCOMMARIGHT(final BinaryNode binaryNode) {
-        return end(ensureSymbol(binaryNode.rhs().getType(), binaryNode));
+        return leaveComma(binaryNode, binaryNode.rhs());
     }
 
     @Override
     public Node leaveCOMMALEFT(final BinaryNode binaryNode) {
-        return end(ensureSymbol(binaryNode.lhs().getType(), binaryNode));
+        return leaveComma(binaryNode, binaryNode.lhs());
+    }
+
+    private Node leaveComma(final BinaryNode commaNode, final Expression effectiveExpr) {
+        ensureTypeNotUnknown(effectiveExpr);
+        return end(ensureSymbol(effectiveExpr.getType(), commaNode));
     }
 
     @Override

--- a/nashorn/src/jdk/nashorn/internal/codegen/CodeGenerator.java	Thu Nov 07 17:26:46 2013 +0530
+++ b/nashorn/src/jdk/nashorn/internal/codegen/CodeGenerator.java	Mon Nov 11 14:25:01 2013 +0100
@@ -412,6 +412,8 @@
             return method;
         }
 
+        assert !type.isUnknown();
+
         /*
          * The load may be of type IdentNode, e.g. "x", AccessNode, e.g. "x.y"
          * or IndexNode e.g. "x[y]". Both AccessNodes and IndexNodes are

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/nashorn/test/script/basic/JDK-8028020.js	Mon Nov 11 14:25:01 2013 +0100
@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ * 
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ * 
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ * 
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ * 
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/**
+ * JDK-8028020: Function parameter as last expression in comma in return value causes bad type calculation
+ *
+ * @test
+ * @run
+ */
+
+function f(x) {
+    return 1, x
+}
+
+function g(x, y) {
+    return x, y
+}
+
+print(f("'1, x' works."))
+print(g(42, "'x, y' works too."))

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/nashorn/test/script/basic/JDK-8028020.js.EXPECTED	Mon Nov 11 14:25:01 2013 +0100
@@ -0,0 +1,2 @@
+'1, x' works.
+'x, y' works too.