Make __proto__ more closely match the spec

Create test cases for the "ES6" behavior that matches the spec,
and different tests for the backwardly-compatible behavior.

src/org/mozilla/javascript/SpecialRef.java

@@ -98,8 +98,13 @@ public Object set(Context cx, Scriptable scope, Object value)
                     } while (search != null);
                 }
                 if (type == SPECIAL_PROTO) {
-                    if (cx.getLanguageVersion() < Context.VERSION_ES6 ||
-                        !(target instanceof BaseFunction)) {
+                    if (cx.getLanguageVersion() >= Context.VERSION_ES6) {
+                        if ((value != null && !"object".equals(ScriptRuntime.typeof(value))) ||
+                            !"object".equals(ScriptRuntime.typeof(target))) {
+                            return Undefined.instance;
+                        }
+                        target.setPrototype(obj);
+                    } else {
                         target.setPrototype(obj);
                     }
                 } else {

testsrc/jstests/backwardcompat/backward-proto-property.js

@@ -0,0 +1,41 @@
+load('testsrc/assert.js');
+
+// Test old behavior of "__proto__"
+
+// __proto__ can be set to lots of things
+let obj = {};
+assertTrue(obj.__proto__ === Object.prototype);
+obj.__proto__ = undefined;
+assertFalse(obj.__proto__ === Object.prototype);
+obj.__proto__ = true;
+assertFalse(obj.__proto__ === Object.prototype);
+obj.__proto__ = 12345;
+assertFalse(obj.__proto__ === Object.prototype);
+obj.__proto__ = 'foobar';
+assertFalse(obj.__proto__ === Object.prototype);
+
+// __proto__ on an object can indeed be set to another object
+let prot = {
+  bar: function() { print('bar'); }
+};
+obj = {};
+assertEquals(prot, obj.__proto__ = prot);
+assertFalse(obj.__proto__ === Object.prototype);
+assertEquals('function', typeof obj.__proto__.bar);
+assertEquals(prot, obj.__proto__);
+
+// __proto__ on an object can be set to null
+obj = {};
+assertNull(obj.__proto__ = null);
+assertFalse(obj.__proto__ === Object.prototype);
+
+//__proto__ setting on a non-object also works
+function f() {}
+assertTrue(f.__proto__ === Function.prototype);
+f.__proto__ = prot;
+assertFalse(f.__proto__ === Function.prototype);
+assertTrue(f.__proto__ === prot);
+f.__proto__ = null;
+assertFalse(f.__proto__ === Function.prototype);
+
+"success";

testsrc/jstests/es6/proto-property.js

@@ -0,0 +1,41 @@
+load('testsrc/assert.js');
+
+// Test section B.2.2.1.2 of ECMAScript 6.0, which
+// defines the behavior of the "__proto__" special property.
+
+// __proto__ can only be set to an object or to null
+let obj = {};
+assertTrue(obj.__proto__ === Object.prototype);
+assertEquals(undefined, obj.__proto__ = undefined);
+assertTrue(obj.__proto__ === Object.prototype);
+assertEquals(undefined, obj.__proto__ = true);
+assertTrue(obj.__proto__ === Object.prototype);
+assertEquals(undefined, obj.__proto__ = 12345);
+assertTrue(obj.__proto__ === Object.prototype);
+assertEquals(undefined, obj.__proto__ = 'foobar');
+assertTrue(obj.__proto__ === Object.prototype);
+
+// __proto__ on an object can indeed be set to another object
+let prot = {
+  bar: function() { print('bar'); }
+};
+obj = {};
+assertEquals(prot, obj.__proto__ = prot);
+assertFalse(obj.__proto__ === Object.prototype);
+assertEquals('function', typeof obj.__proto__.bar);
+assertEquals(prot, obj.__proto__);
+
+// __proto__ on an object can be set to null
+obj = {};
+assertNull(obj.__proto__ = null);
+assertFalse(obj.__proto__ === Object.prototype);
+
+// However, __proto__ setting on a non-object does nothing
+function f() {}
+assertTrue(f.__proto__ === Function.prototype);
+assertEquals(undefined, f.__proto__ = prot);
+assertTrue(f.__proto__ === Function.prototype);
+assertEquals(undefined, f.__proto__ = null);
+assertTrue(f.__proto__ === Function.prototype);
+
+"success";

testsrc/org/mozilla/javascript/tests/backwardcompat/BackwardProtoPropertyTest.java

@@ -0,0 +1,9 @@
+package org.mozilla.javascript.tests.backwardcompat;
+
+import org.mozilla.javascript.Context;
+import org.mozilla.javascript.drivers.LanguageVersion;
+import org.mozilla.javascript.drivers.RhinoTest;
+
+@RhinoTest("testsrc/jstests/backwardcompat/backward-proto-property.js")
+@LanguageVersion(Context.VERSION_1_8)
+public class BackwardProtoPropertyTest {}

testsrc/org/mozilla/javascript/tests/es6/ProtoPropertyTest.java

@@ -0,0 +1,9 @@
+package org.mozilla.javascript.tests.es6;
+
+import org.mozilla.javascript.Context;
+import org.mozilla.javascript.drivers.LanguageVersion;
+import org.mozilla.javascript.drivers.RhinoTest;
+
+@RhinoTest("testsrc/jstests/es6/proto-property.js")
+@LanguageVersion(Context.VERSION_ES6)
+public class ProtoPropertyTest {}