Make function proxies work as constructors.

src/proxy.js

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 24 matching lines @@
   if (!IS_SPEC_OBJECT(proto)) proto = null  // Mozilla does this...
   return %CreateJSProxy(handler, proto)
 }
 
 $Proxy.createFunction = function(handler, callTrap, constructTrap) {
   if (!IS_SPEC_OBJECT(handler))
     throw MakeTypeError("handler_non_object", ["create"])
   if (!IS_FUNCTION(callTrap))
     throw MakeTypeError("trap_function_expected", ["createFunction", "call"])
   if (IS_UNDEFINED(constructTrap)) {
-    constructTrap = callTrap
-  } else if (!IS_FUNCTION(constructTrap)) {
+    var construct = DerivedConstructTrap(callTrap)
+  } else if (IS_FUNCTION(constructTrap)) {
+    construct = function() {
+      // Make sure the trap gets undefined as this.
+      return $Function.prototype.apply.call(constructTrap, void 0, arguments)
+    }
+  } else {
     throw MakeTypeError("trap_function_expected",
                         ["createFunction", "construct"])
   }
   return %CreateJSFunctionProxy(
-    handler, callTrap, constructTrap, $Function.prototype)
+    handler, callTrap, construct, $Function.prototype)
 }
 
 
 
 ////////////////////////////////////////////////////////////////////////////////
 // Builtins
 ////////////////////////////////////////////////////////////////////////////////
 
+function DerivedConstructTrap(callTrap) {
+  return function() {
+    var proto = this.prototype
+    if (!IS_SPEC_OBJECT(proto)) proto = $Object.prototype
+    var obj = new $Object()
+    obj.__proto__ = proto
+    var result = $Function.prototype.apply.call(callTrap, obj, arguments)
+    return IS_SPEC_OBJECT(result) ? result : obj
+  }
+}
+
 function DelegateCallAndConstruct(callTrap, constructTrap) {
   return function() {
     if (%_IsConstructCall()) {
-      // return constructTrap.apply(this, arguments)
       return $Function.prototype.apply.call(constructTrap, this, arguments)
     } else {
-      // return callTrap.apply(this, arguments)
       return $Function.prototype.apply.call(callTrap, this, arguments)
     }
   }
 }
 
 function DerivedGetTrap(receiver, name) {
   var desc = this.getPropertyDescriptor(name)
   if (IS_UNDEFINED(desc)) { return desc }
   if ('value' in desc) {
     return desc.value
@@ skipping 63 matching lines @@
   var names = this.getOwnPropertyNames()
   var enumerableNames = []
   for (var i = 0, count = 0; i < names.length; ++i) {
     var name = names[i]
     if (this.getOwnPropertyDescriptor(TO_STRING_INLINE(name)).enumerable) {
       enumerableNames[count++] = names[i]
     }
   }
   return enumerableNames
 }