Fix for subtypes of prototype subtypes

When extending a class that is a subtype of a prototype, it would clobber
___$$vis$$, yielding a useless class. See the test case for a detailed
description.

lib/ClassBuilder.js

@@ -365,7 +365,8 @@ exports.prototype.build = function extend( _, __ )
     // if we are inheriting from a prototype, we must make sure that all
     // properties initialized by the ctor are implicitly public; otherwise,
     // proxying will fail to take place
-    if ( !( prototype instanceof exports.ClassBase ) )
+    // TODO: see Class.isA TODO
+    if ( prototype.___$$vis$$ === undefined )
     {
         this._discoverProtoProps( prototype, prop_init );
     }

lib/class.js

@@ -148,6 +148,9 @@ module.exports.use = function( traits )
 };
 
 
+var _dummyclass = { prototype: {} };
+var _dummyinst  = { constructor: { prototype: {} } };
+
 /**
  * Determines whether the provided object is a class created through ease.js
  *
@@ -157,9 +160,18 @@ module.exports.use = function( traits )
  */
 module.exports.isClass = function( obj )
 {
-    obj = obj || {};
+    obj = obj || _dummyclass;
 
-    return ( obj.prototype instanceof ClassBuilder.ClassBase )
+    if ( !obj.prototype )
+    {
+        return false;
+    }
+
+    // TODO: this just checks one of many internal fields; we need something
+    // more formal (cannot use a strict ClassBase check because it will fail
+    // when extending prototypes)
+    return ( ( obj.prototype.___$$vis$$ !== undefined )
+        || ( obj.prototype instanceof ClassBuilder.ClassBase ) )
         ? true
         : false
     ;
@@ -177,9 +189,16 @@ module.exports.isClass = function( obj )
  */
 module.exports.isClassInstance = function( obj )
 {
-    obj = obj || {};
+    obj = obj || _dummyinst;
 
-    return ( obj instanceof ClassBuilder.ClassBase )
+    if ( !obj.constructor || !obj.constructor.prototype )
+    {
+        return false;
+    }
+
+    // TODO: see isClass TODO
+    return ( ( obj.constructor.prototype.___$$vis$$ !== undefined )
+        || ( obj instanceof ClassBuilder.ClassBase ) )
         ? true
         : false;
 };

test/Class/InteropTest.js

@@ -319,6 +319,42 @@ require( 'common' ).testCase(
     },
 
 
+    /**
+     * This is a regression test for an interesting (and particularily
+     * nasty) bug for a situation that is probably reasonably rare. The
+     * original check for a non-class supertype checked whether the
+     * supertype was an instance of the internal base class. While this
+     * works, it unforunately causes problems for subtypes of the class that
+     * extended the prototype---the check will fail, since there is no
+     * ClassBase in the prototype chain.
+     *
+     * This resulted in it processing the class fields, which ended up
+     * overwriting ___$$vis$$, which clobbered all the methods. Doh.
+     */
+    'Subtypes of prototype subtypes yield stable classes': function()
+    {
+        function P() {};
+
+        // sub-subtype of P
+        var expected = {};
+        var C = this.Class.extend( P, {} ).extend(
+        {
+            foo: function() { return expected; }
+        } );
+
+        var inst = C();
+
+        // this should be recognized as a class (prior to the fix, it was
+        // not), and inst should be an instance of a class
+        this.assertOk( this.Class.isClass( C ) );
+        this.assertOk( this.Class.isClassInstance( inst ) );
+        this.assertOk( this.Class.isA( C, inst ) );
+
+        // before the fix, foo is undefined since ___$$vis$$ was clobbered
+        this.assertStrictEqual( inst.foo(), expected );
+    },
+
+
     /**
      * When prototypally extending a class, it is not wise to invoke the
      * constructor (just like ease.js does not invoke the constructor of