Correct trait->class calling context on class supertype

See test cases for more information.  This was a pretty unfortunate and
nasty bug that I discovered while working on a project that uses easejs; it
wasn't something that was found previously because this support was only
added relatively recently, and this problem does not exist if an interface
is used.

* lib/Trait.js (bindSuperCtx): Add function.
  (tctor): Use it.

* test/Trait/ScopeTest.js: Add calling context tests.

lib/Trait.js

@@ -908,6 +908,10 @@ function tctor( tc, base, privsym )
         // the intimate relationship
         this[ f ] = C( base, this[ privsym ].vis )[ privsym ].vis;
 
+        // rebind trait's supertype context (if any) to our own, causing us
+        // to share state
+        bindSuperCtx( this[ f ], this, privsym );
+
         // this has been previously validated to ensure that it is a
         // function
         this[ f ].__mixin && this[ f ].__mixin.apply(
@@ -920,6 +924,34 @@ function tctor( tc, base, privsym )
 };
 
 
+/**
+ * Bind the supertype context to that of another instance
+ *
+ * The source instance FROM is expected to have a `___$$super$$' property
+ * holding a reference to the supertype.  The context associated with the
+ * supertype's class id X in FROM will then be reassigned by reference to
+ * the context X of instance TO.
+ *
+ * This has the effect of ensuring that, whenever a call enters the
+ * supertype's context in FROM, it will use all state from the context of
+ * TO.  For mixins, this means that---despite the trait itself inheriting
+ * from the class---we will use the private state of the _mixer_, not our
+ * own.
+ *
+ * This is all done without any loss in performance, since calling context
+ * is immediately bound rather than resolved during each method call.
+ *
+ * TODO: make into a standard API for class manipulation
+ */
+function bindSuperCtx( from, to, privsym )
+{
+    var ctx = from[ privsym ].vis,
+        cid = ctx.___$$super$$.__cid;
+
+    ctx[ privsym ].vis[ cid ] = to[ privsym ].vis[ cid ];
+}
+
+
 /**
  * Create trait constructor
  *

test/Trait/ScopeTest.js

@@ -17,14 +17,19 @@
  *
  *  You should have received a copy of the GNU General Public License
  *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * These tests could possibly duplicate tests elsewhere; that's fine, as
+ * this is a vital concept that wouldn't hurt to be reiterated in a
+ * different context (no pun intended).
  */
 
 require( 'common' ).testCase(
 {
     caseSetUp: function()
     {
-        this.Sut   = this.require( 'Trait' );
+        this.Sut       = this.require( 'Trait' );
-        this.Class = this.require( 'class' );
+        this.Class     = this.require( 'class' );
+        this.Interface = this.require( 'interface' );
     },
 
 
@@ -149,4 +154,218 @@ require( 'common' ).testCase(
             } )().callFoo();
         } );
     },
+
+
+    /**
+     * When a class makes a call to a trait method, the calling context
+     * should be that of the trait itself (that is, the trait has its own
+     * internal state).
+     */
+    'Class->trait calling context binds to trait': function()
+    {
+        var T = this.Sut(
+        {
+            'private _foo': [],
+            _givenMixin: null,
+
+            // must be properly bound before mixin
+            __mixin: function()
+            {
+                this._givenMixin = this.get();
+            },
+
+            push: function( item )
+            {
+                this._foo.push( item );
+            },
+
+            // make sure calling context is preserved on override
+            'virtual overridePush': function( item )
+            {
+                this._foo.push( item );
+            },
+
+            get: function()
+            {
+                return this._foo;
+            },
+
+            getGivenMixin: function()
+            {
+                return this._givenMixin;
+            },
+        } );
+
+        var inst = this.Class.use( T ).extend(
+        {
+            // ensure calling context on T
+            superPush: function( item )
+            {
+                this.push( item );
+            },
+
+            'override overridePush': function( item )
+            {
+                this.__super( item );
+            },
+        } )();
+
+        inst.push( 'a' );
+        inst.superPush( 'b' );
+        inst.overridePush( 'c' );
+
+        this.assertDeepEqual( [ 'a', 'b', 'c' ], inst.get() );
+        this.assertStrictEqual( inst.get(), inst.getGivenMixin() );
+    },
+
+
+    /**
+     * This test focuses on an implementation detail: that traits extending
+     * classes literally extend that class.  The problem there is that,
+     * because of this detail, calling one of the supertypes methods is
+     * going to apply the method within the context of _that
+     * trait_.  Remember: each object has private state associated with each
+     * class in its hierarchy.  So the class C containing the mixin of trait
+     * T has it's own state S_c, and T has its own state T_c because of the
+     * extension.  Given C#Foo, calling T#Foo applies T_c rather than the
+     * intended C_c.  That is, without proper care.
+     *
+     * This tests to make sure the context has been properly rebound to the
+     * mixer.
+     */
+    'Trait->class calling context binds to class': function()
+    {
+        var C = this.Class(
+        {
+            'private _stack': [],
+
+            'virtual push': function( item )
+            {
+                this._stack.push( item );
+            },
+
+            // non-virtual, test fall-through
+            getStack: function()
+            {
+                return this._stack;
+            },
+        } );
+
+        var T = this.Sut.extend( C,
+        {
+            _givenMixin: null,
+
+            // proper context set before __mixin
+            __mixin: function()
+            {
+                this._givenMixin = this.getStack();
+            },
+
+            // proper context to __super
+            'override push': function( item )
+            {
+                this.__super( item );
+            },
+
+            // proper context to parent `getStack'
+            getSuperStack: function()
+            {
+                return this.getStack();
+            },
+
+            getGivenMixin: function()
+            {
+                return this._givenMixin;
+            },
+        } );
+
+        var stack = C.use( T )();
+        stack.push( 'a' );
+
+        // proper context to parent method call (non-__super)
+        this.assertStrictEqual( stack.getStack(), stack.getSuperStack() );
+
+        // proper context to __super
+        this.assertDeepEqual( [ 'a' ], stack.getStack() );
+
+        // context available before __mixin
+        this.assertStrictEqual( stack.getStack(), stack.getGivenMixin() );
+    },
+
+
+    /**
+     * Similar to the above, except that we extend an interface rather than
+     * a base class.
+     *
+     * Notice how T here implements I rather than extending C, and
+     * consequently uses `abstract override' in place of `override'.
+     *
+     * What is interesting in this case is whether this test fails when the
+     * previous does not, or vice-versa (such was the case when this test
+     * was introduced).
+     */
+    'Trait->interface calling context binds to implementing class': function()
+    {
+        var I = this.Interface(
+        {
+            push:     [ 'item' ],
+            getStack: [],
+        } );
+
+        var C = this.Class.implement( I ).extend(
+        {
+            'private _stack': [],
+
+            'virtual push': function( item )
+            {
+                this._stack.push( item );
+            },
+
+            // non-virtual, test fall-through
+            getStack: function()
+            {
+                return this._stack;
+            },
+        } );
+
+        var T = this.Sut.implement( I ).extend(
+        {
+            _givenMixin: null,
+
+            // proper context set before __mixin
+            __mixin: function()
+            {
+                this._givenMixin = this.getStack();
+            },
+
+            // proper context to __super
+            'abstract override push': function( item )
+            {
+                this.__super( item );
+            },
+
+            // proper context to parent `getStack'
+            getSuperStack: function()
+            {
+                return this.getStack();
+            },
+
+            getGivenMixin: function()
+            {
+                return this._givenMixin;
+            },
+        } );
+
+        var stack = C.use( T )();
+        stack.push( 'a' );
+
+        // proper context to parent method call (non-__super)
+        this.assertStrictEqual( stack.getStack(), stack.getSuperStack() );
+
+        // proper context to __super
+        this.assertDeepEqual( [ 'a' ], stack.getStack() );
+
+        // context available before __mixin
+        this.assertStrictEqual( stack.getStack(), stack.getGivenMixin() );
+    },
 } );