Testcase to demonstrate Proxy preincr behavior

tests/ecmascript/test-bi-proxy-preinc-gh309.js

@@ -0,0 +1,37 @@
+/*
+ *  Preincrement applied to a Proxy (GH-309).
+ */
+
+/*===
+get
+set
+4500.5
+get
+9001
+===*/
+
+function test() {
+    p = new Proxy({ val: 4499.5 }, {
+        get: function(t, prop) { print('get'); return t[prop]; },
+        set: function(t, prop, value) { print('set'); t[prop] = value * 2; }
+    });
+
+    // Preincrement is evaluated as:
+    //    (1) read p.val -> 4499.5
+    //    (2) increment 4499.5 -> 4500.5 (this becomes the expression value)
+    //    (3) write 4500.5 to p.val -> 'set' trap writes 9001 to target
+    //
+    // In this case the result may not be intuitive as one might
+    // expect 9001 as a result (i.e. step 4: read p.val back).
+    // The required behavior is described in:
+    // http://www.ecma-international.org/ecma-262/6.0/index.html#sec-prefix-increment-operator
+
+    print(++p.val);  // 4500.5 (from step 2)
+    print(p.val);    // 9001
+}
+
+try {
+    test();
+} catch (e) {
+    print(e.stack || e);
+}