improved alpha channel handling for math ops, removed 'can't substract or divide a color from a number' constraint

lib/less/tree/color.js

@@ -70,16 +70,12 @@ tree.Color.prototype = {
     // we create a new Color node to hold the result.
     //
     operate: function (env, op, other) {
-        var result = [];
-
-        if (! (other instanceof tree.Color)) {
-            other = other.toColor();
-        }
-
+        var rgb = [];
+        var alpha = this.alpha * (1 - other.alpha) + other.alpha;
         for (var c = 0; c < 3; c++) {
-            result[c] = tree.operate(env, op, this.rgb[c], other.rgb[c]);
+            rgb[c] = tree.operate(env, op, this.rgb[c], other.rgb[c]);
         }
-        return new(tree.Color)(result, this.alpha + other.alpha);
+        return new(tree.Color)(rgb, alpha);
     },
 
     toRGB: function () {

lib/less/tree/operation.js

@@ -12,17 +12,14 @@ tree.Operation.prototype = {
     },
     eval: function (env) {
         var a = this.operands[0].eval(env),
-            b = this.operands[1].eval(env),
-            temp;
+            b = this.operands[1].eval(env);
 
         if (env.isMathOn()) {
             if (a instanceof tree.Dimension && b instanceof tree.Color) {
-                if (this.op === '*' || this.op === '+') {
-                    temp = b, b = a, a = temp;
-                } else {
-                    throw { type: "Operation",
-                            message: "Can't substract or divide a color from a number" };
-                }
+                a = a.toColor();
+            }
+            if (b instanceof tree.Dimension && a instanceof tree.Color) {
+                b = b.toColor();
             }
             if (!a.operate) {
                 throw { type: "Operation",

test/less/errors/color-operation-error.less

@@ -1,3 +0,0 @@
-.a {
-  prop: (3 / #fff);
-}

test/less/errors/color-operation-error.txt

@@ -1,2 +0,0 @@
-OperationError: Can't substract or divide a color from a number in {path}color-operation-error.less on line null, column 0:
-1   prop: (3 / #fff);