if contrast first argument does not match, pass it through. Fixes #1090

lib/less/functions.js

@@ -173,6 +173,11 @@ tree.functions = {
         return this.desaturate(color, new(tree.Dimension)(100));
     },
     contrast: function (color, dark, light, threshold) {
+        // filter: contrast(3.2);
+        // should be kept as is, so check for color
+        if (!color.rgb) {
+            return null;
+        }
         if (typeof light === 'undefined') {
             light = this.rgba(255, 255, 255, 1.0);
         }

lib/less/tree/call.js

@@ -14,7 +14,8 @@ tree.Call.prototype = {
     // When evaluating a function call,
     // we either find the function in `tree.functions` [1],
     // in which case we call it, passing the  evaluated arguments,
-    // or we simply print it out as it appeared originally [2].
+    // if this returns null or we cannot find the function, we 
+    // simply print it out as it appeared originally [2].
     //
     // The *functions.js* file contains the built-in functions.
     //
@@ -23,21 +24,26 @@ tree.Call.prototype = {
     // The function should receive the value, not the variable.
     //
     eval: function (env) {
-        var args = this.args.map(function (a) { return a.eval(env) });
+        var args = this.args.map(function (a) { return a.eval(env) }),
+            result;
 
         if (this.name in tree.functions) { // 1.
             try {
-                return tree.functions[this.name].apply(tree.functions, args);
+                result = tree.functions[this.name].apply(tree.functions, args);
+                if (result != null) {
+                    return result;
+                }
             } catch (e) {
                 throw { type: e.type || "Runtime",
                         message: "error evaluating function `" + this.name + "`" +
                                  (e.message ? ': ' + e.message : ''),
                         index: this.index, filename: this.filename };
             }
-        } else { // 2.
-            return new(tree.Anonymous)(this.name +
-                   "(" + args.map(function (a) { return a.toCSS(env) }).join(', ') + ")");
         }
+        
+        // 2.
+        return new(tree.Anonymous)(this.name +
+            "(" + args.map(function (a) { return a.toCSS(env) }).join(', ') + ")");
     },
 
     toCSS: function (env) {

test/css/functions.css

@@ -24,6 +24,7 @@
   luma-yellow: 93%;
   luma-cyan: 79%;
   luma-white-alpha: 50%;
+  contrast-filter: contrast(30%);
   contrast-white: #000000;
   contrast-black: #ffffff;
   contrast-red: #ffffff;

test/less/functions.less

@@ -28,6 +28,7 @@
   luma-yellow: luma(#ffff00);
   luma-cyan: luma(#00ffff);
   luma-white-alpha: luma(rgba(255,255,255,0.5));
+  contrast-filter: contrast(30%);
   contrast-white: contrast(#fff);
   contrast-black: contrast(#000);
   contrast-red: contrast(#ff0000);