Do not evaluate css with guards if the guards fail. Fixes #1873

lib/less/tree/ruleset.js

@@ -20,7 +20,8 @@ tree.Ruleset.prototype = {
     },
     eval: function (env) {
         var thisSelectors = this.selectors, selectors, 
-            selCnt, i, defaultFunc = tree.defaultFunc;
+            selCnt, selector, i, defaultFunc = tree.defaultFunc, hasOnePassingSelector = false;
+
         if (thisSelectors && (selCnt = thisSelectors.length)) {
             selectors = [];
             defaultFunc.error({
@@ -28,9 +29,15 @@ tree.Ruleset.prototype = {
                 message: "it is currently only allowed in parametric mixin guards," 
             });
             for (i = 0; i < selCnt; i++) {
-                selectors.push(thisSelectors[i].eval(env));
+                selector = thisSelectors[i].eval(env);
+                selectors.push(selector);
+                if (selector.evaldCondition) {
+                    hasOnePassingSelector = true;
+                }
             }
             defaultFunc.reset();  
+        } else {
+            hasOnePassingSelector = true;
         }
 
         var rules = this.rules ? this.rules.slice(0) : null,
@@ -45,6 +52,10 @@ tree.Ruleset.prototype = {
         if(this.debugInfo) {
             ruleset.debugInfo = this.debugInfo;
         }
+        
+        if (!hasOnePassingSelector) {
+            rules.length = 0;
+        }
 
         // push the current ruleset to the frames stack
         var envFrames = env.frames;
@@ -122,10 +133,7 @@ tree.Ruleset.prototype = {
                 // check if it can be folded in (e.g. & where)
                 if (rule.selectors[0].isJustParentSelector()) {
                     rsRules.splice(i--, 1);
-                    // cannot call if there is no selector, so we can just continue
-                    if (!rule.selectors[0].evaldCondition) {
-                        continue;
-                    }
+
                     for(var j = 0; j < rule.rules.length; j++) {
                         subRule = rule.rules[j];
                         if (!(subRule instanceof tree.Rule) || !subRule.variable) {

test/less/css-guards.less

@@ -97,3 +97,6 @@
   .scope-check();
   @k:4px;
 }
+.errors-if-called when (@c = never) {
+  .mixin-doesnt-exist();
+}