Fix css guard error. fixes #1796

lib/less/parser.js

@@ -224,7 +224,7 @@ less.Parser = function Parser(env) {
     }
 
     function expect(arg, msg) {
-        // check if it is a function or a regexp (some older browsers don't give 'function' on typeof RegExp)
+        // some older browsers return typeof 'function' for RegExp
         var result = (Object.prototype.toString.call(arg) === '[object Function]') ? arg.call(parsers) : $(arg);
         if (result) {
             return result;
@@ -1447,6 +1447,9 @@ less.Parser = function Parser(env) {
                     }
                     if (selectors) { selectors.push(s); } else { selectors = [ s ]; }
                     this.comments();
+                    if (s.condition && selectors.length > 1) {
+                        error("Guards are only currently allowed on a single selector.");
+                    }
                     if (! $char(',')) { break; }
                     if (s.condition) {
                         error("Guards are only currently allowed on a single selector.");

test/less/errors/multiple-guards-on-css-selectors2.less

@@ -0,0 +1,4 @@
+@ie8: true;
+.a,
+.b when (@ie8 = true) {
+}

test/less/errors/multiple-guards-on-css-selectors2.txt

@@ -0,0 +1,4 @@
+SyntaxError: Guards are only currently allowed on a single selector. in {path}multiple-guards-on-css-selectors2.less on line 3, column 23:
+2 .a,
+3 .b when (@ie8 = true) {
+4 }