throw an error when the unit function is passed bad input. Fixes #1426

lib/less/functions.js

@@ -233,6 +233,9 @@ tree.functions = {
         return new(tree.Quoted)('"' + str + '"', str);
     },
     unit: function (val, unit) {
+        if(!(val instanceof tree.Dimension)) {
+            throw { type: "Argument", message: "the first argument to unit must be a number" + (val instanceof tree.Operation ? ". Have you forgotten parenthesis?" : "") };
+        }
         return new(tree.Dimension)(val.value, unit ? unit.toCSS() : "");
     },
     convert: function (val, unit) {

test/browser/runner-errors-options.js

@@ -1,3 +1,5 @@
-var less = {};
-less.strictUnits = true;
+var less = {
+    strictUnits: true,
+    strictMath: true
+};
 

test/less/errors/unit-function.less

@@ -0,0 +1,3 @@
+.a {
+  font-size: unit(80/16,rem);
+}

test/less/errors/unit-function.txt

@@ -0,0 +1,4 @@
+ArgumentError: error evaluating function `unit`: the first argument to unit must be a number. Have you forgotten parenthesis? in {path}unit-function.less on line 2, column 14:
+1 .a {
+2   font-size: unit(80/16,rem);
+3 }