fix(issue:4258) variable interpolation after math

* Fixes variable interpolation issue after previous variable addition.

Author: puckowski

packages/less/src/less/parser/parser.js

@@ -1707,7 +1707,9 @@ const Parser = function Parser(context, imports, fileInfo, currentIndex) {
                             }
                             // Treat like quoted values, but replace vars like unquoted expressions
                             const quote = new tree.Quoted('\'', item, true, index, fileInfo);
-                            quote.variableRegex = /@([\w-]+)/g;
+                            if (!item.startsWith('@{')) {
+                                quote.variableRegex = /@([\w-]+)/g;
+                            }
                             quote.propRegex = /\$([\w-]+)/g;
                             result.push(quote);
                         }

packages/test-data/css/_main/variables.css

@@ -69,3 +69,6 @@
   mul-px-2: 140px;
   mul-px-3: 140px;
 }
+.radio_checked {
+  border-color: #fff;
+}

packages/test-data/less/_main/variables.less

@@ -125,3 +125,14 @@
     mul-px-3: ((@px * 1) * @cm);
   }
 }
+
+@a1: 1px;
+@b2: 2px;
+@c3: @a1 + @b2;
+
+@radio-cls: radio;
+@radio-cls-checked: @{radio-cls}_checked;
+
+.@{radio-cls-checked} {
+  border-color: #fff;
+}