Fixes less#3675 - LESS detects @apply as variable (ISSUE)

packages/less/src/less/tree/expression.js

@@ -45,7 +45,7 @@ Expression.prototype = Object.assign(new Node(), {
         if (inParenthesis) {
             context.outOfParenthesis();
         }
-        if (this.parens && this.parensInOp && !mathOn && !doubleParen 
+        if (this.parens && this.parensInOp && !mathOn && !doubleParen
             && (!(returnValue instanceof Dimension))) {
             returnValue = new Paren(returnValue);
         }

packages/less/src/less/tree/quoted.js

@@ -34,7 +34,15 @@ Quoted.prototype = Object.assign(new Node(), {
         const that = this;
         let value = this.value;
         const variableReplacement = function (_, name) {
-            const v = new Variable(`@${name}`, that.getIndex(), that.fileInfo()).eval(context, true);
+            let v;
+            try {
+                v = new Variable(`@${name}`, that.getIndex(), that.fileInfo()).eval(context, true);
+            } catch (e) {
+                if (e.type !== 'Name') {
+                    throw e;
+                }
+                return `@${name}`;
+            }
             return (v instanceof Quoted) ? v.value : v.toCSS();
         };
         const propertyReplacement = function (_, name) {

packages/less/test/browser/generator/runner.config.js

@@ -183,4 +183,4 @@ module.exports = {
             outfile: 'tmp/browser/test-runner-filemanager-plugin.html'
         }
     }
-}
+}

packages/test-data/css/_main/atrule-passthrough.css

@@ -0,0 +1,5 @@
+.test {
+  --iron-autogrow-textarea: {
+    @apply --app-font-monospace;
+    };
+}

packages/test-data/less/_main/atrule-passthrough.less

@@ -0,0 +1,5 @@
+.test {
+  --iron-autogrow-textarea: {
+    @apply --app-font-monospace;
+    }
+}