Fixes #1973

lib/less/tree/element.js

@@ -1,4 +1,5 @@
 var Node = require("./node.js"),
+    Paren = require("./paren.js"),
     Combinator = require("./combinator.js");
 
 var Element = function (combinator, value, index, currentFileInfo) {
@@ -34,11 +35,19 @@ Element.prototype.genCSS = function (env, output) {
     output.add(this.toCSS(env), this.currentFileInfo, this.index);
 };
 Element.prototype.toCSS = function (env) {
-    var value = (this.value.toCSS ? this.value.toCSS(env) : this.value);
+    env = env || {};
+    var value = this.value, firstSelector = env.firstSelector;
+    if (value instanceof Paren) {
+        // selector in parens should not be affected by outer selector 
+        // flags (breaks only interpolated selectors - see #1973)
+        env.firstSelector = true;
+    }
+    value = value.toCSS ? value.toCSS(env) : value;
+    env.firstSelector = firstSelector;
     if (value === '' && this.combinator.value.charAt(0) === '&') {
         return '';
     } else {
-        return this.combinator.toCSS(env || {}) + value;
+        return this.combinator.toCSS(env) + value;
     }
 };
 module.exports = Element;

test/css/selectors.css

@@ -125,6 +125,9 @@ p a span {
 :nth-child(3) {
   selector: interpolated;
 }
+.test:nth-child(3) {
+  selector: interpolated;
+}
 .test:nth-child(odd):not(:nth-child(3)) {
   color: #ff0000;
 }

test/less/selectors.less

@@ -126,6 +126,9 @@ a {
     selector: interpolated;
 }
 .test {
+  &:nth-child(@{num}) {
+    selector: interpolated;
+  }
   &:nth-child(odd):not(:nth-child(3)) {
     color: #ff0000;
   }