The guarded namespace can use when the mixin is invoked

packages/less/src/less/tree/ruleset.js

@@ -264,7 +264,8 @@ Ruleset.prototype = Object.assign(new Node(), {
     // lets you call a css selector with a guard
     matchCondition(args, context) {
         const lastSelector = this.selectors[this.selectors.length - 1];
-        if (!lastSelector.evaldCondition) {
+        // fix:3407
+        if (!lastSelector.evaldCondition && !lastSelector.condition) {
             return false;
         }
         if (lastSelector.condition &&

packages/test-data/css/_main/mixins-guards.css

@@ -173,6 +173,18 @@
 #guarded-deeper {
   should: match 1;
 }
+@media (min-width: 700px) {
+  .example1 {
+    success1: 1;
+    success2: 2;
+    success3: 3;
+  }
+}
+@media (min-width: 700px) {
+  .example2 {
+    wut: 1;
+  }
+}
 #parenthesisNot-true {
   parenthesisNot: just-value;
   parenthesisNot: negated twice 1;

packages/test-data/less/_main/mixins-guards.less

@@ -270,6 +270,43 @@
   #top > #deeper > .mixin(1);
 }
 
+// guarded namespaced should pass when true
+#desktop (@ruleset) {
+  @media (min-width: 700px) {
+    @ruleset();
+  }
+}
+
+@switch: demo;
+
+.example1 {
+  #lookup when (@switch = demo) {
+    @not-found: 1;
+  }
+
+  @found: 2;
+
+  #lookup2 {
+    @also-found: 3;
+  }
+
+  #desktop({
+    success1: #lookup[@not-found];
+    success2: @found;
+    success3: #lookup2[@also-found];
+  });
+}
+
+.example2 {
+  #lookup () when (@switch = demo) {
+    @not-found: 1;
+  }
+
+  #desktop({
+    wut: #lookup[@not-found];
+  });
+}
+
 // namespaced & guarded mixin in root
 // outputs nothing but should pass: