use the same environment frames for guards as for evaluating parameters

test/css/mixins-guards.css

@@ -74,3 +74,7 @@
   declare: 4;
   declare: 4px;
 }
+.call-lock-mixin .call-inner-lock-mixin {
+  a: 1;
+  x: 1;
+}

test/less/mixins-guards.less

@@ -134,4 +134,17 @@
 }
 #tryNumberPx {
   .mixin(4px);
+}
+
+.lock-mixin(@a) {
+ .inner-locked-mixin(@x: @a) when (@a = 1) {
+    a: @a;
+    x: @x;
+  }
+}
+.call-lock-mixin {
+  .lock-mixin(1);
+  .call-inner-lock-mixin {
+    .inner-locked-mixin();
+  }
 }