Fix in default params

packages/babel-plugin-proposal-private-property-in-object/src/native-private-fields.js

@@ -61,6 +61,12 @@ export default function pluginPrivateIn({ types: t, template }) {
           return true;
         });
 
+        if (outerClass.parentPath.scope.path.isPattern()) {
+          outerClass.replaceWith(template.ast`(() => ${outerClass.node})()`);
+          // The injected class will be queued and eventually transformed when visited
+          return;
+        }
+
         if (isStatic) {
           if (!outerClass.node.id) {
             outerClass.set("id", path.scope.generateUidIdentifier("class"));

packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-in-default-param/input.js

@@ -0,0 +1,4 @@
+(x = class {
+  #foo;
+  test(other) { return #foo in other }
+}) => {}

packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-in-default-param/output.js

@@ -0,0 +1,12 @@
+(x = (() => {
+  var _brandCheck;
+
+  return _brandCheck = new WeakSet(), class {
+    #foo = void _brandCheck.add(this);
+
+    test(other) {
+      return _brandCheck.has(other);
+    }
+
+  };
+})()) => {};