diff --git a/packages/babel-helper-create-class-features-plugin/src/index.js b/packages/babel-helper-create-class-features-plugin/src/index.js
index 5aa2db723519..19da30a62e03 100644
--- a/packages/babel-helper-create-class-features-plugin/src/index.js
+++ b/packages/babel-helper-create-class-features-plugin/src/index.js
@@ -20,7 +20,7 @@ import {
   isLoose,
 } from "./features";
 
-export { FEATURES, injectInitialization };
+export { FEATURES, enableFeature, injectInitialization };
 
 // Note: Versions are represented as an integer. e.g. 7.1.5 is represented
 //       as 70000100005. This method is easier than using a semver-parsing
diff --git a/packages/babel-plugin-proposal-private-property-in-object/package.json b/packages/babel-plugin-proposal-private-property-in-object/package.json
index 385e50f0436d..464b0c27620d 100644
--- a/packages/babel-plugin-proposal-private-property-in-object/package.json
+++ b/packages/babel-plugin-proposal-private-property-in-object/package.json
@@ -17,8 +17,10 @@
     "babel-plugin"
   ],
   "dependencies": {
+    "@babel/helper-compilation-targets": "workspace:^7.12.17",
     "@babel/helper-create-class-features-plugin": "workspace:^7.13.0",
-    "@babel/helper-plugin-utils": "workspace:^7.13.0"
+    "@babel/helper-plugin-utils": "workspace:^7.13.0",
+    "@babel/plugin-syntax-private-property-in-object": "workspace:^7.13.0"
   },
   "peerDependencies": {
     "@babel/core": "^7.0.0-0"
diff --git a/packages/babel-plugin-proposal-private-property-in-object/src/index.js b/packages/babel-plugin-proposal-private-property-in-object/src/index.js
index c8c5c7bddcc0..2beb98dae688 100644
--- a/packages/babel-plugin-proposal-private-property-in-object/src/index.js
+++ b/packages/babel-plugin-proposal-private-property-in-object/src/index.js
@@ -1,23 +1,158 @@
-/* eslint-disable @babel/development/plugin-name */
-
 import { declare } from "@babel/helper-plugin-utils";
+import syntaxPlugin from "@babel/plugin-syntax-private-property-in-object";
 import {
-  createClassFeaturePlugin,
+  enableFeature,
   FEATURES,
+  injectInitialization as injectConstructorInit,
 } from "@babel/helper-create-class-features-plugin";
 
-export default declare((api, options) => {
-  api.assertVersion(7);
+export default declare(({ assertVersion, types: t, template }, { loose }) => {
+  assertVersion(7);
+
+  // NOTE: When using the class fields or private methods plugins,
+  // they will also take care of '#priv in obj' checks when visiting
+  // the ClassExpression or ClassDeclaration nodes.
+  // The visitor of this plugin is only effective when not compiling
+  // private fields and methods.
+
+  const classWeakSets = new WeakMap();
+  const fieldsWeakSets = new WeakMap();
+
+  function unshadow(name, targetScope, scope) {
+    while (scope !== targetScope) {
+      if (scope.hasOwnBinding(name)) scope.rename(name);
+      scope = scope.parent;
+    }
+  }
+
+  function injectToFieldInit(fieldPath, expr, before = false) {
+    if (fieldPath.node.value) {
+      if (before) {
+        fieldPath.get("value").insertBefore(expr);
+      } else {
+        fieldPath.get("value").insertAfter(expr);
+      }
+    } else {
+      fieldPath.set("value", t.unaryExpression("void", expr));
+    }
+  }
+
+  function injectInitialization(classPath, init) {
+    let firstFieldPath;
+    let consturctorPath;
+
+    for (const el of classPath.get("body.body")) {
+      if (
+        (el.isClassProperty() || el.isClassPrivateProperty()) &&
+        !el.node.static
+      ) {
+        firstFieldPath = el;
+        break;
+      }
+      if (!consturctorPath && el.isClassMethod({ kind: "constructor" })) {
+        consturctorPath = el;
+      }
+    }
+
+    if (firstFieldPath) {
+      injectToFieldInit(firstFieldPath, init, true);
+    } else {
+      injectConstructorInit(classPath, consturctorPath, [
+        t.expressionStatement(init),
+      ]);
+    }
+  }
+
+  function getWeakSetId(weakSets, outerClass, reference, name = "", inject) {
+    let id = classWeakSets.get(reference.node);
+
+    if (!id) {
+      id = outerClass.scope.generateUidIdentifier(`${name || ""} brandCheck`);
+      classWeakSets.set(reference.node, id);
+
+      inject(reference, template.expression.ast`${t.cloneNode(id)}.add(this)`);
+
+      outerClass.insertBefore(template.ast`var ${id} = new WeakSet()`);
+    }
+
+    return t.cloneNode(id);
+  }
+
+  return {
+    name: "proposal-private-property-in-object",
+    inherits: syntaxPlugin,
+    pre() {
+      // Enable this in @babel/helper-create-class-features-plugin, so that it
+      // can be handled by the private fields and methods transform.
+      enableFeature(this.file, FEATURES.privateIn, loose);
+    },
+    visitor: {
+      BinaryExpression(path) {
+        const { node } = path;
+        if (node.operator !== "in") return;
+        if (!t.isPrivateName(node.left)) return;
+
+        const { name } = node.left.id;
+
+        let privateElement;
+        const outerClass = path.findParent(path => {
+          if (!path.isClass()) return false;
+
+          privateElement = path
+            .get("body.body")
+            .find(({ node }) => t.isPrivate(node) && node.key.id.name === name);
+
+          return !!privateElement;
+        });
+
+        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 (privateElement.isMethod()) {
+          if (privateElement.node.static) {
+            if (outerClass.node.id) {
+              unshadow(outerClass.node.id.name, outerClass.scope, path.scope);
+            } else {
+              outerClass.set("id", path.scope.generateUidIdentifier("class"));
+            }
+            path.replaceWith(
+              template.expression.ast`
+                ${t.cloneNode(outerClass.node.id)} === ${path.node.right}
+              `,
+            );
+          } else {
+            const id = getWeakSetId(
+              classWeakSets,
+              outerClass,
+              outerClass,
+              outerClass.node.id?.name,
+              injectInitialization,
+            );
 
-  return createClassFeaturePlugin({
-    name: "proposal-class-properties",
+            path.replaceWith(
+              template.expression.ast`${id}.has(${path.node.right})`,
+            );
+          }
+        } else {
+          // Private fields might not all be initialized: see the 'halfConstructed'
+          // example at https://v8.dev/features/private-brand-checks.
 
-    api,
-    feature: FEATURES.privateIn,
-    loose: options.loose,
+          const id = getWeakSetId(
+            fieldsWeakSets,
+            outerClass,
+            privateElement,
+            privateElement.node.key.id.name,
+            injectToFieldInit,
+          );
 
-    manipulateOptions(opts, parserOpts) {
-      parserOpts.plugins.push("privateIn");
+          path.replaceWith(
+            template.expression.ast`${id}.has(${path.node.right})`,
+          );
+        }
+      },
     },
-  });
+  };
 });
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/accessor/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/accessor/input.js
new file mode 100644
index 000000000000..c0e9000615fd
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/accessor/input.js
@@ -0,0 +1,7 @@
+class Foo {
+  get #foo() {}
+
+  test(other) {
+    return #foo in other;
+  }
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/accessor/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/accessor/output.js
new file mode 100644
index 000000000000..7a64679f5a1a
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/accessor/output.js
@@ -0,0 +1,14 @@
+var _FooBrandCheck = new WeakSet();
+
+class Foo {
+  constructor() {
+    _FooBrandCheck.add(this);
+  }
+
+  get #foo() {}
+
+  test(other) {
+    return _FooBrandCheck.has(other);
+  }
+
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-in-default-param/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-in-default-param/input.js
new file mode 100644
index 000000000000..e7f65a90b196
--- /dev/null
+++ b/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 }
+}) => {}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-in-default-param/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-in-default-param/output.js
new file mode 100644
index 000000000000..5289f0c5e834
--- /dev/null
+++ b/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 _fooBrandCheck;
+
+  return _fooBrandCheck = new WeakSet(), class {
+    #foo = void _fooBrandCheck.add(this);
+
+    test(other) {
+      return _fooBrandCheck.has(other);
+    }
+
+  };
+})()) => {};
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-instance/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-instance/input.js
new file mode 100644
index 000000000000..3d817b01b547
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-instance/input.js
@@ -0,0 +1,9 @@
+function fn() {
+  return new class {
+    #priv;
+
+    method(obj) {
+      return #priv in obj;
+    }
+  }
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-instance/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-instance/output.js
new file mode 100644
index 000000000000..495153fae073
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-instance/output.js
@@ -0,0 +1,12 @@
+function fn() {
+  var _privBrandCheck;
+
+  return new (_privBrandCheck = new WeakSet(), class {
+    #priv = void _privBrandCheck.add(this);
+
+    method(obj) {
+      return _privBrandCheck.has(obj);
+    }
+
+  })();
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-static/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-static/input.js
new file mode 100644
index 000000000000..8f394f3d4c76
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-static/input.js
@@ -0,0 +1,9 @@
+function fn() {
+  return new class {
+    static #priv;
+
+    method(obj) {
+      return #priv in obj;
+    }
+  }
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-static/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-static/output.js
new file mode 100644
index 000000000000..39716aed530a
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/class-expression-static/output.js
@@ -0,0 +1,12 @@
+function fn() {
+  var _privBrandCheck;
+
+  return new (_privBrandCheck = new WeakSet(), class {
+    static #priv = void _privBrandCheck.add(this);
+
+    method(obj) {
+      return _privBrandCheck.has(obj);
+    }
+
+  })();
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/field/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/field/input.js
new file mode 100644
index 000000000000..a682a1a15c5b
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/field/input.js
@@ -0,0 +1,7 @@
+class Foo {
+  #foo = 1;
+
+  test(other) {
+    return #foo in other;
+  }
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/field/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/field/output.js
new file mode 100644
index 000000000000..7ff1de3959f6
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/field/output.js
@@ -0,0 +1,12 @@
+var _temp;
+
+var _fooBrandCheck = new WeakSet();
+
+class Foo {
+  #foo = (_temp = 1, _fooBrandCheck.add(this), _temp);
+
+  test(other) {
+    return _fooBrandCheck.has(other);
+  }
+
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-instance/exec.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-instance/exec.js
new file mode 100644
index 000000000000..733062328585
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-instance/exec.js
@@ -0,0 +1,28 @@
+let hasW, hasX, hasY, hasZ;
+let halfConstructed;
+
+class F {
+  m() {
+    hasW = #w in this;
+    hasX = #x in this;
+    hasY = #y in this;
+    hasZ = #z in this;
+  }
+  get #w() {}
+  #x = 0;
+  #y = (() => {
+    halfConstructed = this;
+    throw "error";
+  })();
+  #z() {}
+}
+
+try {
+  new F();
+} catch {}
+halfConstructed.m();
+
+expect(hasW).toBe(true);
+expect(hasX).toBe(true);
+expect(hasY).toBe(false);
+expect(hasZ).toBe(true);
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-instance/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-instance/input.js
new file mode 100644
index 000000000000..048cedcd5bf4
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-instance/input.js
@@ -0,0 +1,14 @@
+class F {
+  m() {
+    #w in this;
+    #x in this;
+    #y in this;
+    #z in this;
+  }
+  get #w() {}
+  #x = 0;
+  #y = (() => {
+    throw 'error';
+  })();
+  #z() {}
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-instance/options.json b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-instance/options.json
new file mode 100644
index 000000000000..1655c55f42cc
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-instance/options.json
@@ -0,0 +1,3 @@
+{
+  "minNodeVersion": "14.0.0"
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-instance/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-instance/output.js
new file mode 100644
index 000000000000..4d8bbd2a8b80
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-instance/output.js
@@ -0,0 +1,29 @@
+var _temp, _temp2;
+
+var _FBrandCheck = new WeakSet();
+
+var _xBrandCheck = new WeakSet();
+
+var _yBrandCheck = new WeakSet();
+
+class F {
+  m() {
+    _FBrandCheck.has(this);
+
+    _xBrandCheck.has(this);
+
+    _yBrandCheck.has(this);
+
+    _FBrandCheck.has(this);
+  }
+
+  get #w() {}
+
+  #x = (_temp = (_FBrandCheck.add(this), 0), _xBrandCheck.add(this), _temp);
+  #y = (_temp2 = (() => {
+    throw 'error';
+  })(), _yBrandCheck.add(this), _temp2);
+
+  #z() {}
+
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-static/exec.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-static/exec.js
new file mode 100644
index 000000000000..f60e0b24fde7
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-static/exec.js
@@ -0,0 +1,27 @@
+let hasW, hasX, hasY, hasZ;
+let halfConstructed;
+
+try {
+  class F {
+    static m() {
+      hasW = #w in this;
+      hasX = #x in this;
+      hasY = #y in this;
+      hasZ = #z in this;
+    }
+    static get #w() {}
+    static #x = 0;
+    static #y = (() => {
+      halfConstructed = this;
+      throw "error";
+    })();
+    static #z() {}
+  }
+} catch {}
+
+halfConstructed.m();
+
+expect(hasW).toBe(true);
+expect(hasX).toBe(true);
+expect(hasY).toBe(false);
+expect(hasZ).toBe(true);
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-static/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-static/input.js
new file mode 100644
index 000000000000..9b6ff923f33a
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-static/input.js
@@ -0,0 +1,12 @@
+class F {
+  static m() {
+    #x in this;
+    #y in this;
+    #z in this;
+  }
+  static #x = 0;
+  static #y = (() => {
+    throw 'error';
+  })();
+  static #z() {}
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-static/options.json b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-static/options.json
new file mode 100644
index 000000000000..1655c55f42cc
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-static/options.json
@@ -0,0 +1,3 @@
+{
+  "minNodeVersion": "14.0.0"
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-static/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-static/output.js
new file mode 100644
index 000000000000..e0ebbc3e2758
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/half-constructed-static/output.js
@@ -0,0 +1,23 @@
+var _temp, _temp2;
+
+var _xBrandCheck = new WeakSet();
+
+var _yBrandCheck = new WeakSet();
+
+class F {
+  static m() {
+    _xBrandCheck.has(this);
+
+    _yBrandCheck.has(this);
+
+    F === this;
+  }
+
+  static #x = (_temp = 0, _xBrandCheck.add(this), _temp);
+  static #y = (_temp2 = (() => {
+    throw 'error';
+  })(), _yBrandCheck.add(this), _temp2);
+
+  static #z() {}
+
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/method/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/method/input.js
new file mode 100644
index 000000000000..365843c59708
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/method/input.js
@@ -0,0 +1,7 @@
+class Foo {
+  #foo() {}
+
+  test(other) {
+    return #foo in other;
+  }
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/method/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/method/output.js
new file mode 100644
index 000000000000..59a6fb1062f8
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/method/output.js
@@ -0,0 +1,14 @@
+var _FooBrandCheck = new WeakSet();
+
+class Foo {
+  constructor() {
+    _FooBrandCheck.add(this);
+  }
+
+  #foo() {}
+
+  test(other) {
+    return _FooBrandCheck.has(other);
+  }
+
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/multiple-checks/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/multiple-checks/input.js
new file mode 100644
index 000000000000..bb22d4fbdb0e
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/multiple-checks/input.js
@@ -0,0 +1,11 @@
+class A {
+  #x;
+  #m() {}
+
+  test() {
+    #x in this;
+    #m in this;
+    #x in this;
+    #m in this;
+  }
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/multiple-checks/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/multiple-checks/output.js
new file mode 100644
index 000000000000..98dc5cd7d891
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/multiple-checks/output.js
@@ -0,0 +1,20 @@
+var _xBrandCheck = new WeakSet();
+
+var _ABrandCheck = new WeakSet();
+
+class A {
+  #x = (_ABrandCheck.add(this), void _xBrandCheck.add(this));
+
+  #m() {}
+
+  test() {
+    _xBrandCheck.has(this);
+
+    _ABrandCheck.has(this);
+
+    _xBrandCheck.has(this);
+
+    _ABrandCheck.has(this);
+  }
+
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class-other-redeclared/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class-other-redeclared/input.js
new file mode 100644
index 000000000000..a084dd06afd6
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class-other-redeclared/input.js
@@ -0,0 +1,18 @@
+class Foo {
+  #foo = 1;
+  #bar = 1;
+
+  test() {
+    class Nested {
+      #bar = 2;
+
+      test() {
+        #foo in this;
+        #bar in this;
+      }
+    }
+
+    #foo in this;
+    #bar in this;
+  }
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class-other-redeclared/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class-other-redeclared/output.js
new file mode 100644
index 000000000000..2f11e248f111
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class-other-redeclared/output.js
@@ -0,0 +1,32 @@
+var _temp, _temp3;
+
+var _fooBrandCheck = new WeakSet();
+
+var _barBrandCheck2 = new WeakSet();
+
+class Foo {
+  #foo = (_temp = 1, _fooBrandCheck.add(this), _temp);
+  #bar = (_temp3 = 1, _barBrandCheck2.add(this), _temp3);
+
+  test() {
+    var _temp2;
+
+    var _barBrandCheck = new WeakSet();
+
+    class Nested {
+      #bar = (_temp2 = 2, _barBrandCheck.add(this), _temp2);
+
+      test() {
+        _fooBrandCheck.has(this);
+
+        _barBrandCheck.has(this);
+      }
+
+    }
+
+    _fooBrandCheck.has(this);
+
+    _barBrandCheck2.has(this);
+  }
+
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class-redeclared/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class-redeclared/input.js
new file mode 100644
index 000000000000..2258be35c5fd
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class-redeclared/input.js
@@ -0,0 +1,15 @@
+class Foo {
+  #foo = 1;
+
+  test() {
+    class Nested {
+      #foo = 2;
+
+      test() {
+        #foo in this;
+      }
+    }
+
+    #foo in this;
+  }
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class-redeclared/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class-redeclared/output.js
new file mode 100644
index 000000000000..87094388d2ab
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class-redeclared/output.js
@@ -0,0 +1,25 @@
+var _temp2;
+
+var _fooBrandCheck2 = new WeakSet();
+
+class Foo {
+  #foo = (_temp2 = 1, _fooBrandCheck2.add(this), _temp2);
+
+  test() {
+    var _temp;
+
+    var _fooBrandCheck = new WeakSet();
+
+    class Nested {
+      #foo = (_temp = 2, _fooBrandCheck.add(this), _temp);
+
+      test() {
+        _fooBrandCheck.has(this);
+      }
+
+    }
+
+    _fooBrandCheck2.has(this);
+  }
+
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class/input.js
new file mode 100644
index 000000000000..e17e438b314d
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class/input.js
@@ -0,0 +1,13 @@
+class Foo {
+  #foo = 1;
+
+  test() {
+    class Nested {
+      test() {
+        #foo in this;
+      }
+    }
+
+    #foo in this;
+  }
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class/output.js
new file mode 100644
index 000000000000..c897b8d6c8e1
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/nested-class/output.js
@@ -0,0 +1,19 @@
+var _temp;
+
+var _fooBrandCheck = new WeakSet();
+
+class Foo {
+  #foo = (_temp = 1, _fooBrandCheck.add(this), _temp);
+
+  test() {
+    class Nested {
+      test() {
+        _fooBrandCheck.has(this);
+      }
+
+    }
+
+    _fooBrandCheck.has(this);
+  }
+
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/options.json b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/options.json
new file mode 100644
index 000000000000..6a19479b9c2b
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/options.json
@@ -0,0 +1,3 @@
+{
+  "plugins": ["proposal-private-property-in-object", "syntax-class-properties"]
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-accessor/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-accessor/input.js
new file mode 100644
index 000000000000..e74333f2055f
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-accessor/input.js
@@ -0,0 +1,7 @@
+class Foo {
+  static get #foo() {}
+
+  test(other) {
+    return #foo in other;
+  }
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-accessor/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-accessor/output.js
new file mode 100644
index 000000000000..cdec0d002314
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-accessor/output.js
@@ -0,0 +1,8 @@
+class Foo {
+  static get #foo() {}
+
+  test(other) {
+    return Foo === other;
+  }
+
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-field/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-field/input.js
new file mode 100644
index 000000000000..a4f771844fe9
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-field/input.js
@@ -0,0 +1,7 @@
+class Foo {
+  static #foo = 1;
+
+  test(other) {
+    return #foo in other;
+  }
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-field/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-field/output.js
new file mode 100644
index 000000000000..ceadc64ac495
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-field/output.js
@@ -0,0 +1,12 @@
+var _temp;
+
+var _fooBrandCheck = new WeakSet();
+
+class Foo {
+  static #foo = (_temp = 1, _fooBrandCheck.add(this), _temp);
+
+  test(other) {
+    return _fooBrandCheck.has(other);
+  }
+
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-method/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-method/input.js
new file mode 100644
index 000000000000..9fffe7a01622
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-method/input.js
@@ -0,0 +1,7 @@
+class Foo {
+  static #foo() {}
+
+  test(other) {
+    return #foo in other;
+  }
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-method/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-method/output.js
new file mode 100644
index 000000000000..bb657d68eb23
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-method/output.js
@@ -0,0 +1,8 @@
+class Foo {
+  static #foo() {}
+
+  test(other) {
+    return Foo === other;
+  }
+
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-shadowed-binding/input.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-shadowed-binding/input.js
new file mode 100644
index 000000000000..67206379b189
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-shadowed-binding/input.js
@@ -0,0 +1,9 @@
+class A {
+  static #foo;
+
+  test() {
+    let A = function fn(A) {
+      return #foo in A;
+    };
+  }
+}
diff --git a/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-shadowed-binding/output.js b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-shadowed-binding/output.js
new file mode 100644
index 000000000000..b89f6ea5a3cb
--- /dev/null
+++ b/packages/babel-plugin-proposal-private-property-in-object/test/fixtures/to-native-fields/static-shadowed-binding/output.js
@@ -0,0 +1,12 @@
+var _fooBrandCheck = new WeakSet();
+
+class A {
+  static #foo = void _fooBrandCheck.add(this);
+
+  test() {
+    let A = function fn(A) {
+      return _fooBrandCheck.has(A);
+    };
+  }
+
+}
diff --git a/packages/babel-plugin-syntax-private-property-in-object/.npmignore b/packages/babel-plugin-syntax-private-property-in-object/.npmignore
new file mode 100644
index 000000000000..f9806945836e
--- /dev/null
+++ b/packages/babel-plugin-syntax-private-property-in-object/.npmignore
@@ -0,0 +1,3 @@
+src
+test
+*.log
diff --git a/packages/babel-plugin-syntax-private-property-in-object/README.md b/packages/babel-plugin-syntax-private-property-in-object/README.md
new file mode 100644
index 000000000000..8dd2f6b32cb2
--- /dev/null
+++ b/packages/babel-plugin-syntax-private-property-in-object/README.md
@@ -0,0 +1,19 @@
+# @babel/plugin-syntax-class-static-block
+
+> Allow parsing of class static blocks
+
+See our website [@babel/plugin-syntax-class-static-block](https://babeljs.io/docs/en/babel-plugin-syntax-class-static-block) for more information.
+
+## Install
+
+Using npm:
+
+```sh
+npm install --save-dev @babel/plugin-syntax-class-static-block
+```
+
+or using yarn:
+
+```sh
+yarn add @babel/plugin-syntax-class-static-block --dev
+```
diff --git a/packages/babel-plugin-syntax-private-property-in-object/package.json b/packages/babel-plugin-syntax-private-property-in-object/package.json
new file mode 100644
index 000000000000..544e52ee4bea
--- /dev/null
+++ b/packages/babel-plugin-syntax-private-property-in-object/package.json
@@ -0,0 +1,28 @@
+{
+  "name": "@babel/plugin-syntax-private-property-in-object",
+  "version": "7.13.0",
+  "description": "Allow parsing of '#foo in obj' brand checks",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/babel/babel.git",
+    "directory": "packages/babel-plugin-syntax-private-property-in-object"
+  },
+  "homepage": "https://babel.dev/docs/en/next/babel-plugin-syntax-private-property-in-object",
+  "license": "MIT",
+  "publishConfig": {
+    "access": "public"
+  },
+  "main": "./lib/index.js",
+  "exports": {
+    ".": "./lib/index.js"
+  },
+  "keywords": [
+    "babel-plugin"
+  ],
+  "dependencies": {
+    "@babel/helper-plugin-utils": "workspace:^7.12.13"
+  },
+  "peerDependencies": {
+    "@babel/core": "^7.0.0-0"
+  }
+}
diff --git a/packages/babel-plugin-syntax-private-property-in-object/src/index.js b/packages/babel-plugin-syntax-private-property-in-object/src/index.js
new file mode 100644
index 000000000000..a716fe39e301
--- /dev/null
+++ b/packages/babel-plugin-syntax-private-property-in-object/src/index.js
@@ -0,0 +1,13 @@
+import { declare } from "@babel/helper-plugin-utils";
+
+export default declare(api => {
+  api.assertVersion(7);
+
+  return {
+    name: "syntax-private-property-in-object",
+
+    manipulateOptions(opts, parserOpts) {
+      parserOpts.plugins.push("privateIn");
+    },
+  };
+});
diff --git a/yarn.lock b/yarn.lock
index dab65b379707..cf71ba6c9b3f 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -393,7 +393,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"@babel/helper-compilation-targets@workspace:^7.13.13, @babel/helper-compilation-targets@workspace:^7.13.16, @babel/helper-compilation-targets@workspace:^7.13.8, @babel/helper-compilation-targets@workspace:packages/babel-helper-compilation-targets":
+"@babel/helper-compilation-targets@workspace:^7.12.17, @babel/helper-compilation-targets@workspace:^7.13.13, @babel/helper-compilation-targets@workspace:^7.13.16, @babel/helper-compilation-targets@workspace:^7.13.8, @babel/helper-compilation-targets@workspace:packages/babel-helper-compilation-targets":
   version: 0.0.0-use.local
   resolution: "@babel/helper-compilation-targets@workspace:packages/babel-helper-compilation-targets"
   dependencies:
@@ -1439,9 +1439,11 @@ __metadata:
   resolution: "@babel/plugin-proposal-private-property-in-object@workspace:packages/babel-plugin-proposal-private-property-in-object"
   dependencies:
     "@babel/core": "workspace:*"
+    "@babel/helper-compilation-targets": "workspace:^7.12.17"
     "@babel/helper-create-class-features-plugin": "workspace:^7.13.0"
     "@babel/helper-plugin-test-runner": "workspace:*"
     "@babel/helper-plugin-utils": "workspace:^7.13.0"
+    "@babel/plugin-syntax-private-property-in-object": "workspace:^7.13.0"
   peerDependencies:
     "@babel/core": ^7.0.0-0
   languageName: unknown
@@ -1816,6 +1818,16 @@ __metadata:
   languageName: unknown
   linkType: soft
 
+"@babel/plugin-syntax-private-property-in-object@workspace:^7.13.0, @babel/plugin-syntax-private-property-in-object@workspace:packages/babel-plugin-syntax-private-property-in-object":
+  version: 0.0.0-use.local
+  resolution: "@babel/plugin-syntax-private-property-in-object@workspace:packages/babel-plugin-syntax-private-property-in-object"
+  dependencies:
+    "@babel/helper-plugin-utils": "workspace:^7.12.13"
+  peerDependencies:
+    "@babel/core": ^7.0.0-0
+  languageName: unknown
+  linkType: soft
+
 "@babel/plugin-syntax-record-and-tuple@workspace:*, @babel/plugin-syntax-record-and-tuple@workspace:^7.12.1, @babel/plugin-syntax-record-and-tuple@workspace:packages/babel-plugin-syntax-record-and-tuple":
   version: 0.0.0-use.local
   resolution: "@babel/plugin-syntax-record-and-tuple@workspace:packages/babel-plugin-syntax-record-and-tuple"