diff --git a/packages/babel-helper-member-expression-to-functions/src/index.js b/packages/babel-helper-member-expression-to-functions/src/index.js
index 62d844b9f177..0e5ef5ecb8e1 100644
--- a/packages/babel-helper-member-expression-to-functions/src/index.js
+++ b/packages/babel-helper-member-expression-to-functions/src/index.js
@@ -84,7 +84,7 @@ const handle = {
   },
 
   handle(member) {
-    const { node, parent, parentPath } = member;
+    const { node, parent, parentPath, scope } = member;
 
     if (member.isOptionalMemberExpression()) {
       // Transforming optional chaining requires we replace ancestors.
@@ -118,6 +118,17 @@ const handle = {
         return true;
       });
 
+      // Replace `function (a, x = a.b?.#c) {}` to `function (a, x = (() => a.b?.#c)() ){}`
+      // so the temporary variable can be injected in correct scope
+      // This can be further optimized to avoid unecessary IIFE
+      if (scope.path.isPattern()) {
+        endPath.replaceWith(
+          // The injected member will be queued and eventually transformed when visited
+          t.callExpression(t.arrowFunctionExpression([], endPath.node), []),
+        );
+        return;
+      }
+
       const rootParentPath = endPath.parentPath;
       if (
         rootParentPath.isUpdateExpression({ argument: node }) ||
@@ -165,7 +176,6 @@ const handle = {
         );
       }
 
-      const { scope } = member;
       const startingProp = startingOptional.isOptionalMemberExpression()
         ? "object"
         : "callee";
@@ -362,6 +372,16 @@ const handle = {
 
     // MEMBER?.(ARGS) -> _optionalCall(MEMBER, ARGS)
     if (parentPath.isOptionalCallExpression({ callee: node })) {
+      // Replace `function (a, x = a.b.#c?.()) {}` to `function (a, x = (() => a.b.#c?.())() ){}`
+      // so the temporary variable can be injected in correct scope
+      // This can be further optimized to avoid unecessary IIFE
+      if (scope.path.isPattern()) {
+        parentPath.replaceWith(
+          // The injected member will be queued and eventually transformed when visited
+          t.callExpression(t.arrowFunctionExpression([], parentPath.node), []),
+        );
+        return;
+      }
       parentPath.replaceWith(this.optionalCall(member, parent.arguments));
       return;
     }
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param-with-transform/exec.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param-with-transform/exec.js
new file mode 100644
index 000000000000..6eb26ab6e01a
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param-with-transform/exec.js
@@ -0,0 +1,46 @@
+class Foo {
+  static #x = 1;
+  static #m = function() { return this.#x; };
+  static #self = Foo;
+  static self = Foo;
+  static getSelf() { return this }
+
+  static test() {
+    const o = { Foo: Foo };
+    const deep = { very: { o } };
+    function fn() {
+      return o;
+    }
+    function fnDeep() {
+      return deep;
+    }
+
+    function f(o, r = o?.Foo.#m()) {
+      return r;
+    }
+
+    function g(o, r = o?.Foo.#self.getSelf().#m()) {
+      return r;
+    }
+
+    function h(fnDeep, r = fnDeep?.().very.o?.Foo?.#m()) {
+      return r;
+    }
+
+    function i(fn, r = fn?.().Foo.#self?.getSelf()?.self.#m()) {
+      return r;
+    }
+
+    function j(fn, r = (fn().Foo.#self.getSelf().self.#m)?.()) {
+      return r;
+    }
+
+    expect(f(o)).toBe(1);
+    expect(g(o)).toBe(1);
+    expect(h(fnDeep)).toBe(1);
+    expect(i(fn)).toBe(1);
+    expect(j(fn)).toBe(1);
+  }
+}
+
+Foo.test();
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param-with-transform/input.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param-with-transform/input.js
new file mode 100644
index 000000000000..c5fe33636eca
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param-with-transform/input.js
@@ -0,0 +1,46 @@
+class Foo {
+  static #x = 1;
+  static #m = function() { return this.#x; };
+  static #self = Foo;
+  static self = Foo;
+  static getSelf() { return this }
+
+  static test() {
+    const o = { Foo: Foo };
+    const deep = { very: { o } };
+    function fn() {
+      return o;
+    }
+    function fnDeep() {
+      return deep;
+    }
+
+    function f(o, r = o?.Foo.#m()) {
+      return r;
+    }
+
+    function g(o, r = o?.Foo.#self.getSelf().#m?.()) {
+      return r;
+    }
+
+    function h(fnDeep, r = fnDeep?.().very.o?.Foo?.#m()) {
+      return r;
+    }
+
+    function i(fn, r = fn?.().Foo.#self?.getSelf()?.self.#m()) {
+      return r;
+    }
+
+    function j(fn, r = (fn().Foo.#self.getSelf().self.#m)?.()) {
+      return r;
+    }
+
+    f(o);
+    g(o);
+    h(fnDeep);
+    i(fn);
+    j(fn);
+  }
+}
+
+Foo.test();
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param-with-transform/options.json b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param-with-transform/options.json
new file mode 100644
index 000000000000..124133b5af2b
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param-with-transform/options.json
@@ -0,0 +1,6 @@
+{
+  "plugins": [
+    ["proposal-optional-chaining", { "loose": true }],
+    ["proposal-class-properties", { "loose": true }]
+  ]
+}
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param-with-transform/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param-with-transform/output.js
new file mode 100644
index 000000000000..fb3516c4b811
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param-with-transform/output.js
@@ -0,0 +1,96 @@
+function _classPrivateFieldLooseBase(receiver, privateKey) { if (!Object.prototype.hasOwnProperty.call(receiver, privateKey)) { throw new TypeError("attempted to use private field on non-instance"); } return receiver; }
+
+var id = 0;
+
+function _classPrivateFieldLooseKey(name) { return "__private_" + id++ + "_" + name; }
+
+var _x = _classPrivateFieldLooseKey("x");
+
+var _m = _classPrivateFieldLooseKey("m");
+
+var _self = _classPrivateFieldLooseKey("self");
+
+class Foo {
+  static getSelf() {
+    return this;
+  }
+
+  static test() {
+    const o = {
+      Foo: Foo
+    };
+    const deep = {
+      very: {
+        o
+      }
+    };
+
+    function fn() {
+      return o;
+    }
+
+    function fnDeep() {
+      return deep;
+    }
+
+    function f(o, r = (() => o === null || o === void 0 ? void 0 : _classPrivateFieldLooseBase(o.Foo, _m)[_m]())()) {
+      return r;
+    }
+
+    function g(o, r = (() => {
+      var _ref;
+
+      return (_ref = (() => o === null || o === void 0 ? void 0 : _classPrivateFieldLooseBase(_classPrivateFieldLooseBase(o.Foo, _self)[_self].getSelf(), _m)[_m])()) == null ? void 0 : _ref();
+    })()) {
+      return r;
+    }
+
+    function h(fnDeep, r = (() => {
+      var _fnDeep$very$o$Foo, _fnDeep$very$o;
+
+      return (_fnDeep$very$o$Foo = fnDeep == null ? void 0 : (_fnDeep$very$o = fnDeep().very.o) == null ? void 0 : _fnDeep$very$o.Foo) === null || _fnDeep$very$o$Foo === void 0 ? void 0 : _classPrivateFieldLooseBase(_fnDeep$very$o$Foo, _m)[_m]();
+    })()) {
+      return r;
+    }
+
+    function i(fn, r = (() => {
+      var _getSelf, _ref2;
+
+      return (_getSelf = (_ref2 = (() => fn === null || fn === void 0 ? void 0 : _classPrivateFieldLooseBase(fn().Foo, _self)[_self])()) == null ? void 0 : _ref2.getSelf()) === null || _getSelf === void 0 ? void 0 : _classPrivateFieldLooseBase(_getSelf.self, _m)[_m]();
+    })()) {
+      return r;
+    }
+
+    function j(fn, r = (() => {
+      var _classPrivateFieldLoo, _classPrivateFieldLoo2;
+
+      return (_classPrivateFieldLoo = (_classPrivateFieldLoo2 = _classPrivateFieldLooseBase(_classPrivateFieldLooseBase(fn().Foo, _self)[_self].getSelf().self, _m))[_m]) == null ? void 0 : _classPrivateFieldLoo.call(_classPrivateFieldLoo2);
+    })()) {
+      return r;
+    }
+
+    f(o);
+    g(o);
+    h(fnDeep);
+    i(fn);
+    j(fn);
+  }
+
+}
+
+Object.defineProperty(Foo, _x, {
+  writable: true,
+  value: 1
+});
+Object.defineProperty(Foo, _m, {
+  writable: true,
+  value: function () {
+    return _classPrivateFieldLooseBase(this, _x)[_x];
+  }
+});
+Object.defineProperty(Foo, _self, {
+  writable: true,
+  value: Foo
+});
+Foo.self = Foo;
+Foo.test();
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param/exec.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param/exec.js
new file mode 100644
index 000000000000..6eb26ab6e01a
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param/exec.js
@@ -0,0 +1,46 @@
+class Foo {
+  static #x = 1;
+  static #m = function() { return this.#x; };
+  static #self = Foo;
+  static self = Foo;
+  static getSelf() { return this }
+
+  static test() {
+    const o = { Foo: Foo };
+    const deep = { very: { o } };
+    function fn() {
+      return o;
+    }
+    function fnDeep() {
+      return deep;
+    }
+
+    function f(o, r = o?.Foo.#m()) {
+      return r;
+    }
+
+    function g(o, r = o?.Foo.#self.getSelf().#m()) {
+      return r;
+    }
+
+    function h(fnDeep, r = fnDeep?.().very.o?.Foo?.#m()) {
+      return r;
+    }
+
+    function i(fn, r = fn?.().Foo.#self?.getSelf()?.self.#m()) {
+      return r;
+    }
+
+    function j(fn, r = (fn().Foo.#self.getSelf().self.#m)?.()) {
+      return r;
+    }
+
+    expect(f(o)).toBe(1);
+    expect(g(o)).toBe(1);
+    expect(h(fnDeep)).toBe(1);
+    expect(i(fn)).toBe(1);
+    expect(j(fn)).toBe(1);
+  }
+}
+
+Foo.test();
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param/input.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param/input.js
new file mode 100644
index 000000000000..c5fe33636eca
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param/input.js
@@ -0,0 +1,46 @@
+class Foo {
+  static #x = 1;
+  static #m = function() { return this.#x; };
+  static #self = Foo;
+  static self = Foo;
+  static getSelf() { return this }
+
+  static test() {
+    const o = { Foo: Foo };
+    const deep = { very: { o } };
+    function fn() {
+      return o;
+    }
+    function fnDeep() {
+      return deep;
+    }
+
+    function f(o, r = o?.Foo.#m()) {
+      return r;
+    }
+
+    function g(o, r = o?.Foo.#self.getSelf().#m?.()) {
+      return r;
+    }
+
+    function h(fnDeep, r = fnDeep?.().very.o?.Foo?.#m()) {
+      return r;
+    }
+
+    function i(fn, r = fn?.().Foo.#self?.getSelf()?.self.#m()) {
+      return r;
+    }
+
+    function j(fn, r = (fn().Foo.#self.getSelf().self.#m)?.()) {
+      return r;
+    }
+
+    f(o);
+    g(o);
+    h(fnDeep);
+    i(fn);
+    j(fn);
+  }
+}
+
+Foo.test();
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param/options.json b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param/options.json
new file mode 100644
index 000000000000..2d5cfe8e8095
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param/options.json
@@ -0,0 +1,4 @@
+{
+  "plugins": [["proposal-class-properties", { "loose": true }]],
+  "minNodeVersion": "14.0.0"
+}
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param/output.js
new file mode 100644
index 000000000000..2b7d864e3b04
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private-loose/optional-chain-in-function-param/output.js
@@ -0,0 +1,88 @@
+function _classPrivateFieldLooseBase(receiver, privateKey) { if (!Object.prototype.hasOwnProperty.call(receiver, privateKey)) { throw new TypeError("attempted to use private field on non-instance"); } return receiver; }
+
+var id = 0;
+
+function _classPrivateFieldLooseKey(name) { return "__private_" + id++ + "_" + name; }
+
+var _x = _classPrivateFieldLooseKey("x");
+
+var _m = _classPrivateFieldLooseKey("m");
+
+var _self = _classPrivateFieldLooseKey("self");
+
+class Foo {
+  static getSelf() {
+    return this;
+  }
+
+  static test() {
+    const o = {
+      Foo: Foo
+    };
+    const deep = {
+      very: {
+        o
+      }
+    };
+
+    function fn() {
+      return o;
+    }
+
+    function fnDeep() {
+      return deep;
+    }
+
+    function f(o, r = (() => o === null || o === void 0 ? void 0 : _classPrivateFieldLooseBase(o.Foo, _m)[_m]())()) {
+      return r;
+    }
+
+    function g(o, r = (() => o === null || o === void 0 ? void 0 : _classPrivateFieldLooseBase(_classPrivateFieldLooseBase(o.Foo, _self)[_self].getSelf(), _m)[_m])()?.()) {
+      return r;
+    }
+
+    function h(fnDeep, r = (() => {
+      var _fnDeep$very$o$Foo;
+
+      return (_fnDeep$very$o$Foo = fnDeep?.().very.o?.Foo) === null || _fnDeep$very$o$Foo === void 0 ? void 0 : _classPrivateFieldLooseBase(_fnDeep$very$o$Foo, _m)[_m]();
+    })()) {
+      return r;
+    }
+
+    function i(fn, r = (() => {
+      var _getSelf;
+
+      return (_getSelf = (() => fn === null || fn === void 0 ? void 0 : _classPrivateFieldLooseBase(fn().Foo, _self)[_self])()?.getSelf()) === null || _getSelf === void 0 ? void 0 : _classPrivateFieldLooseBase(_getSelf.self, _m)[_m]();
+    })()) {
+      return r;
+    }
+
+    function j(fn, r = (() => _classPrivateFieldLooseBase(_classPrivateFieldLooseBase(fn().Foo, _self)[_self].getSelf().self, _m)[_m]?.())()) {
+      return r;
+    }
+
+    f(o);
+    g(o);
+    h(fnDeep);
+    i(fn);
+    j(fn);
+  }
+
+}
+
+Object.defineProperty(Foo, _x, {
+  writable: true,
+  value: 1
+});
+Object.defineProperty(Foo, _m, {
+  writable: true,
+  value: function () {
+    return _classPrivateFieldLooseBase(this, _x)[_x];
+  }
+});
+Object.defineProperty(Foo, _self, {
+  writable: true,
+  value: Foo
+});
+Foo.self = Foo;
+Foo.test();
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param-with-transform/exec.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param-with-transform/exec.js
new file mode 100644
index 000000000000..6eb26ab6e01a
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param-with-transform/exec.js
@@ -0,0 +1,46 @@
+class Foo {
+  static #x = 1;
+  static #m = function() { return this.#x; };
+  static #self = Foo;
+  static self = Foo;
+  static getSelf() { return this }
+
+  static test() {
+    const o = { Foo: Foo };
+    const deep = { very: { o } };
+    function fn() {
+      return o;
+    }
+    function fnDeep() {
+      return deep;
+    }
+
+    function f(o, r = o?.Foo.#m()) {
+      return r;
+    }
+
+    function g(o, r = o?.Foo.#self.getSelf().#m()) {
+      return r;
+    }
+
+    function h(fnDeep, r = fnDeep?.().very.o?.Foo?.#m()) {
+      return r;
+    }
+
+    function i(fn, r = fn?.().Foo.#self?.getSelf()?.self.#m()) {
+      return r;
+    }
+
+    function j(fn, r = (fn().Foo.#self.getSelf().self.#m)?.()) {
+      return r;
+    }
+
+    expect(f(o)).toBe(1);
+    expect(g(o)).toBe(1);
+    expect(h(fnDeep)).toBe(1);
+    expect(i(fn)).toBe(1);
+    expect(j(fn)).toBe(1);
+  }
+}
+
+Foo.test();
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param-with-transform/input.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param-with-transform/input.js
new file mode 100644
index 000000000000..c5fe33636eca
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param-with-transform/input.js
@@ -0,0 +1,46 @@
+class Foo {
+  static #x = 1;
+  static #m = function() { return this.#x; };
+  static #self = Foo;
+  static self = Foo;
+  static getSelf() { return this }
+
+  static test() {
+    const o = { Foo: Foo };
+    const deep = { very: { o } };
+    function fn() {
+      return o;
+    }
+    function fnDeep() {
+      return deep;
+    }
+
+    function f(o, r = o?.Foo.#m()) {
+      return r;
+    }
+
+    function g(o, r = o?.Foo.#self.getSelf().#m?.()) {
+      return r;
+    }
+
+    function h(fnDeep, r = fnDeep?.().very.o?.Foo?.#m()) {
+      return r;
+    }
+
+    function i(fn, r = fn?.().Foo.#self?.getSelf()?.self.#m()) {
+      return r;
+    }
+
+    function j(fn, r = (fn().Foo.#self.getSelf().self.#m)?.()) {
+      return r;
+    }
+
+    f(o);
+    g(o);
+    h(fnDeep);
+    i(fn);
+    j(fn);
+  }
+}
+
+Foo.test();
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param-with-transform/options.json b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param-with-transform/options.json
new file mode 100644
index 000000000000..63b4c77cc8e8
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param-with-transform/options.json
@@ -0,0 +1,3 @@
+{
+  "plugins": ["proposal-optional-chaining", "proposal-class-properties"]
+}
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param-with-transform/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param-with-transform/output.js
new file mode 100644
index 000000000000..1c52675c3085
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param-with-transform/output.js
@@ -0,0 +1,94 @@
+function _defineProperty(obj, key, value) { if (key in obj) { Object.defineProperty(obj, key, { value: value, enumerable: true, configurable: true, writable: true }); } else { obj[key] = value; } return obj; }
+
+function _classStaticPrivateFieldSpecGet(receiver, classConstructor, descriptor) { if (receiver !== classConstructor) { throw new TypeError("Private static access of wrong provenance"); } if (descriptor.get) { return descriptor.get.call(receiver); } return descriptor.value; }
+
+class Foo {
+  static getSelf() {
+    return this;
+  }
+
+  static test() {
+    const o = {
+      Foo: Foo
+    };
+    const deep = {
+      very: {
+        o
+      }
+    };
+
+    function fn() {
+      return o;
+    }
+
+    function fnDeep() {
+      return deep;
+    }
+
+    function f(o, r = (() => {
+      var _o$Foo;
+
+      return o === null || o === void 0 ? void 0 : _classStaticPrivateFieldSpecGet(_o$Foo = o.Foo, Foo, _m).call(_o$Foo);
+    })()) {
+      return r;
+    }
+
+    function g(o, r = (() => {
+      var _ref;
+
+      return (_ref = (() => o === null || o === void 0 ? void 0 : _classStaticPrivateFieldSpecGet(_classStaticPrivateFieldSpecGet(o.Foo, Foo, _self).getSelf(), Foo, _m))()) === null || _ref === void 0 ? void 0 : _ref();
+    })()) {
+      return r;
+    }
+
+    function h(fnDeep, r = (() => {
+      var _fnDeep$very$o$Foo, _fnDeep$very$o;
+
+      return (_fnDeep$very$o$Foo = fnDeep === null || fnDeep === void 0 ? void 0 : (_fnDeep$very$o = fnDeep().very.o) === null || _fnDeep$very$o === void 0 ? void 0 : _fnDeep$very$o.Foo) === null || _fnDeep$very$o$Foo === void 0 ? void 0 : _classStaticPrivateFieldSpecGet(_fnDeep$very$o$Foo, Foo, _m).call(_fnDeep$very$o$Foo);
+    })()) {
+      return r;
+    }
+
+    function i(fn, r = (() => {
+      var _getSelf, _getSelf$self, _ref2;
+
+      return (_getSelf = (_ref2 = (() => fn === null || fn === void 0 ? void 0 : _classStaticPrivateFieldSpecGet(fn().Foo, Foo, _self))()) === null || _ref2 === void 0 ? void 0 : _ref2.getSelf()) === null || _getSelf === void 0 ? void 0 : _classStaticPrivateFieldSpecGet(_getSelf$self = _getSelf.self, Foo, _m).call(_getSelf$self);
+    })()) {
+      return r;
+    }
+
+    function j(fn, r = (() => {
+      var _classStaticPrivateFi, _classStaticPrivateFi2;
+
+      return (_classStaticPrivateFi2 = _classStaticPrivateFieldSpecGet(_classStaticPrivateFi = _classStaticPrivateFieldSpecGet(fn().Foo, Foo, _self).getSelf().self, Foo, _m)) === null || _classStaticPrivateFi2 === void 0 ? void 0 : _classStaticPrivateFi2.call(_classStaticPrivateFi);
+    })()) {
+      return r;
+    }
+
+    f(o);
+    g(o);
+    h(fnDeep);
+    i(fn);
+    j(fn);
+  }
+
+}
+
+var _x = {
+  writable: true,
+  value: 1
+};
+var _m = {
+  writable: true,
+  value: function () {
+    return _classStaticPrivateFieldSpecGet(this, Foo, _x);
+  }
+};
+var _self = {
+  writable: true,
+  value: Foo
+};
+
+_defineProperty(Foo, "self", Foo);
+
+Foo.test();
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param/exec.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param/exec.js
new file mode 100644
index 000000000000..6eb26ab6e01a
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param/exec.js
@@ -0,0 +1,46 @@
+class Foo {
+  static #x = 1;
+  static #m = function() { return this.#x; };
+  static #self = Foo;
+  static self = Foo;
+  static getSelf() { return this }
+
+  static test() {
+    const o = { Foo: Foo };
+    const deep = { very: { o } };
+    function fn() {
+      return o;
+    }
+    function fnDeep() {
+      return deep;
+    }
+
+    function f(o, r = o?.Foo.#m()) {
+      return r;
+    }
+
+    function g(o, r = o?.Foo.#self.getSelf().#m()) {
+      return r;
+    }
+
+    function h(fnDeep, r = fnDeep?.().very.o?.Foo?.#m()) {
+      return r;
+    }
+
+    function i(fn, r = fn?.().Foo.#self?.getSelf()?.self.#m()) {
+      return r;
+    }
+
+    function j(fn, r = (fn().Foo.#self.getSelf().self.#m)?.()) {
+      return r;
+    }
+
+    expect(f(o)).toBe(1);
+    expect(g(o)).toBe(1);
+    expect(h(fnDeep)).toBe(1);
+    expect(i(fn)).toBe(1);
+    expect(j(fn)).toBe(1);
+  }
+}
+
+Foo.test();
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param/input.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param/input.js
new file mode 100644
index 000000000000..c5fe33636eca
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param/input.js
@@ -0,0 +1,46 @@
+class Foo {
+  static #x = 1;
+  static #m = function() { return this.#x; };
+  static #self = Foo;
+  static self = Foo;
+  static getSelf() { return this }
+
+  static test() {
+    const o = { Foo: Foo };
+    const deep = { very: { o } };
+    function fn() {
+      return o;
+    }
+    function fnDeep() {
+      return deep;
+    }
+
+    function f(o, r = o?.Foo.#m()) {
+      return r;
+    }
+
+    function g(o, r = o?.Foo.#self.getSelf().#m?.()) {
+      return r;
+    }
+
+    function h(fnDeep, r = fnDeep?.().very.o?.Foo?.#m()) {
+      return r;
+    }
+
+    function i(fn, r = fn?.().Foo.#self?.getSelf()?.self.#m()) {
+      return r;
+    }
+
+    function j(fn, r = (fn().Foo.#self.getSelf().self.#m)?.()) {
+      return r;
+    }
+
+    f(o);
+    g(o);
+    h(fnDeep);
+    i(fn);
+    j(fn);
+  }
+}
+
+Foo.test();
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param/options.json b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param/options.json
new file mode 100644
index 000000000000..3b59e1bbfcc8
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param/options.json
@@ -0,0 +1,4 @@
+{
+  "plugins": ["proposal-class-properties"],
+  "minNodeVersion": "14.0.0"
+}
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param/output.js
new file mode 100644
index 000000000000..782d93a5793a
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/private/optional-chain-in-function-param/output.js
@@ -0,0 +1,90 @@
+function _defineProperty(obj, key, value) { if (key in obj) { Object.defineProperty(obj, key, { value: value, enumerable: true, configurable: true, writable: true }); } else { obj[key] = value; } return obj; }
+
+function _classStaticPrivateFieldSpecGet(receiver, classConstructor, descriptor) { if (receiver !== classConstructor) { throw new TypeError("Private static access of wrong provenance"); } if (descriptor.get) { return descriptor.get.call(receiver); } return descriptor.value; }
+
+class Foo {
+  static getSelf() {
+    return this;
+  }
+
+  static test() {
+    const o = {
+      Foo: Foo
+    };
+    const deep = {
+      very: {
+        o
+      }
+    };
+
+    function fn() {
+      return o;
+    }
+
+    function fnDeep() {
+      return deep;
+    }
+
+    function f(o, r = (() => {
+      var _o$Foo;
+
+      return o === null || o === void 0 ? void 0 : _classStaticPrivateFieldSpecGet(_o$Foo = o.Foo, Foo, _m).call(_o$Foo);
+    })()) {
+      return r;
+    }
+
+    function g(o, r = (() => o === null || o === void 0 ? void 0 : _classStaticPrivateFieldSpecGet(_classStaticPrivateFieldSpecGet(o.Foo, Foo, _self).getSelf(), Foo, _m))()?.()) {
+      return r;
+    }
+
+    function h(fnDeep, r = (() => {
+      var _fnDeep$very$o$Foo;
+
+      return (_fnDeep$very$o$Foo = fnDeep?.().very.o?.Foo) === null || _fnDeep$very$o$Foo === void 0 ? void 0 : _classStaticPrivateFieldSpecGet(_fnDeep$very$o$Foo, Foo, _m).call(_fnDeep$very$o$Foo);
+    })()) {
+      return r;
+    }
+
+    function i(fn, r = (() => {
+      var _getSelf, _getSelf$self;
+
+      return (_getSelf = (() => fn === null || fn === void 0 ? void 0 : _classStaticPrivateFieldSpecGet(fn().Foo, Foo, _self))()?.getSelf()) === null || _getSelf === void 0 ? void 0 : _classStaticPrivateFieldSpecGet(_getSelf$self = _getSelf.self, Foo, _m).call(_getSelf$self);
+    })()) {
+      return r;
+    }
+
+    function j(fn, r = (() => {
+      var _classStaticPrivateFi;
+
+      return _classStaticPrivateFieldSpecGet(_classStaticPrivateFi = _classStaticPrivateFieldSpecGet(fn().Foo, Foo, _self).getSelf().self, Foo, _m)?.call(_classStaticPrivateFi);
+    })()) {
+      return r;
+    }
+
+    f(o);
+    g(o);
+    h(fnDeep);
+    i(fn);
+    j(fn);
+  }
+
+}
+
+var _x = {
+  writable: true,
+  value: 1
+};
+var _m = {
+  writable: true,
+  value: function () {
+    return _classStaticPrivateFieldSpecGet(this, Foo, _x);
+  }
+};
+var _self = {
+  writable: true,
+  value: Foo
+};
+
+_defineProperty(Foo, "self", Foo);
+
+Foo.test();
diff --git a/packages/babel-plugin-proposal-nullish-coalescing-operator/src/index.js b/packages/babel-plugin-proposal-nullish-coalescing-operator/src/index.js
index 848a6c3ee3cd..12ddbc7cba97 100644
--- a/packages/babel-plugin-proposal-nullish-coalescing-operator/src/index.js
+++ b/packages/babel-plugin-proposal-nullish-coalescing-operator/src/index.js
@@ -1,6 +1,6 @@
 import { declare } from "@babel/helper-plugin-utils";
 import syntaxNullishCoalescingOperator from "@babel/plugin-syntax-nullish-coalescing-operator";
-import { types as t } from "@babel/core";
+import { types as t, template } from "@babel/core";
 
 export default declare((api, { loose = false }) => {
   api.assertVersion(7);
@@ -16,13 +16,21 @@ export default declare((api, { loose = false }) => {
           return;
         }
 
-        let ref = scope.maybeGenerateMemoised(node.left);
+        let ref;
         let assignment;
         // skip creating extra reference when `left` is static
-        if (ref === null) {
+        if (scope.isStatic(node.left)) {
           ref = node.left;
           assignment = t.cloneNode(node.left);
+        } else if (scope.path.isPattern()) {
+          // Replace `function (a, x = a.b ?? c) {}` to `function (a, x = (() => a.b ?? c)() ){}`
+          // so the temporary variable can be injected in correct scope
+          path.replaceWith(template.ast`(() => ${path.node})()`);
+          // The injected nullish expression will be queued and eventually transformed when visited
+          return;
         } else {
+          ref = scope.generateUidIdentifierBasedOnNode(node.left);
+          scope.push({ id: t.cloneNode(ref) });
           assignment = t.assignmentExpression("=", ref, node.left);
         }
 
diff --git a/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-destructuring/input.js b/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-destructuring/input.js
new file mode 100644
index 000000000000..876e05c1a625
--- /dev/null
+++ b/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-destructuring/input.js
@@ -0,0 +1 @@
+var { qux = foo.bar ?? "qux" } = {};
diff --git a/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default/options.json b/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-destructuring/options.json
similarity index 100%
rename from packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default/options.json
rename to packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-destructuring/options.json
diff --git a/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-destructuring/output.js b/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-destructuring/output.js
new file mode 100644
index 000000000000..e6dcd5e32e2c
--- /dev/null
+++ b/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-destructuring/output.js
@@ -0,0 +1,5 @@
+var _foo$bar;
+
+var {
+  qux = (_foo$bar = foo.bar) !== null && _foo$bar !== void 0 ? _foo$bar : "qux"
+} = {};
diff --git a/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default/input.js b/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-param/input.js
similarity index 51%
rename from packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default/input.js
rename to packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-param/input.js
index 0c13164800af..65b7719722bc 100644
--- a/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default/input.js
+++ b/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-param/input.js
@@ -1 +1,3 @@
 function foo(foo, qux = foo.bar ?? "qux") {}
+
+function bar(bar, qux = bar ?? "qux") {}
diff --git a/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-param/options.json b/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-param/options.json
new file mode 100644
index 000000000000..47e490d2afcf
--- /dev/null
+++ b/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-param/options.json
@@ -0,0 +1,3 @@
+{
+  "plugins": ["proposal-nullish-coalescing-operator"]
+}
diff --git a/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-param/output.js b/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-param/output.js
new file mode 100644
index 000000000000..b746a752e57a
--- /dev/null
+++ b/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default-param/output.js
@@ -0,0 +1,7 @@
+function foo(foo, qux = (() => {
+  var _foo$bar;
+
+  return (_foo$bar = foo.bar) !== null && _foo$bar !== void 0 ? _foo$bar : "qux";
+})()) {}
+
+function bar(bar, qux = bar !== null && bar !== void 0 ? bar : "qux") {}
diff --git a/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default/output.js b/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default/output.js
deleted file mode 100644
index 2b2f2314b366..000000000000
--- a/packages/babel-plugin-proposal-nullish-coalescing-operator/test/fixtures/nullish-coalescing/transform-in-default/output.js
+++ /dev/null
@@ -1,3 +0,0 @@
-function foo(foo, qux = (_foo$bar = foo.bar) !== null && _foo$bar !== void 0 ? _foo$bar : "qux") {
-  var _foo$bar;
-}
diff --git a/packages/babel-plugin-proposal-optional-chaining/src/index.js b/packages/babel-plugin-proposal-optional-chaining/src/index.js
index f3d32be4338a..f9f8f99ab84d 100644
--- a/packages/babel-plugin-proposal-optional-chaining/src/index.js
+++ b/packages/babel-plugin-proposal-optional-chaining/src/index.js
@@ -4,7 +4,7 @@ import {
   skipTransparentExprWrappers,
 } from "@babel/helper-skip-transparent-expression-wrappers";
 import syntaxOptionalChaining from "@babel/plugin-syntax-optional-chaining";
-import { types as t } from "@babel/core";
+import { types as t, template } from "@babel/core";
 
 export default declare((api, options) => {
   api.assertVersion(7);
@@ -22,6 +22,31 @@ export default declare((api, options) => {
     );
   }
 
+  /**
+   * Test if a given optional chain `path` needs to be memoized
+   * @param {NodePath} path
+   * @returns {boolean}
+   */
+  function needsMemoize(path) {
+    let optionalPath = path;
+    const { scope } = path;
+    while (
+      optionalPath.isOptionalMemberExpression() ||
+      optionalPath.isOptionalCallExpression()
+    ) {
+      const { node } = optionalPath;
+      const childKey = optionalPath.isOptionalMemberExpression()
+        ? "object"
+        : "callee";
+      const childPath = skipTransparentExprWrappers(optionalPath.get(childKey));
+      if (node.optional) {
+        return !scope.isStatic(childPath.node);
+      }
+
+      optionalPath = childPath;
+    }
+  }
+
   return {
     name: "proposal-optional-chaining",
     inherits: syntaxOptionalChaining,
@@ -46,6 +71,13 @@ export default declare((api, options) => {
         const optionals = [];
 
         let optionalPath = path;
+        // Replace `function (a, x = a.b?.c) {}` to `function (a, x = (() => a.b?.c)() ){}`
+        // so the temporary variable can be injected in correct scope
+        if (scope.path.isPattern() && needsMemoize(optionalPath)) {
+          path.replaceWith(template.ast`(() => ${path.node})()`);
+          // The injected optional chain will be queued and eventually transformed when visited
+          return;
+        }
         while (
           optionalPath.isOptionalMemberExpression() ||
           optionalPath.isOptionalCallExpression()
diff --git a/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params-loose/input.js b/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params-loose/input.js
new file mode 100644
index 000000000000..882511b447f8
--- /dev/null
+++ b/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params-loose/input.js
@@ -0,0 +1,9 @@
+function f(a = x?.y) {}
+
+function g({ a, b = a?.c }) {}
+
+function h(a, { b = a.b?.c?.d.e }) {}
+
+function i(a, { b = (a.b?.c?.d).e }) {}
+
+function j(a, { b = a?.b?.c().d.e }) {}
diff --git a/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params-loose/options.json b/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params-loose/options.json
new file mode 100644
index 000000000000..39ea3f99c7ff
--- /dev/null
+++ b/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params-loose/options.json
@@ -0,0 +1,3 @@
+{
+  "plugins": [["proposal-optional-chaining", { "loose": true }]]
+}
diff --git a/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params-loose/output.js b/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params-loose/output.js
new file mode 100644
index 000000000000..f5cda260cbfe
--- /dev/null
+++ b/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params-loose/output.js
@@ -0,0 +1,34 @@
+function f(a = (() => {
+  var _x;
+
+  return (_x = x) == null ? void 0 : _x.y;
+})()) {}
+
+function g({
+  a,
+  b = a == null ? void 0 : a.c
+}) {}
+
+function h(a, {
+  b = (() => {
+    var _a$b, _a$b$c;
+
+    return (_a$b = a.b) == null ? void 0 : (_a$b$c = _a$b.c) == null ? void 0 : _a$b$c.d.e;
+  })()
+}) {}
+
+function i(a, {
+  b = (() => {
+    var _a$b2, _a$b2$c;
+
+    return (_a$b2 = a.b) == null ? void 0 : (_a$b2$c = _a$b2.c) == null ? void 0 : _a$b2$c.d;
+  })().e
+}) {}
+
+function j(a, {
+  b = (() => {
+    var _a$b3;
+
+    return a == null ? void 0 : (_a$b3 = a.b) == null ? void 0 : _a$b3.c().d.e;
+  })()
+}) {}
diff --git a/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params/input.js b/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params/input.js
new file mode 100644
index 000000000000..882511b447f8
--- /dev/null
+++ b/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params/input.js
@@ -0,0 +1,9 @@
+function f(a = x?.y) {}
+
+function g({ a, b = a?.c }) {}
+
+function h(a, { b = a.b?.c?.d.e }) {}
+
+function i(a, { b = (a.b?.c?.d).e }) {}
+
+function j(a, { b = a?.b?.c().d.e }) {}
diff --git a/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params/output.js b/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params/output.js
new file mode 100644
index 000000000000..8d7909dc76fa
--- /dev/null
+++ b/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-function-params/output.js
@@ -0,0 +1,34 @@
+function f(a = (() => {
+  var _x;
+
+  return (_x = x) === null || _x === void 0 ? void 0 : _x.y;
+})()) {}
+
+function g({
+  a,
+  b = a === null || a === void 0 ? void 0 : a.c
+}) {}
+
+function h(a, {
+  b = (() => {
+    var _a$b, _a$b$c;
+
+    return (_a$b = a.b) === null || _a$b === void 0 ? void 0 : (_a$b$c = _a$b.c) === null || _a$b$c === void 0 ? void 0 : _a$b$c.d.e;
+  })()
+}) {}
+
+function i(a, {
+  b = (() => {
+    var _a$b2, _a$b2$c;
+
+    return (_a$b2 = a.b) === null || _a$b2 === void 0 ? void 0 : (_a$b2$c = _a$b2.c) === null || _a$b2$c === void 0 ? void 0 : _a$b2$c.d;
+  })().e
+}) {}
+
+function j(a, {
+  b = (() => {
+    var _a$b3;
+
+    return a === null || a === void 0 ? void 0 : (_a$b3 = a.b) === null || _a$b3 === void 0 ? void 0 : _a$b3.c().d.e;
+  })()
+}) {}
diff --git a/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-var-destructuring/input.js b/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-var-destructuring/input.js
new file mode 100644
index 000000000000..d88b60a0e957
--- /dev/null
+++ b/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-var-destructuring/input.js
@@ -0,0 +1 @@
+var { a = x?.y } = {};
diff --git a/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-var-destructuring/output.js b/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-var-destructuring/output.js
new file mode 100644
index 000000000000..0e70d31a12bf
--- /dev/null
+++ b/packages/babel-plugin-proposal-optional-chaining/test/fixtures/general/in-var-destructuring/output.js
@@ -0,0 +1,5 @@
+var _x;
+
+var {
+  a = (_x = x) === null || _x === void 0 ? void 0 : _x.y
+} = {};