Derived constructors should not be allowed to return primitives (#13571)

Closes gh-13571

packages/babel-helpers/src/helpers.js

@@ -617,7 +617,10 @@ helpers.possibleConstructorReturn = helper("7.0.0-beta.0")`
   export default function _possibleConstructorReturn(self, call) {
     if (call && (typeof call === "object" || typeof call === "function")) {
       return call;
+    } else if (call !== void 0) {
+      throw new TypeError("Derived constructors may only return object or undefined");
     }
+
     return assertThisInitialized(self);
   }
 `;

packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-no-primitive-return/exec.js

@@ -0,0 +1,18 @@
+class Bar {}
+
+class Foo extends Bar {
+  constructor() {
+    super();
+    return 3;
+  }
+}
+
+class Foo2 extends Bar {
+  constructor() {
+    super();
+    return null;
+  }
+}
+
+expect(() => new Foo()).toThrow("Derived constructors may only return object or undefined");
+expect(() => new Foo2()).toThrow("Derived constructors may only return object or undefined");

packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-no-primitive-return/input.js

@@ -0,0 +1,6 @@
+class Foo extends Bar {
+  constructor() {
+    super();
+    return 3;
+  }
+}

packages/babel-plugin-transform-classes/test/fixtures/spec/derived-constructor-no-primitive-return/output.js

@@ -0,0 +1,17 @@
+var Foo = /*#__PURE__*/function (_Bar) {
+  "use strict";
+
+  babelHelpers.inherits(Foo, _Bar);
+
+  var _super = babelHelpers.createSuper(Foo);
+
+  function Foo() {
+    var _this;
+
+    babelHelpers.classCallCheck(this, Foo);
+    _this = _super.call(this);
+    return babelHelpers.possibleConstructorReturn(_this, 3);
+  }
+
+  return Foo;
+}(Bar);

packages/babel-plugin-transform-classes/test/fixtures/spec/returning-from-derived-constructor/exec.js

@@ -43,16 +43,16 @@ expect(instance).toBe(singleton);
 instance = new Sub;
 expect(instance).toBe(singleton);
 
-class Null extends Foo {
+class Undefined extends Foo {
   constructor() {
     if (false) {
       super();
     }
-    return null;
+    return;
     super();
   }
 }
 
 expect(() => {
-  new Null();
+  new Undefined();
 }).toThrow("this");