Warn about ReactDOM.createPortal usage within ReactTestRenderer (#12895)

packages/react-test-renderer/src/ReactTestHostConfig.js

@@ -7,6 +7,7 @@
  * @flow
  */
 
+import warning from 'shared/warning';
 import * as TestRendererScheduling from './ReactTestRendererScheduling';
 
 export type Type = string;
@@ -62,6 +63,15 @@ export function appendChild(
   parentInstance: Instance | Container,
   child: Instance | TextInstance,
 ): void {
+  if (__DEV__) {
+    warning(
+      Array.isArray(parentInstance.children),
+      'An invalid container has been provided. ' +
+        'This may indicate that another render is being used in addition to the test renderer. ' +
+        '(For example, ReactDOM.createPortal inside of a ReactTestRenderer tree.) ' +
+        'This is not supported.',
+    );
+  }
   const index = parentInstance.children.indexOf(child);
   if (index !== -1) {
     parentInstance.children.splice(index, 1);

packages/react-test-renderer/src/__tests__/ReactTestRenderer-test.js

@@ -0,0 +1,29 @@
+/**
+ * Copyright (c) 2013-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ */
+
+'use strict';
+
+const ReactDOM = require('react-dom');
+
+// Isolate test renderer.
+jest.resetModules();
+const ReactTestRenderer = require('react-test-renderer');
+
+describe('ReactTestRenderer', () => {
+  it('should warn if used to render a ReactDOM portal', () => {
+    const container = document.createElement('div');
+    expect(() => {
+      expect(() => {
+        ReactTestRenderer.create(ReactDOM.createPortal('foo', container));
+      }).toThrow();
+    }).toWarnDev('An invalid container has been provided.', {
+      withoutStack: true,
+    });
+  });
+});