Warn about rendering Generators (#13312)

* Warn about rendering Generators

* Fix Flow

* Add an explicit test for iterable

* Moar test coverage

packages/react-dom/src/__tests__/ReactMultiChild-test.js

@@ -294,6 +294,67 @@ describe('ReactMultiChild', () => {
     );
   });
 
+  it('should warn for using generators as children', () => {
+    function* Foo() {
+      yield <h1 key="1">Hello</h1>;
+      yield <h1 key="2">World</h1>;
+    }
+
+    const div = document.createElement('div');
+    expect(() => {
+      ReactDOM.render(<Foo />, div);
+    }).toWarnDev(
+      'Using Generators as children is unsupported and will likely yield ' +
+        'unexpected results because enumerating a generator mutates it. You may ' +
+        'convert it to an array with `Array.from()` or the `[...spread]` operator ' +
+        'before rendering. Keep in mind you might need to polyfill these features for older browsers.\n' +
+        '    in Foo (at **)',
+    );
+
+    // Test de-duplication
+    ReactDOM.render(<Foo />, div);
+  });
+
+  it('should not warn for using generators in legacy iterables', () => {
+    const fooIterable = {
+      '@@iterator': function*() {
+        yield <h1 key="1">Hello</h1>;
+        yield <h1 key="2">World</h1>;
+      },
+    };
+
+    function Foo() {
+      return fooIterable;
+    }
+
+    const div = document.createElement('div');
+    ReactDOM.render(<Foo />, div);
+    expect(div.textContent).toBe('HelloWorld');
+
+    ReactDOM.render(<Foo />, div);
+    expect(div.textContent).toBe('HelloWorld');
+  });
+
+  it('should not warn for using generators in modern iterables', () => {
+    const fooIterable = {
+      [Symbol.iterator]: function*() {
+        yield <h1 key="1">Hello</h1>;
+        yield <h1 key="2">World</h1>;
+      },
+    };
+
+    function Foo() {
+      return fooIterable;
+    }
+
+    const div = document.createElement('div');
+    ReactDOM.render(<Foo />, div);
+    expect(div.textContent).toBe('HelloWorld');
+
+    ReactDOM.render(<Foo />, div);
+    expect(div.textContent).toBe('HelloWorld');
+  });
+
   it('should reorder bailed-out children', () => {
     class LetterInner extends React.Component {
       render() {

packages/react-dom/src/__tests__/ReactMultiChildReconcile-test.js

@@ -248,7 +248,7 @@ function prepareChildrenArray(childrenArray) {
   return childrenArray;
 }
 
-function prepareChildrenIterable(childrenArray) {
+function prepareChildrenLegacyIterable(childrenArray) {
   return {
     '@@iterator': function*() {
       // eslint-disable-next-line no-for-of-loops/no-for-of-loops
@@ -259,9 +259,27 @@ function prepareChildrenIterable(childrenArray) {
   };
 }
 
+function prepareChildrenModernIterable(childrenArray) {
+  return {
+    [Symbol.iterator]: function*() {
+      // eslint-disable-next-line no-for-of-loops/no-for-of-loops
+      for (const child of childrenArray) {
+        yield child;
+      }
+    },
+  };
+}
+
 function testPropsSequence(sequence) {
   testPropsSequenceWithPreparedChildren(sequence, prepareChildrenArray);
-  testPropsSequenceWithPreparedChildren(sequence, prepareChildrenIterable);
+  testPropsSequenceWithPreparedChildren(
+    sequence,
+    prepareChildrenLegacyIterable,
+  );
+  testPropsSequenceWithPreparedChildren(
+    sequence,
+    prepareChildrenModernIterable,
+  );
 }
 
 describe('ReactMultiChildReconcile', () => {
@@ -311,7 +329,7 @@ describe('ReactMultiChildReconcile', () => {
     );
   });
 
-  it('should reset internal state if removed then readded in an iterable', () => {
+  it('should reset internal state if removed then readded in a legacy iterable', () => {
     // Test basics.
     const props = {
       usernameToStatus: {
@@ -323,7 +341,7 @@ describe('ReactMultiChildReconcile', () => {
     const parentInstance = ReactDOM.render(
       <FriendsStatusDisplay
         {...props}
-        prepareChildren={prepareChildrenIterable}
+        prepareChildren={prepareChildrenLegacyIterable}
       />,
       container,
     );
@@ -332,7 +350,7 @@ describe('ReactMultiChildReconcile', () => {
 
     // Now remove the child.
     ReactDOM.render(
-      <FriendsStatusDisplay prepareChildren={prepareChildrenIterable} />,
+      <FriendsStatusDisplay prepareChildren={prepareChildrenLegacyIterable} />,
       container,
     );
     statusDisplays = parentInstance.getStatusDisplays();
@@ -342,7 +360,49 @@ describe('ReactMultiChildReconcile', () => {
     ReactDOM.render(
       <FriendsStatusDisplay
         {...props}
-        prepareChildren={prepareChildrenIterable}
+        prepareChildren={prepareChildrenLegacyIterable}
+      />,
+      container,
+    );
+    statusDisplays = parentInstance.getStatusDisplays();
+    expect(statusDisplays.jcw).toBeTruthy();
+    expect(statusDisplays.jcw.getInternalState()).not.toBe(
+      startingInternalState,
+    );
+  });
+
+  it('should reset internal state if removed then readded in a modern iterable', () => {
+    // Test basics.
+    const props = {
+      usernameToStatus: {
+        jcw: 'jcwStatus',
+      },
+    };
+
+    const container = document.createElement('div');
+    const parentInstance = ReactDOM.render(
+      <FriendsStatusDisplay
+        {...props}
+        prepareChildren={prepareChildrenModernIterable}
+      />,
+      container,
+    );
+    let statusDisplays = parentInstance.getStatusDisplays();
+    const startingInternalState = statusDisplays.jcw.getInternalState();
+
+    // Now remove the child.
+    ReactDOM.render(
+      <FriendsStatusDisplay prepareChildren={prepareChildrenModernIterable} />,
+      container,
+    );
+    statusDisplays = parentInstance.getStatusDisplays();
+    expect(statusDisplays.jcw).toBeFalsy();
+
+    // Now reset the props that cause there to be a child
+    ReactDOM.render(
+      <FriendsStatusDisplay
+        {...props}
+        prepareChildren={prepareChildrenModernIterable}
       />,
       container,
     );

packages/react-reconciler/src/ReactChildFiber.js

@@ -46,6 +46,7 @@ import {
 import {StrictMode} from './ReactTypeOfMode';
 
 let didWarnAboutMaps;
+let didWarnAboutGenerators;
 let didWarnAboutStringRefInStrictMode;
 let ownerHasKeyUseWarning;
 let ownerHasFunctionTypeWarning;
@@ -53,6 +54,7 @@ let warnForMissingKey = (child: mixed) => {};
 
 if (__DEV__) {
   didWarnAboutMaps = false;
+  didWarnAboutGenerators = false;
   didWarnAboutStringRefInStrictMode = {};
 
   /**
@@ -903,6 +905,24 @@ function ChildReconciler(shouldTrackSideEffects) {
     );
 
     if (__DEV__) {
+      // We don't support rendering Generators because it's a mutation.
+      // See https://github.com/facebook/react/issues/12995
+      if (
+        typeof Symbol === 'function' &&
+        // $FlowFixMe Flow doesn't know about toStringTag
+        newChildrenIterable[Symbol.toStringTag] === 'Generator'
+      ) {
+        warning(
+          didWarnAboutGenerators,
+          'Using Generators as children is unsupported and will likely yield ' +
+            'unexpected results because enumerating a generator mutates it. ' +
+            'You may convert it to an array with `Array.from()` or the ' +
+            '`[...spread]` operator before rendering. Keep in mind ' +
+            'you might need to polyfill these features for older browsers.',
+        );
+        didWarnAboutGenerators = true;
+      }
+
       // Warn about using Maps as children
       if ((newChildrenIterable: any).entries === iteratorFn) {
         warning(