Suspend Thenable/Lazy if it's used in React.Children and unwrap (#28284)

This pains me because `React.Children` is really already
pseudo-deprecated.

`React.Children` takes any children that `React.Node` takes. We now
support Lazy and Thenable in this position elsewhere, but it errors in
`React.Children`.

This becomes an issue with async Server Components which can resolve
into a Lazy and in the future Lazy will just become Thenables. Which
causes this to error.

There are a few different semantics we could have:

1) Error like we already do (#28280). `React.Children` is about
introspecting children. It was always sketchy because you can't
introspect inside an abstraction anyway. With Server Components we fold
away the components so you can actually introspect inside of them kind
of but what they do is an implementation detail and you should be able
to turn it into a Client Component at any point. The type of an Element
passing the boundary actually reduces to `React.Node`.
2) Suspend and unwrap the Node (this PR). If we assume that Children is
called inside of render, then throwing a Promise if it's not already
loaded or unwrapping would treat it as if it wasn't there. Just like if
you rendered it in React. This lets you introspect what's inside which
isn't really something you should be able to do. This isn't compatible
with deprecating throwing-a-Promise and enable static compilation like
`use()` does. We'd have to deprecate `React.Children` before doing that
which we might anyway.
3) Wrap in a Fragment. If a Server Component was instead a Client
Component, you couldn't introspect through it anyway. Another
alternative might be to let it pass through but then it wouldn't be
given a flat key. We could also wrap it in a Fragment that is keyed.
That way you're always seeing an element. The issue with this solution
is that it wouldn't see the key of the Server Component since that gets
forwarded to the child that is yet to resolve. The nice thing about that
strategy is it doesn't depend on throw-a-Promise but it might not be
keyed correctly when things move.

packages/react-reconciler/src/ReactFiberThenable.js

@@ -212,19 +212,19 @@ export function trackUsedThenable<T>(
             }
           },
         );
+      }
 
-        // Check one more time in case the thenable resolved synchronously.
-        switch (thenable.status) {
-          case 'fulfilled': {
-            const fulfilledThenable: FulfilledThenable<T> = (thenable: any);
-            return fulfilledThenable.value;
-          }
-          case 'rejected': {
-            const rejectedThenable: RejectedThenable<T> = (thenable: any);
-            const rejectedError = rejectedThenable.reason;
-            checkIfUseWrappedInAsyncCatch(rejectedError);
-            throw rejectedError;
-          }
+      // Check one more time in case the thenable resolved synchronously.
+      switch (thenable.status) {
+        case 'fulfilled': {
+          const fulfilledThenable: FulfilledThenable<T> = (thenable: any);
+          return fulfilledThenable.value;
+        }
+        case 'rejected': {
+          const rejectedThenable: RejectedThenable<T> = (thenable: any);
+          const rejectedError = rejectedThenable.reason;
+          checkIfUseWrappedInAsyncCatch(rejectedError);
+          throw rejectedError;
         }
       }
 

packages/react/src/ReactChildren.js

@@ -7,7 +7,13 @@
  * @flow
  */
 
-import type {ReactNodeList} from 'shared/ReactTypes';
+import type {
+  ReactNodeList,
+  Thenable,
+  PendingThenable,
+  FulfilledThenable,
+  RejectedThenable,
+} from 'shared/ReactTypes';
 
 import isArray from 'shared/isArray';
 import {
@@ -75,6 +81,68 @@ function getElementKey(element: any, index: number): string {
   return index.toString(36);
 }
 
+function noop() {}
+
+function resolveThenable<T>(thenable: Thenable<T>): T {
+  switch (thenable.status) {
+    case 'fulfilled': {
+      const fulfilledValue: T = thenable.value;
+      return fulfilledValue;
+    }
+    case 'rejected': {
+      const rejectedError = thenable.reason;
+      throw rejectedError;
+    }
+    default: {
+      if (typeof thenable.status === 'string') {
+        // Only instrument the thenable if the status if not defined. If
+        // it's defined, but an unknown value, assume it's been instrumented by
+        // some custom userspace implementation. We treat it as "pending".
+        // Attach a dummy listener, to ensure that any lazy initialization can
+        // happen. Flight lazily parses JSON when the value is actually awaited.
+        thenable.then(noop, noop);
+      } else {
+        // This is an uncached thenable that we haven't seen before.
+
+        // TODO: Detect infinite ping loops caused by uncached promises.
+
+        const pendingThenable: PendingThenable<T> = (thenable: any);
+        pendingThenable.status = 'pending';
+        pendingThenable.then(
+          fulfilledValue => {
+            if (thenable.status === 'pending') {
+              const fulfilledThenable: FulfilledThenable<T> = (thenable: any);
+              fulfilledThenable.status = 'fulfilled';
+              fulfilledThenable.value = fulfilledValue;
+            }
+          },
+          (error: mixed) => {
+            if (thenable.status === 'pending') {
+              const rejectedThenable: RejectedThenable<T> = (thenable: any);
+              rejectedThenable.status = 'rejected';
+              rejectedThenable.reason = error;
+            }
+          },
+        );
+      }
+
+      // Check one more time in case the thenable resolved synchronously.
+      switch (thenable.status) {
+        case 'fulfilled': {
+          const fulfilledThenable: FulfilledThenable<T> = (thenable: any);
+          return fulfilledThenable.value;
+        }
+        case 'rejected': {
+          const rejectedThenable: RejectedThenable<T> = (thenable: any);
+          const rejectedError = rejectedThenable.reason;
+          throw rejectedError;
+        }
+      }
+    }
+  }
+  throw thenable;
+}
+
 function mapIntoArray(
   children: ?ReactNodeList,
   array: Array<React$Node>,
@@ -106,9 +174,14 @@ function mapIntoArray(
             invokeCallback = true;
             break;
           case REACT_LAZY_TYPE:
-            throw new Error(
-              'Cannot render an Async Component, Promise or React.Lazy inside React.Children. ' +
-                'We recommend not iterating over children and just rendering them plain.',
+            const payload = (children: any)._payload;
+            const init = (children: any)._init;
+            return mapIntoArray(
+              init(payload),
+              array,
+              escapedPrefix,
+              nameSoFar,
+              callback,
             );
         }
     }
@@ -211,16 +284,19 @@ function mapIntoArray(
         );
       }
     } else if (type === 'object') {
-      // eslint-disable-next-line react-internal/safe-string-coercion
-      const childrenString = String((children: any));
-
       if (typeof (children: any).then === 'function') {
-        throw new Error(
-          'Cannot render an Async Component, Promise or React.Lazy inside React.Children. ' +
-            'We recommend not iterating over children and just rendering them plain.',
+        return mapIntoArray(
+          resolveThenable((children: any)),
+          array,
+          escapedPrefix,
+          nameSoFar,
+          callback,
         );
       }
 
+      // eslint-disable-next-line react-internal/safe-string-coercion
+      const childrenString = String((children: any));
+
       throw new Error(
         `Objects are not valid as a React child (found: ${
           childrenString === '[object Object]'

packages/react/src/__tests__/ReactChildren-test.js

@@ -948,26 +948,36 @@ describe('ReactChildren', () => {
     );
   });
 
-  it('should throw on React.lazy', async () => {
+  it('should render React.lazy after suspending', async () => {
     const lazyElement = React.lazy(async () => ({default: <div />}));
-    await expect(() => {
-      React.Children.forEach([lazyElement], () => {}, null);
-    }).toThrowError(
-      'Cannot render an Async Component, Promise or React.Lazy inside React.Children. ' +
-        'We recommend not iterating over children and just rendering them plain.',
-      {withoutStack: true}, // There's nothing on the stack
-    );
+    function Component() {
+      return React.Children.map([lazyElement], c =>
+        React.cloneElement(c, {children: 'hi'}),
+      );
+    }
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<Component />);
+    });
+
+    expect(container.innerHTML).toBe('<div>hi</div>');
   });
 
-  it('should throw on Promises', async () => {
+  it('should render cached Promises after suspending', async () => {
     const promise = Promise.resolve(<div />);
-    await expect(() => {
-      React.Children.forEach([promise], () => {}, null);
-    }).toThrowError(
-      'Cannot render an Async Component, Promise or React.Lazy inside React.Children. ' +
-        'We recommend not iterating over children and just rendering them plain.',
-      {withoutStack: true}, // There's nothing on the stack
-    );
+    function Component() {
+      return React.Children.map([promise], c =>
+        React.cloneElement(c, {children: 'hi'}),
+      );
+    }
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await act(() => {
+      root.render(<Component />);
+    });
+
+    expect(container.innerHTML).toBe('<div>hi</div>');
   });
 
   it('should throw on regex', () => {