Special case the error message for generator components

Author: sebmarkbage

packages/react-reconciler/src/ReactChildFiber.js

@@ -2129,6 +2129,23 @@ export function validateSuspenseListChildren(
           // TODO: Technically we should warn for nested arrays inside the
           // async iterable but it would require unwrapping the array.
           // However, this mistake is not as easy to make so it's ok not to warn.
+        } else if (
+          enableAsyncIterableChildren &&
+          children.$$typeof === REACT_ELEMENT_TYPE &&
+          typeof children.type === 'function' &&
+          // $FlowFixMe
+          (Object.prototype.toString.call(children.type) ===
+            '[object GeneratorFunction]' ||
+            // $FlowFixMe
+            Object.prototype.toString.call(children.type) ===
+              '[object AsyncGeneratorFunction]')
+        ) {
+          console.error(
+            'A generator Component was passed to a <SuspenseList revealOrder="%s" />. ' +
+              'This is not supported as a way to generate lists. Instead, pass an ' +
+              'iterable as the children.',
+            revealOrder,
+          );
         } else {
           console.error(
             'A single row was passed to a <SuspenseList revealOrder="%s" />. ' +

packages/react-reconciler/src/__tests__/ReactSuspenseList-test.js

@@ -3120,6 +3120,44 @@ describe('ReactSuspenseList', () => {
     },
   );
 
+  // @gate enableSuspenseList && enableAsyncIterableChildren
+  it('warns for async generator components in "forwards" order', async () => {
+    async function* Generator() {
+      yield 'A';
+      yield 'B';
+    }
+    function Foo() {
+      return (
+        <SuspenseList revealOrder="forwards">
+          <Generator />
+        </SuspenseList>
+      );
+    }
+
+    await act(() => {
+      React.startTransition(() => {
+        ReactNoop.render(<Foo />);
+      });
+    });
+    assertConsoleErrorDev([
+      'A generator Component was passed to a <SuspenseList revealOrder="forwards" />. ' +
+        'This is not supported as a way to generate lists. Instead, pass an ' +
+        'iterable as the children.' +
+        '\n    in SuspenseList (at **)' +
+        '\n    in Foo (at **)',
+      '<Generator> is an async Client Component. ' +
+        'Only Server Components can be async at the moment. ' +
+        "This error is often caused by accidentally adding `'use client'` " +
+        'to a module that was originally written for the server.\n' +
+        '    in Foo (at **)',
+      // We get this warning because the generator's promise themselves are not cached.
+      'A component was suspended by an uncached promise. ' +
+        'Creating promises inside a Client Component or hook is not yet supported, ' +
+        'except via a Suspense-compatible library or framework.\n' +
+        '    in Foo (at **)',
+    ]);
+  });
+
   // @gate enableSuspenseList && enableAsyncIterableChildren
   it('can display async iterable in "forwards" order', async () => {
     const A = createAsyncText('A');