Revert "Warn about undefined return value for memo and forwardRef (#19550)"

This reverts commit a63893f.

Author: gaearon

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

@@ -316,24 +316,4 @@ describe('ReactEmptyComponent', () => {
     const noscript2 = container.firstChild;
     expect(noscript2).toBe(null);
   });
-
-  it('should warn about React.forwardRef that returns undefined', () => {
-    const Empty = () => {};
-    const EmptyForwardRef = React.forwardRef(Empty);
-
-    expect(() => {
-      ReactTestUtils.renderIntoDocument(<EmptyForwardRef />);
-    }).toThrowError(
-      'ForwardRef(Empty)(...): Nothing was returned from render.',
-    );
-  });
-
-  it('should warn about React.memo that returns undefined', () => {
-    const Empty = () => {};
-    const EmptyMemo = React.memo(Empty);
-
-    expect(() => {
-      ReactTestUtils.renderIntoDocument(<EmptyMemo />);
-    }).toThrowError('Empty(...): Nothing was returned from render.');
-  });
 });

packages/react-reconciler/src/ReactChildFiber.new.js

@@ -29,9 +29,7 @@ import {
   ClassComponent,
   HostText,
   HostPortal,
-  ForwardRef,
   Fragment,
-  SimpleMemoComponent,
   Block,
 } from './ReactWorkTags';
 import invariant from 'shared/invariant';
@@ -1395,16 +1393,14 @@ function ChildReconciler(shouldTrackSideEffects) {
         // Intentionally fall through to the next case, which handles both
         // functions and classes
         // eslint-disable-next-lined no-fallthrough
-        case Block:
-        case FunctionComponent:
-        case ForwardRef:
-        case SimpleMemoComponent: {
+        case FunctionComponent: {
+          const Component = returnFiber.type;
           invariant(
             false,
             '%s(...): Nothing was returned from render. This usually means a ' +
               'return statement is missing. Or, to render nothing, ' +
               'return null.',
-            getComponentName(returnFiber.type) || 'Component',
+            Component.displayName || Component.name || 'Component',
           );
         }
       }

packages/react-reconciler/src/ReactChildFiber.old.js

@@ -29,9 +29,7 @@ import {
   ClassComponent,
   HostText,
   HostPortal,
-  ForwardRef,
   Fragment,
-  SimpleMemoComponent,
   Block,
 } from './ReactWorkTags';
 import invariant from 'shared/invariant';
@@ -1387,16 +1385,14 @@ function ChildReconciler(shouldTrackSideEffects) {
         // Intentionally fall through to the next case, which handles both
         // functions and classes
         // eslint-disable-next-lined no-fallthrough
-        case Block:
-        case FunctionComponent:
-        case ForwardRef:
-        case SimpleMemoComponent: {
+        case FunctionComponent: {
+          const Component = returnFiber.type;
           invariant(
             false,
             '%s(...): Nothing was returned from render. This usually means a ' +
               'return statement is missing. Or, to render nothing, ' +
               'return null.',
-            getComponentName(returnFiber.type) || 'Component',
+            Component.displayName || Component.name || 'Component',
           );
         }
       }