React has deprecated module pattern Function Components for many years at this point. Supporting this pattern required React to have a concept of an indeterminate component so that when a component first renders it can turn into either a ClassComponent or a FunctionComponent depending on what it returns. While this feature was deprecated and put behind a flag it is still in stable. This change remvoes the flag, removes the warnings, and removes the concept of IndeterminateComponent from the React codebase.

While removing IndeterminateComponent type Seb and I discovered that we needed a concept of IncompleteFunctionComponent to support Suspense in legacy mode. This new work tag is only needed as long as legacy mode is around and ideally any code that considers this tag will be excludable from OSS builds once we land extra gates using `disableLegacyMode` flag.

Author: gnoff

packages/react-devtools-shared/src/backend/renderer.js

@@ -225,7 +225,8 @@ export function getInternalReactConstants(version: string): {
       HostSingleton: 27, // Same as above
       HostText: 6,
       IncompleteClassComponent: 17,
-      IndeterminateComponent: 2,
+      IncompleteFunctionComponent: 28,
+      IndeterminateComponent: 2, // removed in 19.0.0
       LazyComponent: 16,
       LegacyHiddenComponent: 23,
       MemoComponent: 14,
@@ -259,6 +260,7 @@ export function getInternalReactConstants(version: string): {
       HostSingleton: -1, // Doesn't exist yet
       HostText: 6,
       IncompleteClassComponent: 17,
+      IncompleteFunctionComponent: -1, // Doesn't exist yet
       IndeterminateComponent: 2,
       LazyComponent: 16,
       LegacyHiddenComponent: 24,
@@ -292,6 +294,7 @@ export function getInternalReactConstants(version: string): {
       HostSingleton: -1, // Doesn't exist yet
       HostText: 6,
       IncompleteClassComponent: 17,
+      IncompleteFunctionComponent: -1, // Doesn't exist yet
       IndeterminateComponent: 2,
       LazyComponent: 16,
       LegacyHiddenComponent: -1,
@@ -325,6 +328,7 @@ export function getInternalReactConstants(version: string): {
       HostSingleton: -1, // Doesn't exist yet
       HostText: 8,
       IncompleteClassComponent: -1, // Doesn't exist yet
+      IncompleteFunctionComponent: -1, // Doesn't exist yet
       IndeterminateComponent: 4,
       LazyComponent: -1, // Doesn't exist yet
       LegacyHiddenComponent: -1,
@@ -358,6 +362,7 @@ export function getInternalReactConstants(version: string): {
       HostSingleton: -1, // Doesn't exist yet
       HostText: 6,
       IncompleteClassComponent: -1, // Doesn't exist yet
+      IncompleteFunctionComponent: -1, // Doesn't exist yet
       IndeterminateComponent: 0,
       LazyComponent: -1, // Doesn't exist yet
       LegacyHiddenComponent: -1,
@@ -391,6 +396,7 @@ export function getInternalReactConstants(version: string): {
     CacheComponent,
     ClassComponent,
     IncompleteClassComponent,
+    IncompleteFunctionComponent,
     FunctionComponent,
     IndeterminateComponent,
     ForwardRef,
@@ -459,6 +465,7 @@ export function getInternalReactConstants(version: string): {
         return 'Cache';
       case ClassComponent:
       case IncompleteClassComponent:
+      case IncompleteFunctionComponent:
       case FunctionComponent:
       case IndeterminateComponent:
         return getDisplayName(resolvedType);
@@ -624,6 +631,7 @@ export function attach(
     HostComponent,
     HostText,
     IncompleteClassComponent,
+    IncompleteFunctionComponent,
     IndeterminateComponent,
     LegacyHiddenComponent,
     MemoComponent,
@@ -1061,6 +1069,7 @@ export function attach(
       case ClassComponent:
       case IncompleteClassComponent:
         return ElementTypeClass;
+      case IncompleteFunctionComponent:
       case FunctionComponent:
       case IndeterminateComponent:
         return ElementTypeFunction;
@@ -3059,6 +3068,7 @@ export function attach(
     switch (tag) {
       case ClassComponent:
       case IncompleteClassComponent:
+      case IncompleteFunctionComponent:
       case IndeterminateComponent:
       case FunctionComponent:
         global.$type = type;
@@ -3193,6 +3203,7 @@ export function attach(
       tag === ClassComponent ||
       tag === FunctionComponent ||
       tag === IncompleteClassComponent ||
+      tag === IncompleteFunctionComponent ||
       tag === IndeterminateComponent ||
       tag === MemoComponent ||
       tag === ForwardRef ||
@@ -3540,6 +3551,7 @@ export function attach(
       case IndeterminateComponent:
         global.$r = stateNode;
         break;
+      case IncompleteFunctionComponent:
       case FunctionComponent:
         global.$r = {
           hooks,

packages/react-devtools-shared/src/backend/types.js

@@ -58,6 +58,7 @@ export type WorkTagMap = {
   HostSingleton: WorkTag,
   HostText: WorkTag,
   IncompleteClassComponent: WorkTag,
+  IncompleteFunctionComponent: WorkTag,
   IndeterminateComponent: WorkTag,
   LazyComponent: WorkTag,
   LegacyHiddenComponent: WorkTag,

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

@@ -223,23 +223,16 @@ describe('ReactCompositeComponent', () => {
     const el = document.createElement('div');
     const root = ReactDOMClient.createRoot(el);
     await expect(async () => {
-      await expect(async () => {
-        await act(() => {
-          root.render(<Child test="test" />);
-        });
-      }).rejects.toThrow(
-        'Objects are not valid as a React child (found: object with keys {render}).',
-      );
-    }).toErrorDev(
-      'Warning: The <Child /> component appears to be a function component that returns a class instance. ' +
-        'Change Child to a class that extends React.Component instead. ' +
-        "If you can't use a class try assigning the prototype on the function as a workaround. " +
-        '`Child.prototype = React.Component.prototype`. ' +
-        "Don't use an arrow function since it cannot be called with `new` by React.",
+      await act(() => {
+        root.render(<Child test="test" />);
+      });
+    }).rejects.toThrow(
+      'Objects are not valid as a React child (found: object with keys {render}).',
     );
 
     expect(el.textContent).toBe('');
   });
+
   it('should use default values for undefined props', async () => {
     class Component extends React.Component {
       static defaultProps = {prop: 'testKey'};

packages/react-reconciler/src/ReactFiber.js

@@ -42,7 +42,6 @@ import {
 import {NoFlags, Placement, StaticMask} from './ReactFiberFlags';
 import {ConcurrentRoot} from './ReactRootTags';
 import {
-  IndeterminateComponent,
   ClassComponent,
   HostRoot,
   HostComponent,
@@ -248,19 +247,10 @@ export function isSimpleFunctionComponent(type: any): boolean {
   );
 }
 
-export function resolveLazyComponentTag(Component: Function): WorkTag {
-  if (typeof Component === 'function') {
-    return shouldConstruct(Component) ? ClassComponent : FunctionComponent;
-  } else if (Component !== undefined && Component !== null) {
-    const $$typeof = Component.$$typeof;
-    if ($$typeof === REACT_FORWARD_REF_TYPE) {
-      return ForwardRef;
-    }
-    if ($$typeof === REACT_MEMO_TYPE) {
-      return MemoComponent;
-    }
-  }
-  return IndeterminateComponent;
+export function isFunctionClassComponent(
+  type: (...args: Array<any>) => mixed,
+): boolean {
+  return shouldConstruct(type);
 }
 
 // This is used to create an alternate fiber to do work on.
@@ -351,7 +341,6 @@ export function createWorkInProgress(current: Fiber, pendingProps: any): Fiber {
     workInProgress._debugInfo = current._debugInfo;
     workInProgress._debugNeedsRemount = current._debugNeedsRemount;
     switch (workInProgress.tag) {
-      case IndeterminateComponent:
       case FunctionComponent:
       case SimpleMemoComponent:
         workInProgress.type = resolveFunctionForHotReloading(current.type);
@@ -492,7 +481,7 @@ export function createFiberFromTypeAndProps(
   mode: TypeOfMode,
   lanes: Lanes,
 ): Fiber {
-  let fiberTag = IndeterminateComponent;
+  let fiberTag = FunctionComponent;
   // The resolved type is set if we know what the final type will be. I.e. it's not lazy.
   let resolvedType = type;
   if (typeof type === 'function') {