Rename the react.element symbol to react.transitional.element

packages/react-devtools-shared/src/__tests__/legacy/inspectElement-test.js

@@ -290,9 +290,23 @@ describe('InspectedElementContext', () => {
             "preview_long": {boolean: true, number: 123, string: "abc"},
           },
         },
-        "react_element": Dehydrated {
-          "preview_short": <span />,
-          "preview_long": <span />,
+        "react_element": {
+          "$$typeof": Dehydrated {
+            "preview_short": Symbol(react.element),
+            "preview_long": Symbol(react.element),
+          },
+          "_owner": null,
+          "_store": Dehydrated {
+            "preview_short": {…},
+            "preview_long": {},
+          },
+          "key": null,
+          "props": Dehydrated {
+            "preview_short": {…},
+            "preview_long": {},
+          },
+          "ref": null,
+          "type": "span",
         },
         "regexp": Dehydrated {
           "preview_short": /abc/giu,

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

@@ -23,8 +23,9 @@ export const SERVER_CONTEXT_SYMBOL_STRING = 'Symbol(react.server_context)';
 
 export const DEPRECATED_ASYNC_MODE_SYMBOL_STRING = 'Symbol(react.async_mode)';
 
-export const ELEMENT_NUMBER = 0xeac7;
-export const ELEMENT_SYMBOL_STRING = 'Symbol(react.element)';
+export const ELEMENT_SYMBOL_STRING = 'Symbol(react.transitional.element)';
+export const LEGACY_ELEMENT_NUMBER = 0xeac7;
+export const LEGACY_ELEMENT_SYMBOL_STRING = 'Symbol(react.element)';
 
 export const DEBUG_TRACING_MODE_NUMBER = 0xeae1;
 export const DEBUG_TRACING_MODE_SYMBOL_STRING =

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

@@ -612,6 +612,31 @@ describe('ReactComponent', () => {
     );
   });
 
+  it('throws if a legacy element is used as a child', async () => {
+    const inlinedElement = {
+      $$typeof: Symbol.for('react.element'),
+      type: 'div',
+      key: null,
+      ref: null,
+      props: {},
+      _owner: null,
+    };
+    const element = <div>{[inlinedElement]}</div>;
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    await expect(
+      act(() => {
+        root.render(element);
+      }),
+    ).rejects.toThrowError(
+      'A React Element from an older version of React was rendered. ' +
+        'This is not supported. It can happen if:\n' +
+        '- Multiple copies of the "react" package is used.\n' +
+        '- A library pre-bundled an old copy of "react" or "react/jsx-runtime".\n' +
+        '- A compiler tries to "inline" JSX instead of using the runtime.',
+    );
+  });
+
   it('throws if a plain object even if it is in an owner', async () => {
     class Foo extends React.Component {
       render() {

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

@@ -138,7 +138,7 @@ describe('ReactDOMOption', () => {
     // This is similar to <fbt>.
     // We don't toString it because you must instead provide a value prop.
     const obj = {
-      $$typeof: Symbol.for('react.element'),
+      $$typeof: Symbol.for('react.transitional.element'),
       type: props => props.content,
       ref: null,
       key: null,

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

@@ -389,7 +389,7 @@ describe('ref swapping', () => {
     await expect(async () => {
       await act(() => {
         root.render({
-          $$typeof: Symbol.for('react.element'),
+          $$typeof: Symbol.for('react.transitional.element'),
           type: 'div',
           props: {
             ref: undefined,

packages/react-reconciler/src/ReactChildFiber.js

@@ -32,6 +32,7 @@ import {
   REACT_PORTAL_TYPE,
   REACT_LAZY_TYPE,
   REACT_CONTEXT_TYPE,
+  REACT_LEGACY_ELEMENT_TYPE,
 } from 'shared/ReactSymbols';
 import {
   HostRoot,
@@ -166,6 +167,16 @@ function coerceRef(
 }
 
 function throwOnInvalidObjectType(returnFiber: Fiber, newChild: Object) {
+  if (newChild.$$typeof === REACT_LEGACY_ELEMENT_TYPE) {
+    throw new Error(
+      'A React Element from an older version of React was rendered. ' +
+        'This is not supported. It can happen if:\n' +
+        '- Multiple copies of the "react" package is used.\n' +
+        '- A library pre-bundled an old copy of "react" or "react/jsx-runtime".\n' +
+        '- A compiler tries to "inline" JSX instead of using the runtime.',
+    );
+  }
+
   // $FlowFixMe[method-unbinding]
   const childString = Object.prototype.toString.call(newChild);
 

packages/react/src/jsx/ReactJSXElement.js

@@ -162,7 +162,7 @@ function elementRefGetterWithDeprecationWarning() {
 /**
  * Factory method to create a new React element. This no longer adheres to
  * the class pattern, so do not use new to call it. Also, instanceof check
- * will not work. Instead test $$typeof field against Symbol.for('react.element') to check
+ * will not work. Instead test $$typeof field against Symbol.for('react.transitional.element') to check
  * if something is a React Element.
  *
  * @param {*} type

packages/shared/ReactSymbols.js

@@ -12,7 +12,10 @@
 // Please consider also adding to 'react-devtools-shared/src/backend/ReactSymbols'
 
 // The Symbol used to tag the ReactElement-like types.
-export const REACT_ELEMENT_TYPE: symbol = Symbol.for('react.element');
+export const REACT_ELEMENT_TYPE: symbol = Symbol.for(
+  'react.transitional.element',
+);
+export const REACT_LEGACY_ELEMENT_TYPE: symbol = Symbol.for('react.element');
 export const REACT_PORTAL_TYPE: symbol = Symbol.for('react.portal');
 export const REACT_FRAGMENT_TYPE: symbol = Symbol.for('react.fragment');
 export const REACT_STRICT_MODE_TYPE: symbol = Symbol.for('react.strict_mode');