Revert "Remove module pattern function component support" (#28670)

This breaks internal tests, so must be something in the refactor. Since
it's the top commit let's revert and split into two PRs, one that
removes the flag and one that does the refactor, so we can find the bug.

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

@@ -225,7 +225,7 @@ export function getInternalReactConstants(version: string): {
       HostSingleton: 27, // Same as above
       HostText: 6,
       IncompleteClassComponent: 17,
-      IndeterminateComponent: 2, // removed in 19.0.0
+      IndeterminateComponent: 2,
       LazyComponent: 16,
       LegacyHiddenComponent: 23,
       MemoComponent: 14,

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

@@ -14,6 +14,7 @@ let act;
 let React;
 let ReactDOM;
 let ReactDOMClient;
+let PropTypes;
 let findDOMNode;
 
 const clone = function (o) {
@@ -98,6 +99,7 @@ describe('ReactComponentLifeCycle', () => {
     findDOMNode =
       ReactDOM.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED.findDOMNode;
     ReactDOMClient = require('react-dom/client');
+    PropTypes = require('prop-types');
   });
 
   it('should not reuse an instance when it has been unmounted', async () => {
@@ -1112,6 +1114,72 @@ describe('ReactComponentLifeCycle', () => {
     });
   });
 
+  if (!require('shared/ReactFeatureFlags').disableModulePatternComponents) {
+    // @gate !disableLegacyContext
+    it('calls effects on module-pattern component', async () => {
+      const log = [];
+
+      function Parent() {
+        return {
+          render() {
+            expect(typeof this.props).toBe('object');
+            log.push('render');
+            return <Child />;
+          },
+          UNSAFE_componentWillMount() {
+            log.push('will mount');
+          },
+          componentDidMount() {
+            log.push('did mount');
+          },
+          componentDidUpdate() {
+            log.push('did update');
+          },
+          getChildContext() {
+            return {x: 2};
+          },
+        };
+      }
+      Parent.childContextTypes = {
+        x: PropTypes.number,
+      };
+      function Child(props, context) {
+        expect(context.x).toBe(2);
+        return <div />;
+      }
+      Child.contextTypes = {
+        x: PropTypes.number,
+      };
+
+      const root = ReactDOMClient.createRoot(document.createElement('div'));
+      await expect(async () => {
+        await act(() => {
+          root.render(<Parent ref={c => c && log.push('ref')} />);
+        });
+      }).toErrorDev(
+        'Warning: The <Parent /> component appears to be a function component that returns a class instance. ' +
+          'Change Parent 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. " +
+          '`Parent.prototype = React.Component.prototype`. ' +
+          "Don't use an arrow function since it cannot be called with `new` by React.",
+      );
+      await act(() => {
+        root.render(<Parent ref={c => c && log.push('ref')} />);
+      });
+
+      expect(log).toEqual([
+        'will mount',
+        'render',
+        'did mount',
+        'ref',
+
+        'render',
+        'did update',
+        'ref',
+      ]);
+    });
+  }
+
   it('should warn if getDerivedStateFromProps returns undefined', async () => {
     class MyComponent extends React.Component {
       state = {};

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

@@ -211,27 +211,63 @@ describe('ReactCompositeComponent', () => {
     });
   });
 
-  it('should not support module pattern components', async () => {
-    function Child({test}) {
-      return {
-        render() {
-          return <div>{test}</div>;
-        },
-      };
-    }
+  if (require('shared/ReactFeatureFlags').disableModulePatternComponents) {
+    it('should not support module pattern components', async () => {
+      function Child({test}) {
+        return {
+          render() {
+            return <div>{test}</div>;
+          },
+        };
+      }
 
-    const el = document.createElement('div');
-    const root = ReactDOMClient.createRoot(el);
-    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}).',
-    );
+      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.",
+      );
 
-    expect(el.textContent).toBe('');
-  });
+      expect(el.textContent).toBe('');
+    });
+  } else {
+    it('should support module pattern components', () => {
+      function Child({test}) {
+        return {
+          render() {
+            return <div>{test}</div>;
+          },
+        };
+      }
+
+      const el = document.createElement('div');
+      const root = ReactDOMClient.createRoot(el);
+      expect(() => {
+        ReactDOM.flushSync(() => {
+          root.render(<Child test="test" />);
+        });
+      }).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.",
+      );
+
+      expect(el.textContent).toBe('test');
+    });
+  }
 
   it('should use default values for undefined props', async () => {
     class Component extends React.Component {

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

@@ -527,6 +527,72 @@ describe('ReactCompositeComponent-state', () => {
     ]);
   });
 
+  if (!require('shared/ReactFeatureFlags').disableModulePatternComponents) {
+    it('should support stateful module pattern components', async () => {
+      function Child() {
+        return {
+          state: {
+            count: 123,
+          },
+          render() {
+            return <div>{`count:${this.state.count}`}</div>;
+          },
+        };
+      }
+
+      const el = document.createElement('div');
+      const root = ReactDOMClient.createRoot(el);
+      expect(() => {
+        ReactDOM.flushSync(() => {
+          root.render(<Child />);
+        });
+      }).toErrorDev(
+        'Warning: The <Child /> component appears to be a function component that returns a class instance. ' +
+          'Change Child to a class that extends React.Component instead. ' +
+          "If you can't use a class try assigning the prototype on the function as a workaround. " +
+          '`Child.prototype = React.Component.prototype`. ' +
+          "Don't use an arrow function since it cannot be called with `new` by React.",
+      );
+
+      expect(el.textContent).toBe('count:123');
+    });
+
+    it('should support getDerivedStateFromProps for module pattern components', async () => {
+      function Child() {
+        return {
+          state: {
+            count: 1,
+          },
+          render() {
+            return <div>{`count:${this.state.count}`}</div>;
+          },
+        };
+      }
+      Child.getDerivedStateFromProps = (props, prevState) => {
+        return {
+          count: prevState.count + props.incrementBy,
+        };
+      };
+
+      const el = document.createElement('div');
+      const root = ReactDOMClient.createRoot(el);
+      await act(() => {
+        root.render(<Child incrementBy={0} />);
+      });
+
+      expect(el.textContent).toBe('count:1');
+      await act(() => {
+        root.render(<Child incrementBy={2} />);
+      });
+      expect(el.textContent).toBe('count:3');
+
+      await act(() => {
+        root.render(<Child incrementBy={1} />);
+      });
+      expect(el.textContent).toBe('count:4');
+    });
+  }
+
   it('should not support setState in componentWillUnmount', async () => {
     let subscription;
     class A extends React.Component {

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

@@ -627,20 +627,33 @@ describe('ReactDOMServerIntegration', () => {
         checkFooDiv(await render(<ClassComponent />));
       });
 
-      itThrowsWhenRendering(
-        'factory components',
-        async render => {
+      if (require('shared/ReactFeatureFlags').disableModulePatternComponents) {
+        itThrowsWhenRendering(
+          'factory components',
+          async render => {
+            const FactoryComponent = () => {
+              return {
+                render: function () {
+                  return <div>foo</div>;
+                },
+              };
+            };
+            await render(<FactoryComponent />, 1);
+          },
+          'Objects are not valid as a React child (found: object with keys {render})',
+        );
+      } else {
+        itRenders('factory components', async render => {
           const FactoryComponent = () => {
             return {
               render: function () {
                 return <div>foo</div>;
               },
             };
           };
-          await render(<FactoryComponent />, 1);
-        },
-        'Objects are not valid as a React child (found: object with keys {render})',
-      );
+          checkFooDiv(await render(<FactoryComponent />, 1));
+        });
+      }
     });
 
     describe('component hierarchies', function () {

packages/react-dom/src/__tests__/ReactErrorBoundaries-test.internal.js

@@ -879,6 +879,56 @@ describe('ReactErrorBoundaries', () => {
     expect(container.firstChild.textContent).toBe('Caught an error: Hello.');
   });
 
+  // @gate !disableModulePatternComponents
+  it('renders an error state if module-style context provider throws in componentWillMount', async () => {
+    function BrokenComponentWillMountWithContext() {
+      return {
+        getChildContext() {
+          return {foo: 42};
+        },
+        render() {
+          return <div>{this.props.children}</div>;
+        },
+        UNSAFE_componentWillMount() {
+          throw new Error('Hello');
+        },
+      };
+    }
+    BrokenComponentWillMountWithContext.childContextTypes = {
+      foo: PropTypes.number,
+    };
+
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+
+    await expect(async () => {
+      await act(() => {
+        root.render(
+          <ErrorBoundary>
+            <BrokenComponentWillMountWithContext />
+          </ErrorBoundary>,
+        );
+      });
+    }).toErrorDev([
+      'Warning: The <BrokenComponentWillMountWithContext /> component appears to be a function component that ' +
+        'returns a class instance. ' +
+        'Change BrokenComponentWillMountWithContext 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. " +
+        '`BrokenComponentWillMountWithContext.prototype = React.Component.prototype`. ' +
+        "Don't use an arrow function since it cannot be called with `new` by React.",
+      ...gate(flags =>
+        flags.disableLegacyContext
+          ? [
+              'Warning: BrokenComponentWillMountWithContext uses the legacy childContextTypes API which was removed in React 19. Use React.createContext() instead.',
+              'Warning: BrokenComponentWillMountWithContext uses the legacy childContextTypes API which was removed in React 19. Use React.createContext() instead.',
+            ]
+          : [],
+      ),
+    ]);
+
+    expect(container.firstChild.textContent).toBe('Caught an error: Hello.');
+  });
+
   it('mounts the error message if mounting fails', async () => {
     function renderError(error) {
       return <ErrorMessage message={error.message} />;