From 31487dd82e82ef62243806b2e76b23a6fb21d0bc Mon Sep 17 00:00:00 2001
From: Dan Abramov <dan.abramov@gmail.com>
Date: Sat, 18 May 2019 01:38:13 +0100
Subject: [PATCH] [Fresh] Set up initial scaffolding (#15619)

* Add a minimal failing test for hot reload

* Set up scaffolding for React Fresh

* Consider type family when comparing elementType

Rendering an element with stale type should not cause it to remount.

We only do this for FunctionComponent tag since checking is unnecessary for classes or host components.

* Add support for forwardRef()

Initially I thought I would compare families of inner .render functions.

However, there is a corner case where this can create false positives. Such as when you forwardRef(X) the same X twice. Those are supposed to be distinct. But if we compare .render functions, we wouldn't be able to distinguish them after first reload.

It seems safer to rely on explicit registration for those. This should be easy when forwardRef() call is in the same file, and usually it would be. For cases like HOCs and style.div`...` factories that return forwardRef(), we could have the __register__ helper itself "dig deeper" and register the inner function.

* Show how forwardRef inner identity can be inferred

The __register__ implementation can read the inner identity itself.

* Add missing __DEV__ to tests

* Add support for memo() (without fixing bailouts)

This adds rudimentary support for memo components. However, we don't actually skip bailouts yet, so this is not very useful by itself alone. Tests have TODOs that we need to remove after bailout skipping is done.

* Refactor type comparison for clarity

* Hot update shouldn't re-render ancestor components unnecessarily

My code had a bug where it checked for a wrong thing in a wrong set, leading us to always re-render.

This fixes the checks so that we only schedule updates for things that were actually edited.

* Add test coverage for memo(fn, areEqual)

* Explicitly skip bailouts for hot reloading fibers

This forces even memo() with shallow comparison to re-render on hot update.

* Refactor scheduling update to reduce duplication

* Remove unused variable in test

* Don't check presence in a set while not hot reloading

* Make scheduleHotUpdate() take named arguments

* Don't keep unedited component types in the type => family map

It's unnecessary because if they haven't been edited, there's no special reconciliation logic.

* Add signatures that force remounting

Signatures let us force a remount of a type even if from React's point of view, type is the same.

A type has one current signature. If that signature changes during next hot update, all Fibers with that type should be deleted and remounted.

We do this by mutating elementType scheduling a parent.

This will be handy to force remount of mismatching Hooks, as well as failed error boundaries.

For this to fully work, we'll need to add a way to skip built-in bailouts for all Fiber types. This will be the most invasive and annoying change. I did it for HostRoot in this PR but there's more. I'll add an automated test case that catches the missing bailout bailouts.

* Support forced remounting for all component types

This teaches all parent component types to remount their child if necessary.

It also adds tests for them.

* Remount effects while preserving state for hot reloaded components

This makes sure that changes to *code* always propagate.

It can break components that aren't resilient to useEffect over-firing, but that seems like a good constraint since you might need to add a dependency later anyway, and this helps avoid coding yourself into the corner.

* Add missing __DEV__ blocks to tests

* Fix unused variables in tests

* Remove outdated TODO

* Expose scheduleHotUpdate directly

* Inline isCompatibleType

* Run one check per component for invalidating deps

This also makes the bailouts more targeted--no need to remount useEffect for a parent component of remounted fiber.

* Resolve .type early

This moves resolving to set up the right .type early instead of doing this before render.
A bit more future-proof in case we want to restructure the begin phase later.

ForwardRef is special because its type is a wrapper but it can be hot reloaded by itself.
So we have a special overload for it that reconstucts the wrapper type if needed.

* Add a Suspense todo

* Use current.type !== workInProgress.type for ignoring deps

This gets rid of one of the sets.

* Use workInProgress.type !== current.type check for force re-render

We still use a set for forced remount though.

* Use wip.type !== current.type check in more places

This also disables the remounting tests. They need a separate approach.

* Use a dedicated remount mechanism

* Add a test for offscreen trees

It has a TODO because it seems like offscreen updates are incorrectly applied too soon.

* Enable offscreen test now that it is fixed

* Fix corner cases in the new remounting mechanism

* Remount failed error boundaries on hot reload

* Fix test now that act() flushes

This test is manual so I don't actually want act here.

* Nits

* Add comments
---
 .../src/__tests__/ReactFresh-test.internal.js | 2036 +++++++++++++++++
 .../react-reconciler/src/ReactChildFiber.js   |   14 +-
 packages/react-reconciler/src/ReactFiber.js   |   31 +
 .../src/ReactFiberBeginWork.js                |  106 +-
 .../react-reconciler/src/ReactFiberHooks.js   |   15 +
 .../src/ReactFiberHotReloading.js             |  264 +++
 .../src/ReactFiberReconciler.js               |    2 +
 .../src/ReactFiberUnwindWork.js               |    8 +
 ...actIncrementalErrorReplay-test.internal.js |    7 +-
 9 files changed, 2472 insertions(+), 11 deletions(-)
 create mode 100644 packages/react-dom/src/__tests__/ReactFresh-test.internal.js
 create mode 100644 packages/react-reconciler/src/ReactFiberHotReloading.js

diff --git a/packages/react-dom/src/__tests__/ReactFresh-test.internal.js b/packages/react-dom/src/__tests__/ReactFresh-test.internal.js
new file mode 100644
index 0000000000..4aa0d30bfa
--- /dev/null
+++ b/packages/react-dom/src/__tests__/ReactFresh-test.internal.js
@@ -0,0 +1,2036 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ */
+
+/* eslint-disable no-for-of-loops/no-for-of-loops */
+
+'use strict';
+
+let React;
+let ReactDOM;
+let Scheduler;
+let act;
+
+describe('ReactFresh', () => {
+  let container;
+  let familiesByID;
+  let familiesByType;
+  let newFamilies;
+  let updatedFamilies;
+  let performHotReload;
+  let signaturesByType;
+
+  beforeEach(() => {
+    let scheduleHotUpdate;
+    let lastRoot;
+    global.__REACT_DEVTOOLS_GLOBAL_HOOK__ = {
+      supportsFiber: true,
+      inject: injected => {
+        scheduleHotUpdate = injected.scheduleHotUpdate;
+      },
+      onCommitFiberRoot: (id, root) => {
+        lastRoot = root;
+      },
+      onCommitFiberUnmount: () => {},
+    };
+
+    jest.resetModules();
+    React = require('react');
+    ReactDOM = require('react-dom');
+    Scheduler = require('scheduler');
+    act = require('react-dom/test-utils').act;
+    container = document.createElement('div');
+    document.body.appendChild(container);
+
+    familiesByID = new Map();
+    familiesByType = new WeakMap();
+
+    if (__DEV__) {
+      performHotReload = function(staleFamilies) {
+        scheduleHotUpdate({
+          root: lastRoot,
+          familiesByType,
+          updatedFamilies,
+          staleFamilies,
+        });
+      };
+    }
+  });
+
+  afterEach(() => {
+    document.body.removeChild(container);
+  });
+
+  function prepare(version) {
+    newFamilies = new Set();
+    updatedFamilies = new Set();
+    signaturesByType = new Map();
+    const Component = version();
+
+    // Fill in the signatures.
+    for (let family of newFamilies) {
+      const latestSignature = signaturesByType.get(family.currentType) || null;
+      family.currentSignature = latestSignature;
+    }
+
+    newFamilies = null;
+    updatedFamilies = null;
+    signaturesByType = null;
+
+    return Component;
+  }
+
+  function render(version, props) {
+    const Component = prepare(version);
+    act(() => {
+      ReactDOM.render(<Component {...props} />, container);
+    });
+    return Component;
+  }
+
+  function patch(version) {
+    // Will be filled in by __register__ calls in user code.
+    newFamilies = new Set();
+    updatedFamilies = new Set();
+    signaturesByType = new Map();
+    const Component = version();
+
+    // Fill in the signatures.
+    for (let family of newFamilies) {
+      const latestSignature = signaturesByType.get(family.currentType) || null;
+      family.currentSignature = latestSignature;
+    }
+    // Now that all registration and signatures are collected,
+    // find which registrations changed their signatures since last time.
+    const staleFamilies = new Set();
+    for (let family of updatedFamilies) {
+      const latestSignature = signaturesByType.get(family.currentType) || null;
+      if (family.currentSignature !== latestSignature) {
+        family.currentSignature = latestSignature;
+        staleFamilies.add(family);
+      }
+    }
+
+    performHotReload(staleFamilies);
+    newFamilies = null;
+    updatedFamilies = null;
+    signaturesByType = null;
+    return Component;
+  }
+
+  function __register__(type, id) {
+    if (familiesByType.has(type)) {
+      return;
+    }
+    let family = familiesByID.get(id);
+    let isNew = false;
+    if (family === undefined) {
+      isNew = true;
+      family = {currentType: type, currentSignature: null};
+      familiesByID.set(id, family);
+    }
+    const prevType = family.currentType;
+    if (isNew) {
+      // The first time a type is registered, we don't need
+      // any special reconciliation logic. So we won't add it to the map.
+      // Instead, this will happen the firt time it is edited.
+      newFamilies.add(family);
+    } else {
+      family.currentType = type;
+      // Point both previous and next types to this family.
+      familiesByType.set(prevType, family);
+      familiesByType.set(type, family);
+      updatedFamilies.add(family);
+    }
+
+    if (typeof type === 'object' && type !== null) {
+      switch (type.$$typeof) {
+        case Symbol.for('react.forward_ref'):
+          __register__(type.render, id + '$render');
+          break;
+        case Symbol.for('react.memo'):
+          __register__(type.type, id + '$type');
+          break;
+      }
+    }
+  }
+
+  function __signature__(type, signature) {
+    signaturesByType.set(type, signature);
+  }
+
+  it('can preserve state for compatible types', () => {
+    if (__DEV__) {
+      let HelloV1 = render(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+        return Hello;
+      });
+
+      // Bump the state before patching.
+      const el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('blue');
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('1');
+
+      // Perform a hot update.
+      let HelloV2 = patch(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'red'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+        return Hello;
+      });
+
+      // Assert the state was preserved but color changed.
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('1');
+      expect(el.style.color).toBe('red');
+
+      // Bump the state again.
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('2');
+      expect(el.style.color).toBe('red');
+
+      // Perform top-down renders with both fresh and stale types.
+      // Neither should change the state or color.
+      // They should always resolve to the latest version.
+      render(() => HelloV1);
+      render(() => HelloV2);
+      render(() => HelloV1);
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('2');
+      expect(el.style.color).toBe('red');
+
+      // Bump the state again.
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('3');
+      expect(el.style.color).toBe('red');
+
+      // Finally, a render with incompatible type should reset it.
+      render(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        // No register call.
+        // This is considered a new type.
+        return Hello;
+      });
+      expect(container.firstChild).not.toBe(el);
+      const newEl = container.firstChild;
+      expect(newEl.textContent).toBe('0');
+      expect(newEl.style.color).toBe('blue');
+    }
+  });
+
+  it('can preserve state for forwardRef', () => {
+    if (__DEV__) {
+      let OuterV1 = render(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        const Outer = React.forwardRef(() => <Hello />);
+        __register__(Outer, 'Outer');
+        return Outer;
+      });
+
+      // Bump the state before patching.
+      const el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('blue');
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('1');
+
+      // Perform a hot update.
+      let OuterV2 = patch(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'red'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        const Outer = React.forwardRef(() => <Hello />);
+        __register__(Outer, 'Outer');
+        return Outer;
+      });
+
+      // Assert the state was preserved but color changed.
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('1');
+      expect(el.style.color).toBe('red');
+
+      // Bump the state again.
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('2');
+      expect(el.style.color).toBe('red');
+
+      // Perform top-down renders with both fresh and stale types.
+      // Neither should change the state or color.
+      // They should always resolve to the latest version.
+      render(() => OuterV1);
+      render(() => OuterV2);
+      render(() => OuterV1);
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('2');
+      expect(el.style.color).toBe('red');
+
+      // Finally, a render with incompatible type should reset it.
+      render(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        // Note: no forwardRef wrapper this time.
+        return Hello;
+      });
+
+      expect(container.firstChild).not.toBe(el);
+      const newEl = container.firstChild;
+      expect(newEl.textContent).toBe('0');
+      expect(newEl.style.color).toBe('blue');
+    }
+  });
+
+  it('should not consider two forwardRefs around the same type to be equivalent', () => {
+    if (__DEV__) {
+      let ParentV1 = render(
+        () => {
+          function Hello() {
+            const [val, setVal] = React.useState(0);
+            return (
+              <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+                {val}
+              </p>
+            );
+          }
+          __register__(Hello, 'Hello');
+
+          function renderInner() {
+            return <Hello />;
+          }
+          // Both of these are wrappers around the same inner function.
+          // They should be treated as distinct types across reloads.
+          let ForwardRefA = React.forwardRef(renderInner);
+          __register__(ForwardRefA, 'ForwardRefA');
+          let ForwardRefB = React.forwardRef(renderInner);
+          __register__(ForwardRefB, 'ForwardRefB');
+
+          function Parent({cond}) {
+            return cond ? <ForwardRefA /> : <ForwardRefB />;
+          }
+          __register__(Parent, 'Parent');
+
+          return Parent;
+        },
+        {cond: true},
+      );
+
+      // Bump the state before switching up types.
+      let el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('blue');
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('1');
+
+      // Switching up the inner types should reset the state.
+      render(() => ParentV1, {cond: false});
+      expect(el).not.toBe(container.firstChild);
+      el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('blue');
+
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('1');
+
+      // Switch them up back again.
+      render(() => ParentV1, {cond: true});
+      expect(el).not.toBe(container.firstChild);
+      el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('blue');
+
+      // Now bump up the state to prepare for patching.
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('1');
+
+      // Patch to change the color.
+      let ParentV2 = patch(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'red'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        function renderInner() {
+          return <Hello />;
+        }
+        // Both of these are wrappers around the same inner function.
+        // They should be treated as distinct types across reloads.
+        let ForwardRefA = React.forwardRef(renderInner);
+        __register__(ForwardRefA, 'ForwardRefA');
+        let ForwardRefB = React.forwardRef(renderInner);
+        __register__(ForwardRefB, 'ForwardRefB');
+
+        function Parent({cond}) {
+          return cond ? <ForwardRefA /> : <ForwardRefB />;
+        }
+        __register__(Parent, 'Parent');
+
+        return Parent;
+      });
+
+      // The state should be intact; the color should change.
+      expect(el).toBe(container.firstChild);
+      expect(el.textContent).toBe('1');
+      expect(el.style.color).toBe('red');
+
+      // Switching up the condition should still reset the state.
+      render(() => ParentV2, {cond: false});
+      expect(el).not.toBe(container.firstChild);
+      el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('red');
+
+      // Now bump up the state to prepare for top-level renders.
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el).toBe(container.firstChild);
+      expect(el.textContent).toBe('1');
+      expect(el.style.color).toBe('red');
+
+      // Finally, verify using top-level render with stale type keeps state.
+      render(() => ParentV1);
+      render(() => ParentV2);
+      render(() => ParentV1);
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('1');
+      expect(el.style.color).toBe('red');
+    }
+  });
+
+  it('can update forwardRef render function with its wrapper', () => {
+    if (__DEV__) {
+      render(() => {
+        function Hello({color}) {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        const Outer = React.forwardRef(() => <Hello color="blue" />);
+        __register__(Outer, 'Outer');
+        return Outer;
+      });
+
+      // Bump the state before patching.
+      const el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('blue');
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('1');
+
+      // Perform a hot update.
+      patch(() => {
+        function Hello({color}) {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        const Outer = React.forwardRef(() => <Hello color="red" />);
+        __register__(Outer, 'Outer');
+        return Outer;
+      });
+
+      // Assert the state was preserved but color changed.
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('1');
+      expect(el.style.color).toBe('red');
+    }
+  });
+
+  it('can update forwardRef render function in isolation', () => {
+    if (__DEV__) {
+      render(() => {
+        function Hello({color}) {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        function renderHello() {
+          return <Hello color="blue" />;
+        }
+        __register__(renderHello, 'renderHello');
+
+        return React.forwardRef(renderHello);
+      });
+
+      // Bump the state before patching.
+      const el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('blue');
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('1');
+
+      // Perform a hot update of just the rendering function.
+      patch(() => {
+        function Hello({color}) {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        function renderHello() {
+          return <Hello color="red" />;
+        }
+        __register__(renderHello, 'renderHello');
+
+        // Not updating the wrapper.
+      });
+
+      // Assert the state was preserved but color changed.
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('1');
+      expect(el.style.color).toBe('red');
+    }
+  });
+
+  it('can preserve state for simple memo', () => {
+    if (__DEV__) {
+      let OuterV1 = render(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        const Outer = React.memo(Hello);
+        __register__(Outer, 'Outer');
+        return Outer;
+      });
+
+      // Bump the state before patching.
+      const el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('blue');
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('1');
+
+      // Perform a hot update.
+      let OuterV2 = patch(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'red'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        const Outer = React.memo(Hello);
+        __register__(Outer, 'Outer');
+        return Outer;
+      });
+
+      // Assert the state was preserved but color changed.
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('1');
+      expect(el.style.color).toBe('red');
+
+      // Bump the state again.
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('2');
+      expect(el.style.color).toBe('red');
+
+      // Perform top-down renders with both fresh and stale types.
+      // Neither should change the state or color.
+      // They should always resolve to the latest version.
+      render(() => OuterV1);
+      render(() => OuterV2);
+      render(() => OuterV1);
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('2');
+      expect(el.style.color).toBe('red');
+
+      // Finally, a render with incompatible type should reset it.
+      render(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        // Note: no wrapper this time.
+        return Hello;
+      });
+
+      expect(container.firstChild).not.toBe(el);
+      const newEl = container.firstChild;
+      expect(newEl.textContent).toBe('0');
+      expect(newEl.style.color).toBe('blue');
+    }
+  });
+
+  it('can preserve state for memo with custom comparison', () => {
+    if (__DEV__) {
+      let OuterV1 = render(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+
+        const Outer = React.memo(Hello, () => true);
+        __register__(Outer, 'Outer');
+        return Outer;
+      });
+
+      // Bump the state before patching.
+      const el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('blue');
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('1');
+
+      // Perform a hot update.
+      let OuterV2 = patch(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'red'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+
+        const Outer = React.memo(Hello, () => true);
+        __register__(Outer, 'Outer');
+        return Outer;
+      });
+
+      // Assert the state was preserved but color changed.
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('1');
+      expect(el.style.color).toBe('red');
+
+      // Bump the state again.
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('2');
+      expect(el.style.color).toBe('red');
+
+      // Perform top-down renders with both fresh and stale types.
+      // Neither should change the state or color.
+      // They should always resolve to the latest version.
+      render(() => OuterV1);
+      render(() => OuterV2);
+      render(() => OuterV1);
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('2');
+      expect(el.style.color).toBe('red');
+
+      // Finally, a render with incompatible type should reset it.
+      render(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        // Note: no wrapper this time.
+        return Hello;
+      });
+
+      expect(container.firstChild).not.toBe(el);
+      const newEl = container.firstChild;
+      expect(newEl.textContent).toBe('0');
+      expect(newEl.style.color).toBe('blue');
+    }
+  });
+
+  it('can update simple memo function in isolation', () => {
+    if (__DEV__) {
+      render(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        return React.memo(Hello);
+      });
+
+      // Bump the state before patching.
+      const el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('blue');
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('1');
+
+      // Perform a hot update of just the rendering function.
+      patch(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'red'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        // Not updating the wrapper.
+      });
+
+      // Assert the state was preserved but color changed.
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('1');
+      expect(el.style.color).toBe('red');
+    }
+  });
+
+  it('can preserve state for memo(forwardRef)', () => {
+    if (__DEV__) {
+      let OuterV1 = render(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        const Outer = React.memo(React.forwardRef(() => <Hello />));
+        __register__(Outer, 'Outer');
+        return Outer;
+      });
+
+      // Bump the state before patching.
+      const el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('blue');
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('1');
+
+      // Perform a hot update.
+      let OuterV2 = patch(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'red'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        const Outer = React.memo(React.forwardRef(() => <Hello />));
+        __register__(Outer, 'Outer');
+        return Outer;
+      });
+
+      // Assert the state was preserved but color changed.
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('1');
+      expect(el.style.color).toBe('red');
+
+      // Bump the state again.
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('2');
+      expect(el.style.color).toBe('red');
+
+      // Perform top-down renders with both fresh and stale types.
+      // Neither should change the state or color.
+      // They should always resolve to the latest version.
+      render(() => OuterV1);
+      render(() => OuterV2);
+      render(() => OuterV1);
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('2');
+      expect(el.style.color).toBe('red');
+
+      // Finally, a render with incompatible type should reset it.
+      render(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        // Note: no wrapper this time.
+        return Hello;
+      });
+
+      expect(container.firstChild).not.toBe(el);
+      const newEl = container.firstChild;
+      expect(newEl.textContent).toBe('0');
+      expect(newEl.style.color).toBe('blue');
+    }
+  });
+
+  it('does not re-render ancestor components unnecessarily during a hot update', () => {
+    if (__DEV__) {
+      let appRenders = 0;
+
+      render(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+        function App() {
+          appRenders++;
+          return <Hello />;
+        }
+        __register__(App, 'App');
+        return App;
+      });
+
+      expect(appRenders).toBe(1);
+
+      // Bump the state before patching.
+      const el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('blue');
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('1');
+
+      // No re-renders from the top.
+      expect(appRenders).toBe(1);
+
+      // Perform a hot update for Hello only.
+      patch(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'red'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+      });
+
+      // Assert the state was preserved but color changed.
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('1');
+      expect(el.style.color).toBe('red');
+
+      // Still no re-renders from the top.
+      expect(appRenders).toBe(1);
+
+      // Bump the state.
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('2');
+
+      // Still no re-renders from the top.
+      expect(appRenders).toBe(1);
+    }
+  });
+
+  it('can force remount by changing signature', () => {
+    if (__DEV__) {
+      let HelloV1 = render(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+        // When this changes, we'll expect a remount:
+        __signature__(Hello, '1');
+        return Hello;
+      });
+
+      // Bump the state before patching.
+      const el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('blue');
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('1');
+
+      // Perform a hot update.
+      let HelloV2 = patch(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'red'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+        // The signature hasn't changed since the last time:
+        __signature__(Hello, '1');
+        return Hello;
+      });
+
+      // Assert the state was preserved but color changed.
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('1');
+      expect(el.style.color).toBe('red');
+
+      // Perform a hot update.
+      let HelloV3 = patch(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'yellow'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        // We're changing the signature now so it will remount:
+        __register__(Hello, 'Hello');
+        __signature__(Hello, '2');
+        return Hello;
+      });
+
+      // Expect a remount.
+      expect(container.firstChild).not.toBe(el);
+      const newEl = container.firstChild;
+      expect(newEl.textContent).toBe('0');
+      expect(newEl.style.color).toBe('yellow');
+
+      // Bump state again.
+      act(() => {
+        newEl.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(newEl.textContent).toBe('1');
+      expect(newEl.style.color).toBe('yellow');
+
+      // Perform top-down renders with both fresh and stale types.
+      // Neither should change the state or color.
+      // They should always resolve to the latest version.
+      render(() => HelloV1);
+      render(() => HelloV2);
+      render(() => HelloV3);
+      render(() => HelloV2);
+      render(() => HelloV1);
+      expect(container.firstChild).toBe(newEl);
+      expect(newEl.textContent).toBe('1');
+      expect(newEl.style.color).toBe('yellow');
+
+      // Verify we can patch again while preserving the signature.
+      patch(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'purple'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        // Same signature as last time.
+        __register__(Hello, 'Hello');
+        __signature__(Hello, '2');
+        return Hello;
+      });
+
+      expect(container.firstChild).toBe(newEl);
+      expect(newEl.textContent).toBe('1');
+      expect(newEl.style.color).toBe('purple');
+
+      // Check removing the signature also causes a remount.
+      patch(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'orange'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        // No signature this time.
+        __register__(Hello, 'Hello');
+        return Hello;
+      });
+
+      // Expect a remount.
+      expect(container.firstChild).not.toBe(newEl);
+      const finalEl = container.firstChild;
+      expect(finalEl.textContent).toBe('0');
+      expect(finalEl.style.color).toBe('orange');
+    }
+  });
+
+  it('keeps a valid tree when forcing remount', () => {
+    if (__DEV__) {
+      let HelloV1 = prepare(() => {
+        function Hello() {
+          return null;
+        }
+        __register__(Hello, 'Hello');
+        __signature__(Hello, '1');
+        return Hello;
+      });
+
+      const Bailout = React.memo(({children}) => {
+        return children;
+      });
+
+      // Each of those renders three instances of HelloV1,
+      // but in different ways.
+      let trees = [
+        <div>
+          <HelloV1 />
+          <div>
+            <HelloV1 />
+            <Bailout>
+              <HelloV1 />
+            </Bailout>
+          </div>
+        </div>,
+        <div>
+          <div>
+            <HelloV1>
+              <HelloV1 />
+            </HelloV1>
+            <HelloV1 />
+          </div>
+        </div>,
+        <div>
+          <span />
+          <HelloV1 />
+          <HelloV1 />
+          <HelloV1 />
+        </div>,
+        <div>
+          <HelloV1 />
+          <span />
+          <HelloV1 />
+          <HelloV1 />
+        </div>,
+        <div>
+          <div>foo</div>
+          <HelloV1 />
+          <div>
+            <HelloV1 />
+          </div>
+          <HelloV1 />
+          <span />
+        </div>,
+        <div>
+          <HelloV1>
+            <span />
+            Hello
+            <span />
+          </HelloV1>,
+          <HelloV1>
+            <React.Fragment>
+              <HelloV1 />
+            </React.Fragment>
+          </HelloV1>,
+        </div>,
+        <HelloV1>
+          <HelloV1>
+            <Bailout>
+              <span />
+              <HelloV1>
+                <span />
+              </HelloV1>
+              <span />
+            </Bailout>
+          </HelloV1>
+        </HelloV1>,
+        <div>
+          <span />
+          <HelloV1 key="0" />
+          <HelloV1 key="1" />
+          <HelloV1 key="2" />
+          <span />
+        </div>,
+        <div>
+          <span />
+          {null}
+          <HelloV1 key="1" />
+          {null}
+          <HelloV1 />
+          <HelloV1 />
+          <span />
+        </div>,
+        <div>
+          <HelloV1 key="2" />
+          <span />
+          <HelloV1 key="0" />
+          <span />
+          <HelloV1 key="1" />
+        </div>,
+        <div>
+          {[[<HelloV1 key="2" />]]}
+          <span>
+            <HelloV1 key="0" />
+            {[null]}
+            <HelloV1 key="1" />
+          </span>
+        </div>,
+        <div>
+          {['foo', <HelloV1 key="hi" />, null, <HelloV1 key="2" />]}
+          <span>
+            {[null]}
+            <HelloV1 key="x" />
+          </span>
+        </div>,
+        <HelloV1>
+          <HelloV1>
+            <span />
+            <Bailout>
+              <HelloV1>hi</HelloV1>
+              <span />
+            </Bailout>
+          </HelloV1>
+        </HelloV1>,
+      ];
+
+      // First, check that each tree handles remounts in isolation.
+      ReactDOM.render(null, container);
+      for (let i = 0; i < trees.length; i++) {
+        runRemountingStressTest(trees[i]);
+      }
+
+      // Then check that each tree is resilient to updates from another tree.
+      for (let i = 0; i < trees.length; i++) {
+        for (let j = 0; j < trees.length; j++) {
+          ReactDOM.render(null, container);
+          // Intentionally don't clean up between the tests:
+          runRemountingStressTest(trees[i]);
+          runRemountingStressTest(trees[j]);
+          runRemountingStressTest(trees[i]);
+        }
+      }
+    }
+  });
+
+  function runRemountingStressTest(tree) {
+    patch(() => {
+      function Hello({children}) {
+        return <section data-color="blue">{children}</section>;
+      }
+      __register__(Hello, 'Hello');
+      __signature__(Hello, '1');
+      return Hello;
+    });
+
+    ReactDOM.render(tree, container);
+    const elements = container.querySelectorAll('section');
+    // Each tree above products exactly three <section> elements:
+    expect(elements.length).toBe(3);
+    elements.forEach(el => {
+      expect(el.dataset.color).toBe('blue');
+    });
+
+    // Patch color without changing the signature.
+    patch(() => {
+      function Hello({children}) {
+        return <section data-color="red">{children}</section>;
+      }
+      __register__(Hello, 'Hello');
+      __signature__(Hello, '1');
+      return Hello;
+    });
+
+    const elementsAfterPatch = container.querySelectorAll('section');
+    expect(elementsAfterPatch.length).toBe(3);
+    elementsAfterPatch.forEach((el, index) => {
+      // The signature hasn't changed so we expect DOM nodes to stay the same.
+      expect(el).toBe(elements[index]);
+      // However, the color should have changed:
+      expect(el.dataset.color).toBe('red');
+    });
+
+    // Patch color *and* change the signature.
+    patch(() => {
+      function Hello({children}) {
+        return <section data-color="orange">{children}</section>;
+      }
+      __register__(Hello, 'Hello');
+      __signature__(Hello, '2'); // Remount
+      return Hello;
+    });
+
+    const elementsAfterRemount = container.querySelectorAll('section');
+    expect(elementsAfterRemount.length).toBe(3);
+    elementsAfterRemount.forEach((el, index) => {
+      // The signature changed so we expect DOM nodes to be different.
+      expect(el).not.toBe(elements[index]);
+      // They should all be using the new color:
+      expect(el.dataset.color).toBe('orange');
+    });
+
+    // Now patch color but *don't* change the signature.
+    patch(() => {
+      function Hello({children}) {
+        return <section data-color="black">{children}</section>;
+      }
+      __register__(Hello, 'Hello');
+      __signature__(Hello, '2'); // Same signature as before
+      return Hello;
+    });
+
+    expect(container.querySelectorAll('section').length).toBe(3);
+    container.querySelectorAll('section').forEach((el, index) => {
+      // The signature didn't change so DOM nodes should stay the same.
+      expect(el).toBe(elementsAfterRemount[index]);
+      // They should all be using the new color:
+      expect(el.dataset.color).toBe('black');
+    });
+
+    // Do another render just in case.
+    ReactDOM.render(tree, container);
+    expect(container.querySelectorAll('section').length).toBe(3);
+    container.querySelectorAll('section').forEach((el, index) => {
+      expect(el).toBe(elementsAfterRemount[index]);
+      expect(el.dataset.color).toBe('black');
+    });
+  }
+
+  it('can remount on signature change within a <root> wrapper', () => {
+    if (__DEV__) {
+      testRemountingWithWrapper(Hello => Hello);
+    }
+  });
+
+  it('can remount on signature change within a simple memo wrapper', () => {
+    if (__DEV__) {
+      testRemountingWithWrapper(Hello => React.memo(Hello));
+    }
+  });
+
+  it('can remount on signature change within forwardRef', () => {
+    if (__DEV__) {
+      testRemountingWithWrapper(Hello => React.forwardRef(Hello));
+    }
+  });
+
+  it('can remount on signature change within forwardRef render function', () => {
+    if (__DEV__) {
+      testRemountingWithWrapper(Hello => React.forwardRef(() => <Hello />));
+    }
+  });
+
+  it('can remount on signature change within nested memo', () => {
+    if (__DEV__) {
+      testRemountingWithWrapper(Hello =>
+        React.memo(React.memo(React.memo(Hello))),
+      );
+    }
+  });
+
+  it('can remount on signature change within a memo wrapper and custom comparison', () => {
+    if (__DEV__) {
+      testRemountingWithWrapper(Hello => React.memo(Hello, () => true));
+    }
+  });
+
+  it('can remount on signature change within a class', () => {
+    if (__DEV__) {
+      testRemountingWithWrapper(Hello => {
+        const child = <Hello />;
+        return class Wrapper extends React.PureComponent {
+          render() {
+            return child;
+          }
+        };
+      });
+    }
+  });
+
+  it('can remount on signature change within a context provider', () => {
+    if (__DEV__) {
+      testRemountingWithWrapper(Hello => {
+        const Context = React.createContext();
+        const child = (
+          <Context.Provider value="constant">
+            <Hello />
+          </Context.Provider>
+        );
+        return function Wrapper() {
+          return child;
+        };
+      });
+    }
+  });
+
+  it('can remount on signature change within a context consumer', () => {
+    if (__DEV__) {
+      testRemountingWithWrapper(Hello => {
+        const Context = React.createContext();
+        const child = <Context.Consumer>{() => <Hello />}</Context.Consumer>;
+        return function Wrapper() {
+          return child;
+        };
+      });
+    }
+  });
+
+  it('can remount on signature change within a suspense node', () => {
+    if (__DEV__) {
+      testRemountingWithWrapper(Hello => {
+        // TODO: we'll probably want to test fallback trees too.
+        const child = (
+          <React.Suspense>
+            <Hello />
+          </React.Suspense>
+        );
+        return function Wrapper() {
+          return child;
+        };
+      });
+    }
+  });
+
+  it('can remount on signature change within a mode node', () => {
+    if (__DEV__) {
+      testRemountingWithWrapper(Hello => {
+        const child = (
+          <React.StrictMode>
+            <Hello />
+          </React.StrictMode>
+        );
+        return function Wrapper() {
+          return child;
+        };
+      });
+    }
+  });
+
+  it('can remount on signature change within a fragment node', () => {
+    if (__DEV__) {
+      testRemountingWithWrapper(Hello => {
+        const child = (
+          <React.Fragment>
+            <Hello />
+          </React.Fragment>
+        );
+        return function Wrapper() {
+          return child;
+        };
+      });
+    }
+  });
+
+  it('can remount on signature change within multiple siblings', () => {
+    if (__DEV__) {
+      testRemountingWithWrapper(Hello => {
+        const child = (
+          <React.Fragment>
+            <React.Fragment>
+              <React.Fragment />
+            </React.Fragment>
+            <Hello />
+            <React.Fragment />
+          </React.Fragment>
+        );
+        return function Wrapper() {
+          return child;
+        };
+      });
+    }
+  });
+
+  it('can remount on signature change within a profiler node', () => {
+    if (__DEV__) {
+      testRemountingWithWrapper(Hello => {
+        const child = <Hello />;
+        return function Wrapper() {
+          return (
+            <React.Profiler onRender={() => {}} id="foo">
+              {child}
+            </React.Profiler>
+          );
+        };
+      });
+    }
+  });
+
+  function testRemountingWithWrapper(wrap) {
+    render(() => {
+      function Hello() {
+        const [val, setVal] = React.useState(0);
+        return (
+          <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+            {val}
+          </p>
+        );
+      }
+      __register__(Hello, 'Hello');
+      // When this changes, we'll expect a remount:
+      __signature__(Hello, '1');
+
+      // Use the passed wrapper.
+      // This will be different in every test.
+      return wrap(Hello);
+    });
+
+    // Bump the state before patching.
+    const el = container.firstChild;
+    expect(el.textContent).toBe('0');
+    expect(el.style.color).toBe('blue');
+    act(() => {
+      el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+    });
+    expect(el.textContent).toBe('1');
+
+    // Perform a hot update that doesn't remount.
+    patch(() => {
+      function Hello() {
+        const [val, setVal] = React.useState(0);
+        return (
+          <p style={{color: 'red'}} onClick={() => setVal(val + 1)}>
+            {val}
+          </p>
+        );
+      }
+      __register__(Hello, 'Hello');
+      // The signature hasn't changed since the last time:
+      __signature__(Hello, '1');
+      return Hello;
+    });
+
+    // Assert the state was preserved but color changed.
+    expect(container.firstChild).toBe(el);
+    expect(el.textContent).toBe('1');
+    expect(el.style.color).toBe('red');
+
+    // Perform a hot update that remounts.
+    patch(() => {
+      function Hello() {
+        const [val, setVal] = React.useState(0);
+        return (
+          <p style={{color: 'yellow'}} onClick={() => setVal(val + 1)}>
+            {val}
+          </p>
+        );
+      }
+      // We're changing the signature now so it will remount:
+      __register__(Hello, 'Hello');
+      __signature__(Hello, '2');
+      return Hello;
+    });
+
+    // Expect a remount.
+    expect(container.firstChild).not.toBe(el);
+    const newEl = container.firstChild;
+    expect(newEl.textContent).toBe('0');
+    expect(newEl.style.color).toBe('yellow');
+
+    // Bump state again.
+    act(() => {
+      newEl.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+    });
+    expect(newEl.textContent).toBe('1');
+    expect(newEl.style.color).toBe('yellow');
+
+    // Verify we can patch again while preserving the signature.
+    patch(() => {
+      function Hello() {
+        const [val, setVal] = React.useState(0);
+        return (
+          <p style={{color: 'purple'}} onClick={() => setVal(val + 1)}>
+            {val}
+          </p>
+        );
+      }
+      // Same signature as last time.
+      __register__(Hello, 'Hello');
+      __signature__(Hello, '2');
+      return Hello;
+    });
+
+    expect(container.firstChild).toBe(newEl);
+    expect(newEl.textContent).toBe('1');
+    expect(newEl.style.color).toBe('purple');
+
+    // Check removing the signature also causes a remount.
+    patch(() => {
+      function Hello() {
+        const [val, setVal] = React.useState(0);
+        return (
+          <p style={{color: 'orange'}} onClick={() => setVal(val + 1)}>
+            {val}
+          </p>
+        );
+      }
+      // No signature this time.
+      __register__(Hello, 'Hello');
+      return Hello;
+    });
+
+    // Expect a remount.
+    expect(container.firstChild).not.toBe(newEl);
+    const finalEl = container.firstChild;
+    expect(finalEl.textContent).toBe('0');
+    expect(finalEl.style.color).toBe('orange');
+  }
+
+  it('resets hooks with dependencies on hot reload', () => {
+    if (__DEV__) {
+      let useEffectWithEmptyArrayCalls = 0;
+
+      render(() => {
+        function Hello() {
+          const [val, setVal] = React.useState(0);
+          const tranformed = React.useMemo(() => val * 2, [val]);
+          const handleClick = React.useCallback(() => setVal(v => v + 1), []);
+
+          React.useEffect(() => {
+            useEffectWithEmptyArrayCalls++;
+          }, []);
+
+          return (
+            <p style={{color: 'blue'}} onClick={handleClick}>
+              {tranformed}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+        return Hello;
+      });
+
+      // Bump the state before patching.
+      const el = container.firstChild;
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('blue');
+      expect(useEffectWithEmptyArrayCalls).toBe(1); // useEffect ran
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('2'); // val * 2
+      expect(useEffectWithEmptyArrayCalls).toBe(1); // useEffect didn't re-run
+
+      // Perform a hot update.
+      act(() => {
+        patch(() => {
+          function Hello() {
+            const [val, setVal] = React.useState(0);
+            const tranformed = React.useMemo(() => val * 10, [val]);
+            const handleClick = React.useCallback(() => setVal(v => v - 1), []);
+
+            React.useEffect(() => {
+              useEffectWithEmptyArrayCalls++;
+            }, []);
+
+            return (
+              <p style={{color: 'red'}} onClick={handleClick}>
+                {tranformed}
+              </p>
+            );
+          }
+          __register__(Hello, 'Hello');
+          return Hello;
+        });
+      });
+
+      // Assert the state was preserved but memo was evicted.
+      expect(container.firstChild).toBe(el);
+      expect(el.textContent).toBe('10'); // val * 10
+      expect(el.style.color).toBe('red');
+      expect(useEffectWithEmptyArrayCalls).toBe(2); // useEffect re-ran
+
+      // This should fire the new callback which decreases the counter.
+      act(() => {
+        el.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      });
+      expect(el.textContent).toBe('0');
+      expect(el.style.color).toBe('red');
+      expect(useEffectWithEmptyArrayCalls).toBe(2); // useEffect didn't re-run
+    }
+  });
+
+  it('can hot reload offscreen components', () => {
+    if (__DEV__) {
+      const AppV1 = prepare(() => {
+        function Hello() {
+          React.useLayoutEffect(() => {
+            Scheduler.yieldValue('Hello#layout');
+          });
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'blue'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+
+        return function App({offscreen}) {
+          React.useLayoutEffect(() => {
+            Scheduler.yieldValue('App#layout');
+          });
+          return (
+            <div hidden={offscreen}>
+              <Hello />
+            </div>
+          );
+        };
+      });
+
+      const root = ReactDOM.unstable_createRoot(container);
+      root.render(<AppV1 offscreen={true} />);
+      expect(Scheduler).toFlushAndYieldThrough(['App#layout']);
+      const el = container.firstChild;
+      expect(el.hidden).toBe(true);
+      expect(el.firstChild).toBe(null); // Offscreen content not flushed yet.
+
+      // Perform a hot update.
+      patch(() => {
+        function Hello() {
+          React.useLayoutEffect(() => {
+            Scheduler.yieldValue('Hello#layout');
+          });
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'red'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+      });
+
+      // It's still offscreen so we don't see anything.
+      expect(container.firstChild).toBe(el);
+      expect(el.hidden).toBe(true);
+      expect(el.firstChild).toBe(null);
+
+      // Process the offscreen updates.
+      expect(Scheduler).toFlushAndYieldThrough(['Hello#layout']);
+      expect(container.firstChild).toBe(el);
+      expect(el.firstChild.textContent).toBe('0');
+      expect(el.firstChild.style.color).toBe('red');
+
+      el.firstChild.dispatchEvent(new MouseEvent('click', {bubbles: true}));
+      expect(el.firstChild.textContent).toBe('0');
+      expect(el.firstChild.style.color).toBe('red');
+      expect(Scheduler).toFlushAndYieldThrough(['Hello#layout']);
+      expect(el.firstChild.textContent).toBe('1');
+      expect(el.firstChild.style.color).toBe('red');
+
+      // Hot reload while we're offscreen.
+      patch(() => {
+        function Hello() {
+          React.useLayoutEffect(() => {
+            Scheduler.yieldValue('Hello#layout');
+          });
+          const [val, setVal] = React.useState(0);
+          return (
+            <p style={{color: 'orange'}} onClick={() => setVal(val + 1)}>
+              {val}
+            </p>
+          );
+        }
+        __register__(Hello, 'Hello');
+      });
+
+      // It's still offscreen so we don't see the updates.
+      expect(container.firstChild).toBe(el);
+      expect(el.firstChild.textContent).toBe('1');
+      expect(el.firstChild.style.color).toBe('red');
+
+      // Process the offscreen updates.
+      expect(Scheduler).toFlushAndYieldThrough(['Hello#layout']);
+      expect(container.firstChild).toBe(el);
+      expect(el.firstChild.textContent).toBe('1');
+      expect(el.firstChild.style.color).toBe('orange');
+    }
+  });
+
+  it('remounts failed error boundaries (componentDidCatch)', () => {
+    if (__DEV__) {
+      render(() => {
+        function Hello() {
+          return <h1>Hi</h1>;
+        }
+        __register__(Hello, 'Hello');
+
+        class Boundary extends React.Component {
+          state = {error: null};
+          componentDidCatch(error) {
+            this.setState({error});
+          }
+          render() {
+            if (this.state.error) {
+              return <h1>Oops: {this.state.error.message}</h1>;
+            }
+            return this.props.children;
+          }
+        }
+
+        function App() {
+          return (
+            <React.Fragment>
+              <p>A</p>
+              <Boundary>
+                <Hello />
+              </Boundary>
+              <p>B</p>
+            </React.Fragment>
+          );
+        }
+
+        return App;
+      });
+
+      expect(container.innerHTML).toBe('<p>A</p><h1>Hi</h1><p>B</p>');
+      const firstP = container.firstChild;
+      const secondP = firstP.nextSibling.nextSibling;
+
+      // Perform a hot update that fails.
+      patch(() => {
+        function Hello() {
+          throw new Error('No');
+        }
+        __register__(Hello, 'Hello');
+      });
+
+      expect(container.innerHTML).toBe('<p>A</p><h1>Oops: No</h1><p>B</p>');
+      expect(container.firstChild).toBe(firstP);
+      expect(container.firstChild.nextSibling.nextSibling).toBe(secondP);
+
+      // Perform a hot update that fixes the error.
+      patch(() => {
+        function Hello() {
+          return <h1>Fixed!</h1>;
+        }
+        __register__(Hello, 'Hello');
+      });
+
+      // This should remount the error boundary (but not anything above it).
+      expect(container.innerHTML).toBe('<p>A</p><h1>Fixed!</h1><p>B</p>');
+      expect(container.firstChild).toBe(firstP);
+      expect(container.firstChild.nextSibling.nextSibling).toBe(secondP);
+
+      // Verify next hot reload doesn't remount anything.
+      const helloNode = container.firstChild.nextSibling;
+      patch(() => {
+        function Hello() {
+          return <h1>Nice.</h1>;
+        }
+        __register__(Hello, 'Hello');
+      });
+      expect(container.firstChild.nextSibling).toBe(helloNode);
+      expect(helloNode.textContent).toBe('Nice.');
+    }
+  });
+
+  it('remounts failed error boundaries (getDerivedStateFromError)', () => {
+    if (__DEV__) {
+      render(() => {
+        function Hello() {
+          return <h1>Hi</h1>;
+        }
+        __register__(Hello, 'Hello');
+
+        class Boundary extends React.Component {
+          state = {error: null};
+          static getDerivedStateFromError(error) {
+            return {error};
+          }
+          render() {
+            if (this.state.error) {
+              return <h1>Oops: {this.state.error.message}</h1>;
+            }
+            return this.props.children;
+          }
+        }
+
+        function App() {
+          return (
+            <React.Fragment>
+              <p>A</p>
+              <Boundary>
+                <Hello />
+              </Boundary>
+              <p>B</p>
+            </React.Fragment>
+          );
+        }
+
+        return App;
+      });
+
+      expect(container.innerHTML).toBe('<p>A</p><h1>Hi</h1><p>B</p>');
+      const firstP = container.firstChild;
+      const secondP = firstP.nextSibling.nextSibling;
+
+      // Perform a hot update that fails.
+      patch(() => {
+        function Hello() {
+          throw new Error('No');
+        }
+        __register__(Hello, 'Hello');
+      });
+
+      expect(container.innerHTML).toBe('<p>A</p><h1>Oops: No</h1><p>B</p>');
+      expect(container.firstChild).toBe(firstP);
+      expect(container.firstChild.nextSibling.nextSibling).toBe(secondP);
+
+      // Perform a hot update that fixes the error.
+      patch(() => {
+        function Hello() {
+          return <h1>Fixed!</h1>;
+        }
+        __register__(Hello, 'Hello');
+      });
+
+      // This should remount the error boundary (but not anything above it).
+      expect(container.innerHTML).toBe('<p>A</p><h1>Fixed!</h1><p>B</p>');
+      expect(container.firstChild).toBe(firstP);
+      expect(container.firstChild.nextSibling.nextSibling).toBe(secondP);
+
+      // Verify next hot reload doesn't remount anything.
+      const helloNode = container.firstChild.nextSibling;
+      patch(() => {
+        function Hello() {
+          return <h1>Nice.</h1>;
+        }
+        __register__(Hello, 'Hello');
+      });
+      expect(container.firstChild.nextSibling).toBe(helloNode);
+      expect(helloNode.textContent).toBe('Nice.');
+    }
+  });
+
+  it('remounts error boundaries that failed asynchronously after hot update', () => {
+    if (__DEV__) {
+      render(() => {
+        function Hello() {
+          const [x] = React.useState('');
+          React.useEffect(() => {}, []);
+          x.slice(); // Doesn't throw initially.
+          return <h1>Hi</h1>;
+        }
+        __register__(Hello, 'Hello');
+
+        class Boundary extends React.Component {
+          state = {error: null};
+          static getDerivedStateFromError(error) {
+            return {error};
+          }
+          render() {
+            if (this.state.error) {
+              return <h1>Oops: {this.state.error.message}</h1>;
+            }
+            return this.props.children;
+          }
+        }
+
+        function App() {
+          return (
+            <React.Fragment>
+              <p>A</p>
+              <Boundary>
+                <Hello />
+              </Boundary>
+              <p>B</p>
+            </React.Fragment>
+          );
+        }
+
+        return App;
+      });
+
+      expect(container.innerHTML).toBe('<p>A</p><h1>Hi</h1><p>B</p>');
+      const firstP = container.firstChild;
+      const secondP = firstP.nextSibling.nextSibling;
+
+      // Perform a hot update that fails.
+      act(() => {
+        patch(() => {
+          function Hello() {
+            const [x, setX] = React.useState('');
+            React.useEffect(() => {
+              setTimeout(() => {
+                setX(42); // This will crash next render.
+              }, 1);
+            }, []);
+            x.slice();
+            return <h1>Hi</h1>;
+          }
+          __register__(Hello, 'Hello');
+        });
+      });
+
+      expect(container.innerHTML).toBe('<p>A</p><h1>Hi</h1><p>B</p>');
+      // Run timeout inside effect:
+      act(() => {
+        jest.runAllTimers();
+      });
+      expect(container.innerHTML).toBe(
+        '<p>A</p><h1>Oops: x.slice is not a function</h1><p>B</p>',
+      );
+      expect(container.firstChild).toBe(firstP);
+      expect(container.firstChild.nextSibling.nextSibling).toBe(secondP);
+
+      // Perform a hot update that fixes the error.
+      patch(() => {
+        function Hello() {
+          const [x] = React.useState('');
+          React.useEffect(() => {}, []); // Removes the bad effect code.
+          x.slice(); // Doesn't throw initially.
+          return <h1>Fixed!</h1>;
+        }
+        __register__(Hello, 'Hello');
+      });
+
+      // This should remount the error boundary (but not anything above it).
+      expect(container.innerHTML).toBe('<p>A</p><h1>Fixed!</h1><p>B</p>');
+      expect(container.firstChild).toBe(firstP);
+      expect(container.firstChild.nextSibling.nextSibling).toBe(secondP);
+
+      // Verify next hot reload doesn't remount anything.
+      const helloNode = container.firstChild.nextSibling;
+      patch(() => {
+        function Hello() {
+          const [x] = React.useState('');
+          React.useEffect(() => {}, []);
+          x.slice();
+          return <h1>Nice.</h1>;
+        }
+        __register__(Hello, 'Hello');
+      });
+      expect(container.firstChild.nextSibling).toBe(helloNode);
+      expect(helloNode.textContent).toBe('Nice.');
+    }
+  });
+});
diff --git a/packages/react-reconciler/src/ReactChildFiber.js b/packages/react-reconciler/src/ReactChildFiber.js
index f51fdcfe5a..3d9def376b 100644
--- a/packages/react-reconciler/src/ReactChildFiber.js
+++ b/packages/react-reconciler/src/ReactChildFiber.js
@@ -43,6 +43,7 @@ import {
   getCurrentFiberStackInDev,
   getStackByFiberInDevAndProd,
 } from './ReactCurrentFiber';
+import {isCompatibleFamilyForHotReloading} from './ReactFiberHotReloading';
 import {StrictMode} from './ReactTypeOfMode';
 
 let didWarnAboutMaps;
@@ -378,7 +379,12 @@ function ChildReconciler(shouldTrackSideEffects) {
     element: ReactElement,
     expirationTime: ExpirationTime,
   ): Fiber {
-    if (current !== null && current.elementType === element.type) {
+    if (
+      current !== null &&
+      (current.elementType === element.type ||
+        // Keep this check inline so it only runs on the false path:
+        (__DEV__ ? isCompatibleFamilyForHotReloading(current, element) : false))
+    ) {
       // Move based on index
       const existing = useFiber(current, element.props, expirationTime);
       existing.ref = coerceRef(returnFiber, current, element);
@@ -1121,7 +1127,11 @@ function ChildReconciler(shouldTrackSideEffects) {
         if (
           child.tag === Fragment
             ? element.type === REACT_FRAGMENT_TYPE
-            : child.elementType === element.type
+            : child.elementType === element.type ||
+              // Keep this check inline so it only runs on the false path:
+              (__DEV__
+                ? isCompatibleFamilyForHotReloading(child, element)
+                : false)
         ) {
           deleteRemainingChildren(returnFiber, child.sibling);
           const existing = useFiber(
diff --git a/packages/react-reconciler/src/ReactFiber.js b/packages/react-reconciler/src/ReactFiber.js
index 1330887c14..589c482bc1 100644
--- a/packages/react-reconciler/src/ReactFiber.js
+++ b/packages/react-reconciler/src/ReactFiber.js
@@ -45,6 +45,7 @@ import {
   SuspenseComponent,
   FunctionComponent,
   MemoComponent,
+  SimpleMemoComponent,
   LazyComponent,
   EventComponent,
   EventTarget,
@@ -52,6 +53,10 @@ import {
 import getComponentName from 'shared/getComponentName';
 
 import {isDevToolsPresent} from './ReactFiberDevToolsHook';
+import {
+  resolveFunctionForHotReloading,
+  resolveForwardRefForHotReloading,
+} from './ReactFiberHotReloading';
 import {NoWork} from './ReactFiberExpirationTime';
 import {
   NoMode,
@@ -218,6 +223,7 @@ export type Fiber = {|
   _debugSource?: Source | null,
   _debugOwner?: Fiber | null,
   _debugIsCurrentlyTiming?: boolean,
+  _debugNeedsRemount?: boolean,
 
   // Used to verify that the order of hooks does not change between renders.
   _debugHookTypes?: Array<HookType> | null,
@@ -302,6 +308,7 @@ function FiberNode(
     this._debugSource = null;
     this._debugOwner = null;
     this._debugIsCurrentlyTiming = false;
+    this._debugNeedsRemount = false;
     this._debugHookTypes = null;
     if (!hasBadMapPolyfill && typeof Object.preventExtensions === 'function') {
       Object.preventExtensions(this);
@@ -434,6 +441,22 @@ export function createWorkInProgress(
     workInProgress.treeBaseDuration = current.treeBaseDuration;
   }
 
+  if (__DEV__) {
+    workInProgress._debugNeedsRemount = current._debugNeedsRemount;
+    switch (workInProgress.tag) {
+      case IndeterminateComponent:
+      case FunctionComponent:
+      case SimpleMemoComponent:
+        workInProgress.type = resolveFunctionForHotReloading(current.type);
+        break;
+      case ForwardRef:
+        workInProgress.type = resolveForwardRefForHotReloading(current.type);
+        break;
+      default:
+        break;
+    }
+  }
+
   return workInProgress;
 }
 
@@ -473,6 +496,10 @@ export function createFiberFromTypeAndProps(
   if (typeof type === 'function') {
     if (shouldConstruct(type)) {
       fiberTag = ClassComponent;
+    } else {
+      if (__DEV__) {
+        resolvedType = resolveFunctionForHotReloading(resolvedType);
+      }
     }
   } else if (typeof type === 'string') {
     fiberTag = HostComponent;
@@ -509,6 +536,9 @@ export function createFiberFromTypeAndProps(
               break getTag;
             case REACT_FORWARD_REF_TYPE:
               fiberTag = ForwardRef;
+              if (__DEV__) {
+                resolvedType = resolveForwardRefForHotReloading(resolvedType);
+              }
               break getTag;
             case REACT_MEMO_TYPE:
               fiberTag = MemoComponent;
@@ -777,6 +807,7 @@ export function assignFiberPropertiesInDEV(
   target._debugSource = source._debugSource;
   target._debugOwner = source._debugOwner;
   target._debugIsCurrentlyTiming = source._debugIsCurrentlyTiming;
+  target._debugNeedsRemount = source._debugNeedsRemount;
   target._debugHookTypes = source._debugHookTypes;
   return target;
 }
diff --git a/packages/react-reconciler/src/ReactFiberBeginWork.js b/packages/react-reconciler/src/ReactFiberBeginWork.js
index 56208b1c43..71b931f6bb 100644
--- a/packages/react-reconciler/src/ReactFiberBeginWork.js
+++ b/packages/react-reconciler/src/ReactFiberBeginWork.js
@@ -71,6 +71,7 @@ import {
   getCurrentFiberStackInDev,
 } from './ReactCurrentFiber';
 import {startWorkTimer, cancelWorkTimer} from './ReactDebugFiberPerf';
+import {resolveFunctionForHotReloading} from './ReactFiberHotReloading';
 
 import {
   mountChildFibers,
@@ -358,18 +359,22 @@ function updateMemoComponent(
       // SimpleMemoComponent codepath doesn't resolve outer props either.
       Component.defaultProps === undefined
     ) {
+      let resolvedType = type;
+      if (__DEV__) {
+        resolvedType = resolveFunctionForHotReloading(type);
+      }
       // If this is a plain function component without default props,
       // and with only the default shallow comparison, we upgrade it
       // to a SimpleMemoComponent to allow fast path updates.
       workInProgress.tag = SimpleMemoComponent;
-      workInProgress.type = type;
+      workInProgress.type = resolvedType;
       if (__DEV__) {
         validateFunctionComponentInDev(workInProgress, type);
       }
       return updateSimpleMemoComponent(
         current,
         workInProgress,
-        type,
+        resolvedType,
         nextProps,
         updateExpirationTime,
         renderExpirationTime,
@@ -486,7 +491,9 @@ function updateSimpleMemoComponent(
     const prevProps = current.memoizedProps;
     if (
       shallowEqual(prevProps, nextProps) &&
-      current.ref === workInProgress.ref
+      current.ref === workInProgress.ref &&
+      // Prevent bailout if the implementation changed due to hot reload:
+      (__DEV__ ? workInProgress.type === current.type : true)
     ) {
       didReceiveUpdate = false;
       if (updateExpirationTime < renderExpirationTime) {
@@ -1023,6 +1030,9 @@ function mountLazyComponent(
   // Cancel and resume right after we know the tag.
   cancelWorkTimer(workInProgress);
   let Component = readLazyComponentType(elementType);
+  if (__DEV__) {
+    // TODO: resolve type for hot reloading.
+  }
   // Store the unwrapped component in the type.
   workInProgress.type = Component;
   const resolvedTag = (workInProgress.tag = resolveLazyComponentTag(Component));
@@ -2106,6 +2116,68 @@ function bailoutOnAlreadyFinishedWork(
   }
 }
 
+function remountFiber(
+  current: Fiber,
+  oldWorkInProgress: Fiber,
+  newWorkInProgress: Fiber,
+): Fiber | null {
+  if (__DEV__) {
+    const returnFiber = oldWorkInProgress.return;
+    if (returnFiber === null) {
+      throw new Error('Cannot swap the root fiber.');
+    }
+
+    // Disconnect from the old current.
+    // It will get deleted.
+    current.alternate = null;
+    oldWorkInProgress.alternate = null;
+
+    // Connect to the new tree.
+    newWorkInProgress.index = oldWorkInProgress.index;
+    newWorkInProgress.sibling = oldWorkInProgress.sibling;
+    newWorkInProgress.return = oldWorkInProgress.return;
+
+    // Replace the child/sibling pointers above it.
+    if (oldWorkInProgress === returnFiber.child) {
+      returnFiber.child = newWorkInProgress;
+    } else {
+      let prevSibling = returnFiber.child;
+      if (prevSibling === null) {
+        throw new Error('Expected parent to have a child.');
+      }
+      while (prevSibling.sibling !== oldWorkInProgress) {
+        prevSibling = prevSibling.sibling;
+        if (prevSibling === null) {
+          throw new Error('Expected to find the previous sibling.');
+        }
+      }
+      prevSibling.sibling = newWorkInProgress;
+    }
+
+    // Delete the old fiber and place the new one.
+    // Since the old fiber is disconnected, we have to schedule it manually.
+    const last = returnFiber.lastEffect;
+    if (last !== null) {
+      last.nextEffect = current;
+      returnFiber.lastEffect = current;
+    } else {
+      returnFiber.firstEffect = returnFiber.lastEffect = current;
+    }
+    current.nextEffect = null;
+    current.effectTag = Deletion;
+
+    newWorkInProgress.effectTag |= Placement;
+
+    // Restart work from the new fiber.
+    return newWorkInProgress;
+  } else {
+    throw new Error(
+      'Did not expect this call in production. ' +
+        'This is a bug in React. Please file an issue.',
+    );
+  }
+}
+
 function beginWork(
   current: Fiber | null,
   workInProgress: Fiber,
@@ -2113,11 +2185,34 @@ function beginWork(
 ): Fiber | null {
   const updateExpirationTime = workInProgress.expirationTime;
 
+  if (__DEV__) {
+    if (workInProgress._debugNeedsRemount && current !== null) {
+      // This will restart the begin phase with a new fiber.
+      return remountFiber(
+        current,
+        workInProgress,
+        createFiberFromTypeAndProps(
+          workInProgress.type,
+          workInProgress.key,
+          workInProgress.pendingProps,
+          workInProgress._debugOwner || null,
+          workInProgress.mode,
+          workInProgress.expirationTime,
+        ),
+      );
+    }
+  }
+
   if (current !== null) {
     const oldProps = current.memoizedProps;
     const newProps = workInProgress.pendingProps;
 
-    if (oldProps !== newProps || hasLegacyContextChanged()) {
+    if (
+      oldProps !== newProps ||
+      hasLegacyContextChanged() ||
+      // Force a re-render if the implementation changed due to hot reload:
+      (__DEV__ ? workInProgress.type !== current.type : false)
+    ) {
       // If props or context changed, mark the fiber as having performed work.
       // This may be unset if the props are determined to be equal later (memo).
       didReceiveUpdate = true;
@@ -2256,11 +2351,10 @@ function beginWork(
 
   switch (workInProgress.tag) {
     case IndeterminateComponent: {
-      const elementType = workInProgress.elementType;
       return mountIndeterminateComponent(
         current,
         workInProgress,
-        elementType,
+        workInProgress.type,
         renderExpirationTime,
       );
     }
diff --git a/packages/react-reconciler/src/ReactFiberHooks.js b/packages/react-reconciler/src/ReactFiberHooks.js
index 69147a0f71..f7eec92b3d 100644
--- a/packages/react-reconciler/src/ReactFiberHooks.js
+++ b/packages/react-reconciler/src/ReactFiberHooks.js
@@ -189,6 +189,11 @@ let currentHookNameInDev: ?HookType = null;
 let hookTypesDev: Array<HookType> | null = null;
 let hookTypesUpdateIndexDev: number = -1;
 
+// In DEV, this tracks whether currently rendering component needs to ignore
+// the dependencies for Hooks that need them (e.g. useEffect or useMemo).
+// When true, such Hooks will always be "remounted". Only used during hot reload.
+let ignorePreviousDependencies: boolean = false;
+
 function mountHookTypesDev() {
   if (__DEV__) {
     const hookName = ((currentHookNameInDev: any): HookType);
@@ -296,6 +301,13 @@ function areHookInputsEqual(
   nextDeps: Array<mixed>,
   prevDeps: Array<mixed> | null,
 ) {
+  if (__DEV__) {
+    if (ignorePreviousDependencies) {
+      // Only true when this component is being hot reloaded.
+      return false;
+    }
+  }
+
   if (prevDeps === null) {
     if (__DEV__) {
       warning(
@@ -352,6 +364,9 @@ export function renderWithHooks(
         ? ((current._debugHookTypes: any): Array<HookType>)
         : null;
     hookTypesUpdateIndexDev = -1;
+    // Used for hot reloading:
+    ignorePreviousDependencies =
+      current !== null && current.type !== workInProgress.type;
   }
 
   // The following should have already been reset
diff --git a/packages/react-reconciler/src/ReactFiberHotReloading.js b/packages/react-reconciler/src/ReactFiberHotReloading.js
new file mode 100644
index 0000000000..1804ba6fa5
--- /dev/null
+++ b/packages/react-reconciler/src/ReactFiberHotReloading.js
@@ -0,0 +1,264 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import type {ReactElement} from 'shared/ReactElementType';
+import type {Fiber} from './ReactFiber';
+import type {FiberRoot} from './ReactFiberRoot';
+
+import {
+  flushSync,
+  scheduleWork,
+  flushPassiveEffects,
+} from './ReactFiberScheduler';
+import {Sync} from './ReactFiberExpirationTime';
+import {
+  FunctionComponent,
+  ForwardRef,
+  MemoComponent,
+  SimpleMemoComponent,
+} from 'shared/ReactWorkTags';
+import {REACT_FORWARD_REF_TYPE, REACT_MEMO_TYPE} from 'shared/ReactSymbols';
+
+type Family = {|
+  currentType: any,
+  currentSignature: null | string,
+|};
+
+type HotUpdate = {|
+  familiesByType: WeakMap<any, Family>,
+  root: FiberRoot,
+  staleFamilies: Set<Family>,
+  updatedFamilies: Set<Family>,
+|};
+
+let familiesByType: WeakMap<any, Family> | null = null;
+// $FlowFixMe Flow gets confused by a WeakSet feature check below.
+let failedBoundaries: WeakSet<Fiber> | null = null;
+
+export function resolveFunctionForHotReloading(type: any): any {
+  if (__DEV__) {
+    if (familiesByType === null) {
+      // Hot reloading is disabled.
+      return type;
+    }
+    let family = familiesByType.get(type);
+    if (family === undefined) {
+      return type;
+    }
+    // Use the latest known implementation.
+    return family.currentType;
+  } else {
+    return type;
+  }
+}
+
+export function resolveForwardRefForHotReloading(type: any): any {
+  if (__DEV__) {
+    if (familiesByType === null) {
+      // Hot reloading is disabled.
+      return type;
+    }
+    let family = familiesByType.get(type);
+    if (family === undefined) {
+      // Check if we're dealing with a real forwardRef. Don't want to crash early.
+      if (
+        type !== null &&
+        type !== undefined &&
+        typeof type.render === 'function'
+      ) {
+        // ForwardRef is special because its resolved .type is an object,
+        // but it's possible that we only have its inner render function in the map.
+        // If that inner render function is different, we'll build a new forwardRef type.
+        const currentRender = resolveFunctionForHotReloading(type.render);
+        if (type.render !== currentRender) {
+          const syntheticType = {
+            $$typeof: REACT_FORWARD_REF_TYPE,
+            render: currentRender,
+          };
+          if (type.displayName !== undefined) {
+            (syntheticType: any).displayName = type.displayName;
+          }
+          return syntheticType;
+        }
+      }
+      return type;
+    }
+    // Use the latest known implementation.
+    return family.currentType;
+  } else {
+    return type;
+  }
+}
+
+export function isCompatibleFamilyForHotReloading(
+  fiber: Fiber,
+  element: ReactElement,
+): boolean {
+  if (__DEV__) {
+    if (familiesByType === null) {
+      // Hot reloading is disabled.
+      return false;
+    }
+
+    const prevType = fiber.elementType;
+    const nextType = element.type;
+    // If we got here, we know types aren't === equal.
+    let needsCompareFamilies = false;
+    switch (fiber.tag) {
+      case FunctionComponent: {
+        if (typeof nextType === 'function') {
+          needsCompareFamilies = true;
+        }
+        break;
+      }
+      case ForwardRef: {
+        if (
+          typeof nextType === 'object' &&
+          nextType !== null &&
+          nextType.$$typeof === REACT_FORWARD_REF_TYPE
+        ) {
+          needsCompareFamilies = true;
+        }
+        break;
+      }
+      case MemoComponent:
+      case SimpleMemoComponent: {
+        if (
+          typeof nextType === 'object' &&
+          nextType !== null &&
+          nextType.$$typeof === REACT_MEMO_TYPE
+        ) {
+          // TODO: if it was but can no longer be simple,
+          // we shouldn't set this.
+          needsCompareFamilies = true;
+        }
+        break;
+      }
+      // TODO: maybe support lazy?
+      default:
+        return false;
+    }
+
+    // Check if both types have a family and it's the same one.
+    if (needsCompareFamilies) {
+      // Note: memo() and forwardRef() we'll compare outer rather than inner type.
+      // This means both of them need to be registered to preserve state.
+      // If we unwrapped and compared the inner types for wrappers instead,
+      // then we would risk falsely saying two separate memo(Foo)
+      // calls are equivalent because they wrap the same Foo function.
+      const prevFamily = familiesByType.get(prevType);
+      if (
+        prevFamily !== undefined &&
+        prevFamily === familiesByType.get(nextType)
+      ) {
+        return true;
+      }
+    }
+    return false;
+  } else {
+    return false;
+  }
+}
+
+export function markFailedErrorBoundaryForHotReloading(fiber: Fiber) {
+  if (__DEV__) {
+    if (familiesByType === null) {
+      // Not hot reloading.
+      return;
+    }
+    if (typeof WeakSet !== 'function') {
+      return;
+    }
+    if (failedBoundaries === null) {
+      failedBoundaries = new WeakSet();
+    }
+    failedBoundaries.add(fiber);
+  }
+}
+
+export function scheduleHotUpdate(hotUpdate: HotUpdate): void {
+  if (__DEV__) {
+    // TODO: warn if its identity changes over time?
+    familiesByType = hotUpdate.familiesByType;
+
+    const {root, staleFamilies, updatedFamilies} = hotUpdate;
+    flushPassiveEffects();
+    flushSync(() => {
+      scheduleFibersWithFamiliesRecursively(
+        root.current,
+        updatedFamilies,
+        staleFamilies,
+      );
+    });
+  }
+}
+
+function scheduleFibersWithFamiliesRecursively(
+  fiber: Fiber,
+  updatedFamilies: Set<Family>,
+  staleFamilies: Set<Family>,
+) {
+  if (__DEV__) {
+    const {alternate, child, sibling, tag, type} = fiber;
+
+    let candidateType = null;
+    switch (tag) {
+      case FunctionComponent:
+      case SimpleMemoComponent:
+        candidateType = type;
+        break;
+      case ForwardRef:
+        candidateType = type.render;
+        break;
+      default:
+        break;
+    }
+
+    if (familiesByType === null) {
+      throw new Error('Expected familiesByType to be set during hot reload.');
+    }
+
+    if (candidateType !== null) {
+      const family = familiesByType.get(candidateType);
+      if (family !== undefined) {
+        if (staleFamilies.has(family)) {
+          fiber._debugNeedsRemount = true;
+          scheduleWork(fiber, Sync);
+        } else if (updatedFamilies.has(family)) {
+          scheduleWork(fiber, Sync);
+        }
+      }
+    }
+
+    if (failedBoundaries !== null) {
+      if (
+        failedBoundaries.has(fiber) ||
+        (alternate !== null && failedBoundaries.has(alternate))
+      ) {
+        fiber._debugNeedsRemount = true;
+        scheduleWork(fiber, Sync);
+      }
+    }
+
+    if (child !== null) {
+      scheduleFibersWithFamiliesRecursively(
+        child,
+        updatedFamilies,
+        staleFamilies,
+      );
+    }
+    if (sibling !== null) {
+      scheduleFibersWithFamiliesRecursively(
+        sibling,
+        updatedFamilies,
+        staleFamilies,
+      );
+    }
+  }
+}
diff --git a/packages/react-reconciler/src/ReactFiberReconciler.js b/packages/react-reconciler/src/ReactFiberReconciler.js
index ae3d449339..84291f4b7a 100644
--- a/packages/react-reconciler/src/ReactFiberReconciler.js
+++ b/packages/react-reconciler/src/ReactFiberReconciler.js
@@ -67,6 +67,7 @@ import {StrictMode} from './ReactTypeOfMode';
 import {Sync} from './ReactFiberExpirationTime';
 import {revertPassiveEffectsChange} from 'shared/ReactFeatureFlags';
 import {requestCurrentSuspenseConfig} from './ReactFiberSuspenseConfig';
+import {scheduleHotUpdate} from './ReactFiberHotReloading';
 
 type OpaqueRoot = FiberRoot;
 
@@ -460,6 +461,7 @@ export function injectIntoDevTools(devToolsConfig: DevToolsConfig): boolean {
 
   return injectInternals({
     ...devToolsConfig,
+    scheduleHotUpdate: __DEV__ ? scheduleHotUpdate : null,
     overrideHookState,
     overrideProps,
     setSuspenseHandler,
diff --git a/packages/react-reconciler/src/ReactFiberUnwindWork.js b/packages/react-reconciler/src/ReactFiberUnwindWork.js
index 4db7a3a76a..f15815ffe3 100644
--- a/packages/react-reconciler/src/ReactFiberUnwindWork.js
+++ b/packages/react-reconciler/src/ReactFiberUnwindWork.js
@@ -56,6 +56,7 @@ import {
 import {logError} from './ReactFiberCommitWork';
 import {getStackByFiberInDevAndProd} from './ReactCurrentFiber';
 import {popHostContainer, popHostContext} from './ReactFiberHostContext';
+import {markFailedErrorBoundaryForHotReloading} from './ReactFiberHotReloading';
 import {
   suspenseStackCursor,
   InvisibleParentSuspenseContext,
@@ -122,6 +123,9 @@ function createClassErrorUpdate(
   const inst = fiber.stateNode;
   if (inst !== null && typeof inst.componentDidCatch === 'function') {
     update.callback = function callback() {
+      if (__DEV__) {
+        markFailedErrorBoundaryForHotReloading(fiber);
+      }
       if (typeof getDerivedStateFromError !== 'function') {
         // To preserve the preexisting retry behavior of error boundaries,
         // we keep track of which ones already failed during this batch.
@@ -150,6 +154,10 @@ function createClassErrorUpdate(
         }
       }
     };
+  } else if (__DEV__) {
+    update.callback = () => {
+      markFailedErrorBoundaryForHotReloading(fiber);
+    };
   }
   return update;
 }
diff --git a/packages/react-reconciler/src/__tests__/ReactIncrementalErrorReplay-test.internal.js b/packages/react-reconciler/src/__tests__/ReactIncrementalErrorReplay-test.internal.js
index 6e6215efcb..9a497f074d 100644
--- a/packages/react-reconciler/src/__tests__/ReactIncrementalErrorReplay-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/ReactIncrementalErrorReplay-test.internal.js
@@ -10,21 +10,22 @@
 'use strict';
 
 describe('ReactIncrementalErrorReplay-test', () => {
+  const React = require('react');
+  const ReactTestRenderer = require('react-test-renderer');
+
   it('copies all keys when stashing potentially failing work', () => {
     // Note: this test is fragile and relies on internals.
     // We almost always try to avoid such tests, but here the cost of
     // the list getting out of sync (and causing subtle bugs in rare cases)
     // is higher than the cost of maintaining the test.
     const {
-      // Any Fiber factory function will do.
-      createHostRootFiber,
       // This is the method we're going to test.
       // If this is no longer used, you can delete this test file.
       assignFiberPropertiesInDEV,
     } = require('../ReactFiber');
 
     // Get a real fiber.
-    const realFiber = createHostRootFiber(false);
+    const realFiber = ReactTestRenderer.create(<div />).root._currentFiber();
     const stash = assignFiberPropertiesInDEV(null, realFiber);
 
     // Verify we get all the same fields.