Add React Peer dep warning

code/lib/cli/src/build.ts

@@ -2,8 +2,11 @@ import { sync as readUpSync } from 'read-pkg-up';
 import { logger } from '@storybook/node-logger';
 import { buildStaticStandalone, withTelemetry } from '@storybook/core-server';
 import { cache } from '@storybook/core-common';
+import { ensureReactPeerDeps } from './ensure-react-peer-deps';
 
 export const build = async (cliOptions: any) => {
+  ensureReactPeerDeps();
+
   try {
     const options = {
       ...cliOptions,

code/lib/cli/src/dev.ts

@@ -3,10 +3,13 @@ import { sync as readUpSync } from 'read-pkg-up';
 import { logger, instance as npmLog } from '@storybook/node-logger';
 import { buildDevStandalone, withTelemetry } from '@storybook/core-server';
 import { cache } from '@storybook/core-common';
+import { ensureReactPeerDeps } from './ensure-react-peer-deps';
 
 export const dev = async (cliOptions: any) => {
   process.env.NODE_ENV = process.env.NODE_ENV || 'development';
 
+  ensureReactPeerDeps();
+
   try {
     const options = {
       ...cliOptions,

code/lib/cli/src/ensure-react-peer-deps.ts

@@ -0,0 +1,24 @@
+import { logger } from '@storybook/node-logger';
+import dedent from 'ts-dedent';
+
+export function ensureReactPeerDeps() {
+  try {
+    require.resolve('react');
+    require.resolve('react-dom');
+  } catch (e) {
+    logger.error(dedent`
+      Starting in 7.0, react and react-dom are now required peer dependencies of Storybook.
+      Please install react and react-dom in your project.
+
+      npm:
+      $ npm add react react-dom --dev
+
+      yarn:
+      $ yarn add react react-dom --dev
+
+      pnpm:
+      $ pnpm add react react-dom --dev
+    `);
+    process.exit(1);
+  }
+}