Add helpful error for link with multiple children (vercel#25657)

Makes sure a helpful error is shown for `<Link>` with multiple children



## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.

## Documentation / Examples

- [ ] Make sure the linting passes

errors/link-multiple-children.md

@@ -0,0 +1,36 @@
+# Multiple children were passed to <Link>
+
+#### Why This Error Occurred
+
+In your application code multiple children were passed to `next/link` but only one child is supported:
+
+For example:
+
+```js
+import Link from 'next/link'
+
+export default function Home() {
+  return (
+    <Link href="/about">
+      <a>To About</a>
+      <a>Second To About</a>
+    </Link>
+  )
+}
+```
+
+#### Possible Ways to Fix It
+
+Make sure only one child is used when using `<Link>`:
+
+```js
+import Link from 'next/link'
+
+export default function Home() {
+  return (
+    <Link href="/about">
+      <a>To About</a>
+    </Link>
+  )
+}
+```

packages/next/client/link.tsx

@@ -229,7 +229,21 @@ function Link(props: React.PropsWithChildren<LinkProps>) {
   }
 
   // This will return the first child, if multiple are provided it will throw an error
-  const child: any = Children.only(children)
+  let child: any
+  if (process.env.NODE_ENV === 'development') {
+    try {
+      child = Children.only(children)
+    } catch (err) {
+      throw new Error(
+        `Multiple children were passed to <Link> with \`href\` of \`${props.href}\` but only one child is supported https://nextjs.org/docs/messages/link-multiple-children` +
+          (typeof window !== 'undefined'
+            ? "\nOpen your browser's console to view the Component stack trace."
+            : '')
+      )
+    }
+  } else {
+    child = Children.only(children)
+  }
   const childRef: any = child && typeof child === 'object' && child.ref
 
   const [setIntersectionRef, isVisible] = useIntersection({

test/acceptance/ReactRefreshLogBox.dev.test.js

@@ -1099,6 +1099,34 @@ test('logbox: anchors links in error messages', async () => {
   await cleanup()
 })
 
+test('<Link> with multiple children', async () => {
+  const [session, cleanup] = await sandbox()
+
+  console.log({ SANDBOX: session.sandboxDirectory })
+  await session.patch(
+    'index.js',
+    `
+      import Link from 'next/link'
+
+      export default function Index() {
+        return (
+          <Link href="/">
+            <p>One</p>
+            <p>Two</p>
+          </Link>
+        )
+      }
+    `
+  )
+
+  expect(await session.hasRedbox(true)).toBe(true)
+  expect(await session.getRedboxDescription()).toMatchInlineSnapshot(
+    `"Error: Multiple children were passed to <Link> with \`href\` of \`/\` but only one child is supported https://nextjs.org/docs/messages/link-multiple-children"`
+  )
+
+  await cleanup()
+})
+
 test('<Link> component props errors', async () => {
   const [session, cleanup] = await sandbox()