alternates metadata are not correctly merged

[axeleroy]

Link to the code that reproduces this issue

https://github.com/axeleroy/nextjs-alternates-repro

To Reproduce

1. Start the dev server
2. Navigate to http://localhost:3000
3. Inspect the page. The alternates are

<link rel="alternate" hrefLang="en-US" href="https://nextjs.org/en-US"/>
<link rel="alternate" hrefLang="de-DE" href="https://nextjs.org/de-DE"/>

4. Navigate to http://localhost:3000/sub-page-1
5. Inspect the page. The alternates are

<link rel="alternate" type="application/rss+xml" href="https://example.tld/feed.xml"/>

6. Navigate to http://localhost:3000/sub-page-1
7. Inspect the page. The alternates are

<link rel="canonical" href="https://example.tld/sub-page-2"/>

Current vs. Expected behavior

1. Start the dev server
2. Navigate to http://localhost:3000
3. Inspect the page. The alternates should be

<link rel="alternate" type="application/rss+xml" href="https://example.tld/feed.xml"/>
<link rel="alternate" hrefLang="en-US" href="https://nextjs.org/en-US"/>
<link rel="alternate" hrefLang="de-DE" href="https://nextjs.org/de-DE"/>

instead of

<link rel="alternate" hrefLang="en-US" href="https://nextjs.org/en-US"/>
<link rel="alternate" hrefLang="de-DE" href="https://nextjs.org/de-DE"/>

4. Navigate to http://localhost:3000/sub-page-1
5. Inspect the page. The alternates are

<link rel="alternate" type="application/rss+xml" href="https://example.tld/feed.xml"/>

6. Navigate to http://localhost:3000/sub-page-1
7. Inspect the page. The alternates should be

<link rel="alternate" type="application/rss+xml" href="https://example.tld/feed.xml"/>
<link rel="canonical" href="https://example.tld/sub-page-2"/>

instead of

<link rel="canonical" href="https://example.tld/sub-page-2"/>

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #51-Ubuntu SMP PREEMPT_DYNAMIC Sat Nov  9 17:58:29 UTC 2024
  Available memory (MB): 7680
  Available CPU cores: 8
Binaries:
  Node: 20.12.2
  npm: 10.5.0
  Yarn: 1.9.4
  pnpm: 8.15.4
Relevant Packages:
  next: 15.1.3 // Latest available version is detected (15.1.3).
  eslint-config-next: N/A
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.7.2
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Metadata

Which stage(s) are affected? (Select all that apply)

next dev (local), next build (local), next start (local), Vercel (Deployed), Other (Deployed)

Additional context

In addition to the stages mentioned above, I have also noted that the issue happens regardless of whether Turbo is used or not and on Static HTML exports.

[AliRazaDev27]

Read the docs.
https://nextjs.org/docs/app/building-your-application/optimizing/metadata#merging

see how the description field from layout is not merged into page metadata.

[axeleroy]

Oh right, I missed that. Maybe the docs should emphasize this part, for example by putting "Note the absence of openGraph.description" in bold?