Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

I'm migrating a blog from Jekyll. It is necessary for me to keep the exact same url structure because there's external refs to these posts. So I have a

slug: a/b.html

in every blog post frontmatter. On build, this fails with the following error:

[ERROR] Unable to build website for locale en.
[ERROR] Error: Expected output HTML file to be found at /home/projects/kkviyzpmp.github/build/blog/a/b.html/index.html.

Running the site works fine, error only happens on build.

Reproducible demo

https://stackblitz.com/edit/github-iuz7vq?file=blog/2021-08-26-welcome/index.md

Steps to reproduce

1. Set a slug in a blog post frontmatter: /a/b.html
2. Run yarn build

Expected behavior

Build should succeed, especially since running the site poses no problems.

Actual behavior

Build fails with the following error message:

[ERROR] Unable to build website for locale en.
[ERROR] Error: Expected output HTML file to be found at /home/projects/kkviyzpmp.github/build/blog/a/b.html/index.html.

Your environment

On stackblitz playground.

Self-service

• I'd be willing to fix this bug myself.