List project merge request discussions API returning 404

Summary

The "List project merge request discussions" API return a 404 even when the user has full access to the project. Fetching the merge request itself via the API works.

Related links:

• https://gitlab.zendesk.com/agent/tickets/95870 (internal only)
• https://forum.gitlab.com/t/cant-get-merge-request-discussion-endpoint-to-work/15772

Steps to reproduce

1. Create a new merge request
2. Start a discussion on it
3. Try to fetch this discussion using the API: https://docs.gitlab.com/ce/api/discussions.html#list-project-merge-request-discussions. This should return a 404.

What is the current bug behavior?

This bug is preventing users from retrieving merge request discussions via the API. Calling the API results in a 404.

What is the expected correct behavior?

Calling the API should return a list of merge request's discussions as JSON.