Runtime "MissingRefError", with a specific kind of schema, from 4.2.0 onwards

[Danny2462]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

4.2.0

Plugin version

No response

Node.js version

16.3.0

Operating system

Windows

Operating system version (i.e. 20.04, 11.3, 10)

Windows 11 22H2

Description

A schema definition that uses both typebox's Type.Union() and Type.Ref() causes requests to fail with MissingRefError: "can't resolve reference (...) from id (...)"

Only a combination of Type.Union() and Type.Ref() seems to cause this, just either without the other works properly.
Doesn't seem to simply be broken references as the error would suggest.

I've had such API endpoints work fine before 4.2.0 (with 4.0.3, tested to work with 4.1.0), started failing at runtime after bumping to 4.9.2, and I found nothing in the changelog that would point to a breaking change around this.

As this feels like a simple use case, I looked over existing issues, but found nothing, the closest seems to be #4028 which was apparently resolved.

Minimal repro is linked below, with detailed guide in the readme.

If I'm screwing up / misusing something, please point me in the right direction, I'm fresh out of ideas what the mechanism could be behind this!

Steps to Reproduce

https://github.com/Danny2462/fastify-ref-error-after-4.2.0

Expected Behavior

Endpoints with schemas that worked before 4.2.0 would continue to work after a minor version bump too

[Danny2462]

Also would like to grab this chance to say that, otherwise, the TypeScript support is stellar, it's such a comfortable change of pace coming from Express, cheers!

[climba03003]

@ivan-tymoshenko Do you have time to take a look on this issue?

[ivan-tymoshenko]

@Danny2462 I found the bug. There is an issue in the fast-json-stringify that has a failed test for your case. Unfortunately, it might take time for me to fix it.