Fastify "can't resolve reference" error repro, from 4.2.0 onwards

Description:

Using both Type.Union() and Type.Ref() in a schema at the same time,

ERROR: can't resolve reference (...) from id (...)

is thrown at runtime, on request, with no schema error on startup.

Using just one of Type.Union() or Type.Ref() without the other in a schema results in no error.

---

Test:

• npm install

With any fastify version from @4.2.0 to @4.9.2 (latest as of now):

• run npm start
• open http://localhost:3370/documentation
• on /fails1 operation, click Try it out, then Execute, which should result in

{
  "statusCode": 500,
  "error": "Internal Server Error",
  "message": "can't resolve reference RefSchema from id FailingSchema1"
}

• /works1 and /works2 operations both resolve properly
  • /works1 shows that Type.Union() does work next to a non-Type.Ref() property
  • /works2 shows that the Type.Ref() can actually be resolved, even next to some other typebox-provided utility, Type.Array() in this case
• /fails2 is the same as /works2 except it uses Type.Union() inside Type.Array(), causing the same kind of error as in /fails1

---

Version notes:

Happens from fastify@4.2.0 onwards.

Last version before that is 4.1.0:

• Install that exact version (via -E) with

npm install fastify@4.1.0 -E

• run npm start
• open http://localhost:3370/documentation
• on /fails1 operation, click Try it out, then Execute, and it resolves properly with

{
  "id": 1,
  "reference": "hi"
}

• as expected /fails2 resolves properly as well