Fix race in graph checking of type extensions

[majocha]

Description

Fixes#19033

By saving the declared type argument name instead of mutating (possibly concurrently) the source typar.

Checklist

• Test cases added
• Release notes entry updated:

[github-actions]

❗ Release notes required

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/11.0.0.md

[majocha]

To unblock #19028

[nojaf]

It has been too long since I understood all this, so could you high-level explain to me why this works?

Does signature conformance happen after the parallel phase?

[majocha]

It has been too long since I understood all this, so could you high-level explain to me why this works?

Does signature conformance happen after the parallel phase?

No, it seems it just does SetIdent in the opposite direction sig -> impl. Or extended type -> extensions. :) So what is being mutated is naturally isolated by graph checking.

This seems the only place this happens so we can save the declared ident here.

[nojaf]

Thanks, this is a clever solution. Well done!