Type relations cache: Add a failsafe in case of infinite types

[majocha]

Description

This is a speculative fix for #19007

We should not try to get a cache key out of an infinite type.

This adds a failsafe for such cases.

[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]

Unfortunately,, I can't come up with a test case that would repro the issue.

[T-Gro]

I think it would make sense to re-target this to release/dev18.0 and backport, if there is a real chance of it happening for regular projects.

[majocha]

I think it would make sense to re-target this to release/dev18.0 and backport, if there is a real chance of it happening for regular projects.

In the repo from the issue it happens during edits, i.e. the code is not in the state that compiles correctly.
This makes some sense now to me why this could happen. There is the occursCheck that checks for invalid "infinite" types, but various type relation calls can very well happen before that check.

[T-Gro]

Understood.
It would be great if we could get piece of that invalid code as an regression test.

[majocha]

Understood. It would be great if we could get piece of that invalid code as an regression test.

Yeah, tbh I tried a lot of hand crafted and copilot generated code but it all behaves ok. None of the unsolved types hitting getTypeStructure were "infinite":

I'm starting to think this is a red herring and a problem lies somewhere else completely, yet to show up. Sigh.

[T-Gro]

Maybe one of the cases found in issues ( #9523 , #11239 ) or in code comments like type a = a list ?

[T-Gro]

VisualFSharpFull-Unzip-Then-Install.zip

[github-actions]

@T-Gro backporting to "release/dev18.0" failed, the patch most likely resulted in conflicts:

$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch

Applying: Add a failsafe in case of infinite types
Applying: do not cache when invalid keys
Applying: refactor
Applying: rn
.git/rebase-apply/patch:13: trailing whitespace.
* Type relations cache: handle potentially "infinite" types ([PR #19010](https://github.com/dotnet/fsharp/pull/19010))  
warning: 1 line adds whitespace errors.
Using index info to reconstruct a base tree...
M	docs/release-notes/.FSharp.Compiler.Service/11.0.0.md
Falling back to patching base and 3-way merge...
Auto-merging docs/release-notes/.FSharp.Compiler.Service/11.0.0.md
CONFLICT (content): Merge conflict in docs/release-notes/.FSharp.Compiler.Service/11.0.0.md
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Patch failed at 0004 rn
Error: The process '/usr/bin/git' failed with exit code 128

Please backport manually!