error FS0073: internal error: Undefined or unsolved type variable: ^_?82069

I'm creating this issue to track all occurrences of this error for which there is already a fix in the feature/ext branch.

This error is a longstanding one and it's still being reported from time to time with many different repros.

I'm personally tired of wasting my time finding ways to workaround it, which is unfortunately not always possible.

I think the main commit that fixes this issue is here: https://github.com/dotnet/fsharp/pull/6805/commits/f57f9471b08aee19b6575b9b711bad1c7c868a91

And as far as I can tell is totally independent of RFC-1043, the only reason why it was fixed there is because F#+ was used to test the new feature and F#+ finds very frequently that error.

Here's a list of existing reported issues of this error, which I can confirm they work when running them from feature/ext ~~even without the language preview switch which would activate RFC-1043~~.

- https://github.com/dotnet/fsharp/issues/9416

- https://github.com/dotnet/fsharp/issues/9382

- https://github.com/dotnet/fsharp/issues/7384

- https://github.com/fsprojects/FSharpPlus/issues/331

I can't stress how important is to get this bug fixed in the constraint solver area, and AFAIK bugs have priority over features, so I think efforts should be invested in order to get the existing fix merged into master.