Fix an issue with infinite loops while typechecking some expressions

[jasoncarr0]

Some recursive constraints in type signatures could result in infinite loops while typechecking in cases where the typechecking should have succeeded.

This fixes those cases.

Fixes #3658

The fix works by making these cases more coinductive: pushing the assumption before typechecking equality of expressions.

[jasoncarr0]

Only thing WIP I just need to find a good test for it and it could possibly serve to be cleaner. Running manually against the test cases so far seems to work correctly.

[jemc]

Marking as "do not merge" because the testing is WIP.

[mfelsche]

@jasoncarr0 Can you explain someone like me who tried to look into it without success, how this fixes the subtype check recursion and for what kind of types this works? I am very curious but unfortunately unable to understand from this PR. ❤️

[ponylang-main]

Hi @jasoncarr0,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 4274.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[jasoncarr0]

@mfelsche Ultimately the main point is to avoid infinite loops by identifying when we have already started checking a given goal. This PR does that by pushing the goal into the assumptions earlier than it would be before, so we have it in place before certain recursive calls.

[jemc]

Looks good to me assuming tests pass.

[SeanTAllen]

The test should be moved to libponyc-run. We don't do "TEST_COMPILE" tests in libponyc tests anymore.

[SeanTAllen]

Can you add a comment at the top level of this file explaining in general that this is a compile time check so the "additional cases" are covered because this is compile time, not runtime checking?

[jasoncarr0]

Okay

[jasoncarr0]

Added in f16a0ea

[jasoncarr0]

Er, I added the comment locally, not at the top, is that fine?