Fix tuple related compiler segfaults

[Trundle]

It's possible that the method body returns a subtype of the return type.
For example, a return value with type ((U32, U32) | (U32, None)) is
legal for a method with return type (U32, (U32 | None)), but it
requires a cast.

071db10 changed that methods returning a tuple always use a cast as if the method body's value already has the return type. Unfortunately, it doesn't go into detail why. This already caused an issue in #849 and an exception was added if return type and body type are both tuples. I think that assumption doesn't hold: for the above mentioned case, the return type is a tuple, but the body type is an union, and yet the current code continues as if the body produces a tuple.

Fixes #2609, #2808

[SeanTAllen]

@Trundle on a roll!

[ponylang-main]

Hi @Trundle,

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 3723.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.

[SeanTAllen]

Confirmed that #2808 is fixed by this.

[SeanTAllen]

Confirmed #2609 is fixed.

[SeanTAllen]

I'll hold tomorrow's ponyc release until this has release notes and is merged.

Thanks @Trundle.

[SeanTAllen]

@Trundle I think "Fix tuple related compiler segfaults" would be a good release notes title.

[SeanTAllen]

I've added release notes.

[Trundle]

Thank you for adding the release notes, @SeanTAllen. They contain a slight mistake though: the crash happens in the compiled code, not the compiler itself. Is it worth fixing and if so, how does it work? Should I open a new PR with changes to 0.39.0.md?

[SeanTAllen]

@Trundle yeah go ahead and then I'll update the release page as well.