It would be nice if we printed which metadata had a cycle. cc @jckarter, @rjmccall

Is there any workarounds? The bug stop me using Swift 3.1 for development.

This error means you're already outside of valid Swift code. If code was accepted in 3.0 that is now causing this error in 3.1, that's a problem, but it's not something that was ever guaranteed to work.

If you look at the stack trace for the crash, you might be able to find out which types are causing the problem.

not exactly

i have minimised the project and it can reproduce the bug

I found that it fixes the problem if I move the enum Base to the outside of Bezier structure

Huh. It's unfortunate that that runs afoul of the cyclic metadata check, but I think not a surprise today. The layout of 'Bezier' depends on the layout of 'Base', but the metadata for 'Base' has a "parent" pointer that refers to the metadata for 'Bezier'. We ought to be able to handle this since the parent pointer isn't used for layout, but we currently cannot.

I had the same problem, also involving an enum type contained in a struct type. Since it’s apparently a problem with layout, I thought I’d add the “indirect” keyword to that inner enum to decouple the enum layout from the rest, which resolved the issue. Unit tests in my project pass.

Given that my enum uses stdlib types only for associated values, there’s no recursive reference back to the enum. Is “indirect” the recommend way to go in an “ideal” compiler? I wouldn’t want to resolve to “indirect” (seems like a hack) unless that’s intended behaviour in the model.

In any case, a more detailed diagnostic of cycles would be great. :-)