evaluator: structural cycle with optional field regression in v0.6

[heistp]

What version of CUE are you using (cue version)?

# "good" version
$ cue version
cue version v0.5.0

go version go1.21.0
      -buildmode exe
       -compiler gc
  DefaultGODEBUG panicnil=1
     CGO_ENABLED 1
          GOARCH amd64
            GOOS linux
         GOAMD64 v1

# "bad" version
$ cue version
cue version v0.6.0-alpha.1

go version go1.21.0
      -buildmode exe
       -compiler gc
  DefaultGODEBUG panicnil=1
     CGO_ENABLED 1
          GOARCH amd64
            GOOS linux
         GOAMD64 v1

Does this issue reproduce with the latest stable release?

Yes (v0.6.0).

What did you do?

Minimal CUE to reproduce:

package structcycle

#A: {
	B?: #B
}

#B: {
	A: #A
}

To reproduce it, run cue export with the above saved to structcycle.cue, or similar.

What did you expect to see?

Result with v0.5.0 ("good"):

$ cue export
{}

What did you see instead?

Result with v0.6.0-alpha.1 ("bad"):

$ cue export
#B.A: structural cycle

Notes

My antler project is broken by the new behavior. On line 59 of my CUE schema in config.cue, the Run field in #Schedule refers to #Run, which refers to #Schedule through an optional field. This works in v0.5.0, but broke somewhere between v0.5.0 and v0.6.0-alpha.1. I'm unsure of the correct behavior, or if I need to change something in my schema, but I do know the behavior in v0.5.0 was working fine for me. :)