evalv3: unexpected structural cycle regression

[rogpeppe]

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

$ cue version

Does this issue reproduce with the latest stable release?

No (v0.10.0)

What did you do?

exec cue vet x.cue

env CUE_EXPERIMENT=evalv3
exec cue vet x.cue

-- x.cue --
a: #T
a: in: f: true
a: #in: {
	g: #T
	g: #in: f: #T
	g: #in: f: #in: x: true
	f?: _
}

#T: {
	in:  #in
	#in: _
}

What did you expect to see?

Passing test.

What did you see instead?

> exec cue vet x.cue
> env CUE_EXPERIMENT=evalv3
> exec cue vet x.cue
[stderr]
a.in.g.in.f: structural cycle
[exit status 1]
FAIL: /tmp/z.txtar:4: unexpected command failure

This seems like it might well be a bug in evalv3.
As supporting evidence for that, if I duplicate the #T definition and change any one of the references to refer to a difference version, we don't see the structural cycle any more, but that shouldn't make any difference AFAICS.