pkg/struct: MinFields fails when combined with a conditional

[jpluscplusm]

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

$ cue version
cue version v0.11.0-alpha.2

go version go1.23.0
      -buildmode exe
       -compiler gc
       -trimpath true
     CGO_ENABLED 0
          GOARCH amd64
            GOOS linux
         GOAMD64 v1
cue.lang.version v0.11.0

Does this issue reproduce with the latest stable release?

Yes, I've tested it and found it to fail with v0.10.0, v0.9.2, but it passes with v0.8.2. I've not tested v0.9.0.

What did you do?

exec cue vet -c file.cue
env CUE_EXPERIMENT=evalv3
exec cue vet -c file.cue
-- file.cue --
import "struct"

A: struct.MinFields(1)
A: {
	if true {B: true}
}

What did you expect to see?

A passing test.

What did you see instead?

> exec cue-0.11.0-alpha.2 vet -c file.cue
[stderr]
A: incomplete value struct.MinFields(1):
    ./file.cue:3:4
[exit status 1]
FAIL: /tmp/testscript2904768139/minFields.conditional.disagree.txtar/script.txtar:1: unexpected command failure
> env CUE_EXPERIMENT=evalv3
> exec cue-0.11.0-alpha.2 vet -c file.cue
[stderr]
A: incomplete value struct.MinFields(1):
    ./file.cue:3:4
[exit status 1]
FAIL: /tmp/testscript2904768139/minFields.conditional.disagree.txtar/script.txtar:3: unexpected command failure

This feels adjacent to #3450 but, unlike that issue, this issue strikes with both the current and new evaluator.

[rogpeppe]

After bisecting, it looks like the culprit is https://review.gerrithub.io/c/cue-lang/cue/+/1200328