evalv3 "non-concrete value _" regression compared to evalv2

[MarcWort]

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

with eval3 on current master

~/github/cue/cue-c565c1d3 version                      
cue version v0.13.0-alpha.3

go version go1.24.1
      -buildmode exe
       -compiler gc
  DefaultGODEBUG gotestjsonbuildtext=1,multipathtcp=0,randseednop=0,rsa1024min=0,tlsmlkem=0,x509rsacrt=0,x509usepolicies=0
     CGO_ENABLED 1
          GOARCH amd64
            GOOS linux
         GOAMD64 v1
             vcs git
    vcs.revision c565c1d31f2920ec79219ab04699e68fd32af93b
        vcs.time 2025-03-31T13:58:15Z
    vcs.modified false
cue.lang.version v0.13.0

and with experimental eval3 on current stable release:

➜  ~ cue version                          
cue version v0.12.0

go version go1.23.5
      -buildmode exe
       -compiler gc
       -trimpath true
     CGO_ENABLED 0
          GOARCH amd64
            GOOS linux
         GOAMD64 v1
cue.lang.version v0.12.0

Does this issue reproduce with the latest stable release?

Yes, but only with CUE_EXPERIMENT=evalv3

What did you do?

Minimal reproducer

cd $(mktemp -d)
(
set -e
git clone https://github.com/MarcWort/example-cue.git
cd example-cue
git checkout default-string-empty
cue eval standalone.cue -o json:-
CUE_EXPERIMENT=evalv3 cue eval standalone.cue -o json:-
)

What did you expect to see?

No difference between the old and the new eval v3.

The summary field should not be empty or "non-concrete".

What did you see instead?

groups.one_rulegroup.rules.0.annotations.summary: invalid interpolation: non-concrete value _ (type _):
    ./standalone.cue:24:20

or if defining "" as summary default: (see line https://github.com/MarcWort/example-cue/blob/default-string-empty/standalone.cue#L46)

                    "alert": "Stage",
                    "annotations": {
                        "summary": ""
                    }

[mvdan]

Reduced to the following:

# evalv2
env CUE_EXPERIMENT=evalv3=0
exec cue export

# evalv3
env CUE_EXPERIMENT=evalv3=1
exec cue export

-- input.cue --
package p

_global: name: "foo"

out: desc: {
	_hidden: _global.name
	name: _hidden
}
out: #Schema | "never"

#Schema: desc: {...}

As of c565c1d:

# evalv2 (0.009s)
> env CUE_EXPERIMENT=evalv3=0
> exec cue export
[stdout]
{
    "out": {
        "desc": {
            "name": "foo"
        }
    }
}
# evalv3 (0.030s)
> env CUE_EXPERIMENT=evalv3=1
> exec cue export
[stderr]
out.desc.name: incomplete value _
[exit status 1]