eval: regression giving "non-concrete value _ (type _) error"

[myitcv]

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

$ cue version
cue version v0.0.0-20250115193449-0ad6826e46b2

go version go1.23.2
      -buildmode exe
       -compiler gc
  DefaultGODEBUG asynctimerchan=1,gotypesalias=0,httpservecontentkeepheaders=1,tls3des=1,tlskyber=0,x509keypairleaf=0,x509negativeserial=1
     CGO_ENABLED 1
          GOARCH arm64
            GOOS linux
         GOARM64 v8.0
             vcs git
    vcs.revision 0ad6826e46b2dee462a8f307adcb04ef850bcda3
        vcs.time 2025-01-15T19:34:49Z
    vcs.modified false
cue.lang.version v0.12.0

Does this issue reproduce with the latest release?

n/a - testing evalv3 with tip

What did you do?

# evalv2
env CUE_EXPERIMENT=evalv3=0
exec cue export
cmp stdout stdout.golden

# evalv3
env CUE_EXPERIMENT=evalv3=1
exec cue export
cmp stdout stdout.golden

-- release.cue --
package github

workflows: release: _repo.repositoryDispatch
-- workflows.cue --
package github

_repo: {
	botGitHubUserTokenSecretsKey: *"again" | string

	pushTipToTrybotWorkflow: "\(botGitHubUserTokenSecretsKey)"

	repositoryDispatch: {
		_curlGitHubAPI: curlGitHubAPI
		"hello"
	}

	curlGitHubAPI: {
		#tokenSecretsKey: *botGitHubUserTokenSecretsKey | _
	}
}
workflows: doit: _repo.pushTipToTrybotWorkflow
-- stdout.golden --
{
    "workflows": {
        "release": "hello",
        "doit": "again"
    }
}

What did you expect to see?

Passing test.

What did you see instead?

# evalv2 (0.012s)
# evalv3 (0.031s)
> env CUE_EXPERIMENT=evalv3=1
> exec cue export
[stderr]
_repo.pushTipToTrybotWorkflow: invalid interpolation: non-concrete value _ (type _):
    ./workflows.cue:6:27
[exit status 1]
FAIL: /tmp/testscript3475180119/repro.txtar/script.txtar:8: unexpected command failure

This bisects to 80743ca

This was massively reduced from a much more involved case where the non-concrete error was more trickily buried under a matchN-related error.

[mvdan]

Reduced a bit further:

# evalv2
env CUE_EXPERIMENT=evalv3=0
exec cue vet -c input.cue

# evalv3
env CUE_EXPERIMENT=evalv3=1
exec cue vet -c input.cue

-- input.cue --
// Order dependent! These two fields need to go first.
out: one: #strDefaultDef
out: two: "\(strDefault)"

strDefault: *"fallback" | string
#strDefaultDef: strDefault

As of 0ad6826:

# evalv2 (0.010s)
> env CUE_EXPERIMENT=evalv3=0
> exec cue vet -c input.cue
# evalv3 (0.031s)
> env CUE_EXPERIMENT=evalv3=1
> exec cue vet -c input.cue
[stderr]
out.two: invalid interpolation: non-concrete value _ (type _):
    ./input.cue:3:11
[exit status 1]

Note that the reproducer is order dependent, which is worrying.