eval: index out of range for "clone" of list

[myitcv]

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

$ cue version
cue version v0.13.0-alpha.2.0.20250327101324-32d040003e8a

go version go1.24.0
      -buildmode exe
       -compiler gc
  DefaultGODEBUG gotestjsonbuildtext=1,multipathtcp=0,randseednop=0,rsa1024min=0,tlsmlkem=0,x509rsacrt=0,x509usepolicies=0
     CGO_ENABLED 1
          GOARCH arm64
            GOOS linux
         GOARM64 v8.0
             vcs git
    vcs.revision 32d040003e8a43b96d667d1ec2a2a02957853577
        vcs.time 2025-03-27T10:13:24Z
    vcs.modified false
cue.lang.version v0.13.0

Does this issue reproduce with the latest release?

Yes

What did you do?

exec cue export x.cue
cmp stdout stdout.golden

-- x.cue --
_reduce: {
	_defaults: [...]
	_layers: [for i, d in _defaults {d}]
	res: _layers[0]
}

top: _reduce & {
	_defaults: [""]
}

-- stdout.golden --
{
    "top": {
        "res": ""
    }
}

What did you expect to see?

Passing test.

What did you see instead?

> exec cue export x.cue
[stderr]
_reduce.res: index out of range [0] with length 0:
    ./x.cue:4:15
[exit status 1]
FAIL: repro.txtar:1: unexpected command failure

[mvdan]

Still happens as of v0.15.0. My immediate thought was to use an alias, as follows, but it fails the same:

_reduce: this={
	_defaults: [...]
	_layers: [for i, d in this._defaults {d}]
	res: _layers[0]
}