! exec cue vet -c file.cue

cmp stderr expect-stderr

-- expect-stderr --
invalid interpolation: non-concrete value string (type string):
    ./file.cue:12:1
    ./file.cue:3:5
-- file.cue --
#X: {
	x: string
	y: string
	z: string
}

#X: {
	x: "x"
	z: "z"
}

"""
hello
world
\(#X.x)
\(#X.y)
\(#X.z)
"""
