lack of error message in cue export on incomplete value

[joulaud]

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

$ cue version
cue version 0.4.0 linux/amd64

Does this issue reproduce with the latest release?

yes

What did you do?

cat <<'EOF' >x.cue
toto: {
       _current: *"toto" | string
	value:     *_|_ | _current
}
tata: "tata"
EOF

cue export x.cue

What did you expect to see?

either error message on stderr like

toto: value: incomplete value *"toto" | string

or (better?) unified result on stdout

toto: value: "toto"
tata: "tata"

What did you see instead?

unuseful error message on stderr

toto:

full session

$ cat <<'EOF' >y.cue
toto: {
       _current: *"toto" | string
        value:     *_|_ | _current
}
tata: "tata"
EOF

$ cue export y.cue
toto: 

$ cue eval y.cue
toto: {
    value: "toto" | string
}
tata: "tata"

[mpvl]

This seems to be a bug, triggered by the combination of usage of *_|_ and another nested disjunction with default value.

Reproducer:

toto: value: *_|_ | (*"toto" | string)

Judging from the output, it bails printing in an unexpected matter. Given that eval works fine, I expect this is an export bug.