evaluator: comprehension causes failed disjunction elimination

[rogpeppe]

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

17177e96d37f6d504138ff7588cea9157779f687

Does this issue reproduce with the latest stable release?

Yes (v0.13.4)

What did you do?

exec cue vet -c x.cue

-- x.cue --
#Foo: [...] | {
	x?: _
	if false {}
}
x: #Foo & []

What did you expect to see?

A passing test. The list literal should eliminate the struct from the disjunction.

What did you see instead?

> exec cue vet -c x.cue
[stderr]
x: incomplete value [] | [_]
[exit status 1]
FAIL: /tmp/x.txtar:1: unexpected command failure

This is probably a consequence of the fact that a comprehension counts as embedding a non-struct value, so this is valid, for example:

{
    if false {}
} & [1]

Arguably it might be good if a comprehension could not itself change the "scalarness" of a struct, so this would be illegal:

{ if true { 1 } }

requiring an explicit embedded element at the outer level to be considered OK:

{ _, if true { 1 } }

{
    if false {}
} & [1]

This behaviour seems unintuitive to me. It would be better if the comprehension was counted as scalar/list only if its possible output value can also be a scalar/list.