wasm: sets and arrays containing undefined values behave differently from topdown

[srenatus]

When a set contains no element, such as { x | x := input } with undefined input, value_dump will check the number of elements. It finds that the set has one element -- thus it's not printed as set(), but as { + elements dumped + }. When dumping elements, it only finds NULL elements (undefined), and thus doesn't print anything. In value dump's output, there's thus no way to differentiate empty sets from empty objects.

This came up here 👉 #3868 (comment)

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There's more! Extending the existing issue:

$ opa eval -fpretty -twasm 'count({1, 2, input})'
3
$ opa eval -fpretty -trego 'count({1, 2, input})'
undefined
$ opa eval -fpretty -twasm 'count([1, 2, input])'
3
$ opa eval -fpretty -trego 'count([1, 2, input])'
undefined

So, undefined elements in set or array literals don't make them overall undefined.

$ opa eval -fpretty -trego 'count([ 1 | input ])'
0
$ opa eval -fpretty -twasm 'count([ 1 | input ])'
1
$ opa eval -fpretty -trego 'count({ 1 | input })'
0
$ opa eval -fpretty -twasm 'count({ 1 | input })'
1

Set and array comprehensions disregard undefined in their conditions (it seems).