Array subtraction unexpectedly modifies its LHS

[dsharp-pivotal]

Describe the bug

Array subtraction appears to modify its LHS. When the LHS is a variable that is used again later, this has unexpected results.

Version of yq:

$ yq --version
yq (https://github.com/mikefarah/yq/) version v4.44.3

Operating system: macOS
Installed via: homebrew

Input Yaml

["a", "b"]

Command
The command you ran:

$ <<<'["a", "b"]' yq e '["a","b"] as $f | {"0": $f, "1": $f - ., "2": $f}'

Actual behavior

The variable appears to be modified. .2 is empty.

"0":
  - a
  - b
"1": []
"2": []

Expected behavior

.2 should contain the assigned value of $f, ["a", "b"]

Compare to re-using a variable without doing the array subtraction:

$ <<<'["a", "b"]' yq e '["a","b"] as $f | {"0": $f, "2": $f}'
"0":
  - a
  - b
"2":
  - a
  - b

If the LHS is the context ., the same thing occurs:

$ <<<'["a", "b"]' yq e '["a","b"] as $f | {"0": ., "1": . - $f, "2": .}'
"0": ["a", "b"]
"1": []
"2": []

As a point of comparison, jq does not have this behavior:

$ <<<'["a", "b"]' jq '["a","b"] as $f | {"0": $f, "1": ($f - .) , "2": $f}'
{
  "0": [
    "a",
    "b"
  ],
  "1": [],
  "2": [
    "a",
    "b"
  ]
}

Additional context

I think this is due to this line:
https://github.com/mikefarah/yq/blob/master/pkg/yqlib/operator_subtract.go#L41

	// removing children from LHS, parent hasn't changed
	lhs.Content = newLHSArray

I don't understand the intent expressed by the comment.

[mikefarah]

Thanks for raising, will fix in the next release