Relative assign with eval unexpectedly creates new keys

[kknicker]

I have created a YAML "!eval" tag to evaluate embedded yq expressions.

yq '. ref $root | (.. | select(tag == "!eval")) |= (. tag = "" | . as $expr | $root | eval($expr))' t.yaml

x: 1
y: 2
z: !eval .x + .y

x: 1
y: 2
z: 3

If the expression consists of undefined values sometimes the value is unchanged.

x: 1
y: 2
z: !eval .a + .b

x: 1
y: 2
z: .a + .b

In other cases the value is null, but also results in new keys being created for the undefined values.

x: 1
y: 2
z: !eval .a * .b

x: 1
y: 2
z: null
a: null
b: null

I am guessing this has something to do with the expression being evaluated to null versus something else.

I'm not sure if this is a bug, but it seems wrong that it would create new keys.