Here we are merging value into a new JsonObject so we may as well do it in place (hence the True argument). There is no point doing a copy if the inPlace parameter is False.

Also, I think line 31 is wrong, but I cannot add a comment to it on that line. I suspect it should be return merge(target, patch, inPlace);

Hmm... looking closer into the
private Doc merge(Doc doc, Doc patch, Boolean inPlace = False)
implementation I realized, that inPlace argument is not used for anything but a recursive call to itself. The actual value affects absolutely nothing.

Ah, it appears to always be doing an in-place merge, i.e modifying the target directly. Maybe that needs fixing.

Not just that; the "inPlace" flag is not used for anything (e.g. ListMap construction)

Right, it can be removed.

Hmm... but if we remove it, then what is the point of calling this method? Don't we lose the value?

So you can merge one json object into another, or one json array into another, but you cannot merge anything into a primitive. So if the target field is a primitive, it is basically just replaced with the value being merged in.

e.g. If I have

{"foo": 1234}

so "foo" is a primitive number field. The I want to merge the array [1, 2, 3, 4] into field "foo" I end up with

{"foo": [1, 2, 3, 4]}

If "foo" was an array

{"foo": [9, 8, 7]}

and I did the same merge then I believe I would get

{"foo": [9, 8, 7, 1, 2, 3, 4]}

And, yes we lose the original value if it was a primitive

Oh... I got confused by the doc then. The method doc says:

Deeply merge a `Primitive` value into this builder. 
...
@param doc   the value to merge the primitive into

So I thought the current value is supposed to be replaced by the primitive. I think it needs to be improved