For context, here's an earlier thread about the same underlying problem: How does Swift COW manage to avoid a dictionary copy in this simple computed property example?.
Quoting @John_McCall (post):
Yes [confirming the issue]. Eliminating that overhead is a large part of the purpose of
_readand_modify. I would also like to make it possible to conveniently declare storage as an alias for some other piece of storage, which would amount to defining_readand_modifyautomatically, but that's separable.