Why isn’t isUniquelyReferenced(_:) working here?

[taylorswift]

@sighoya

I would expect it because the old subvalue s of the enum is accessed after mutation inducing undefined behavior (UB). To circumvent UB, a safe copy has to made before.

no, the var expression binds the associated value to a new value s, making it independent from the old value in the enum, which can be destroyed at any time without affecting the newly bound value.