Why can a private `<T> void swap(List<T> values, ...)` helper accept a `List<?>` from a p…

Why can a private `<T> void swap(List<T> values, ...)` helper accept a `List<?>` from a public method?

1. Invocation inference names the one captured element type for that helper call.
2. The JVM inspects every list element and chooses their common runtime class.
3. Every unbounded wildcard is replaced with Object before source checking.
4. The helper silently performs an unchecked cast.