Why weak is not part of the type in the first place? Being able to write var delegates: [weak Delegate] would be handy. And this would make contact clear. And if weak reference would be less aggressive about dropping zombie side tables, they would be trivially hashable.
It is now possible to have a struct with a weak reference as a single field. Such struct can be used as a return type and type of arguments, it can be used as a value for generic type parameters. But also, being a single field struct it has a memory layout identical to a weak reference. So, de-facto we can use weak references as types.