SE-0470: Global-actor isolated conformances

Evolution Proposal Reviews
6

I've found a problem with down casting, but it is not specific to global-actor isolated conformances, but rather a general problem with marker protocols:

Normally it is not possible to cast to Sendable:

print(x as? Sendable) // error: marker protocol 'Sendable' cannot be used in a conditional cast

But if you ask nicely, it becomes possible:

func castPlease<T>(_ x: Any, to: T.Type) -> T? {
  return x as? T
}

protocol P {
  func foo() 
}

@MainActor
class MA: @MainActor P {
    func foo() {}
}

class NS {}

@MainActor
func test() {
  print(ObjectIdentifier((any P).self))
  print(ObjectIdentifier((any P & Sendable).self))
  print(castPlease(MA(), to: (any P & Sendable).self))
  print(ObjectIdentifier(Any.self))
  print(ObjectIdentifier((any Sendable).self))
  print(castPlease(NS(), to: (any Sendable).self))
}

// Prints:
// ObjectIdentifier(0x000000010456d340)
// ObjectIdentifier(0x000000010456d340)
// Optional(foo.MA)
// ObjectIdentifier(0x0000000104529de0)
// ObjectIdentifier(0x0000000104529de0)
// Optional(foo.NS)

Since metatypes with and without Sendable look the same in the runtime, this cannot be solved within the scope of the global-actor isolated conformances. I guess this could be solved by introducing another marker protocol (e.g. Castable), but that is out of scope of this feature.

An unexpected `Sendable` pitfall