Structural Sum Types (used to be Anonymous Union Types)

[wadetregaskis]

@toph42

It errors at compile time if you try to put a non-error in an any Error var:

var err: any Error
err = "error"
// Error: Cannot assign value of type 'String' to type 'any Error'

Yes, which is better than nothing (e.g. Python or JavaScript), but still far from ideal in some cases.

See the origin thread for details, but in short if Swift only had "Any" - a bit like Objective-C by convention used id a lot - we'd all be pretty sad, or at least our users would be because of our much buggier apps. It's a bit weird that Swift goes to great lengths to have really good type inference and static typing, and actively discourages unnecessary use of existentials, for everything except exceptions.