Reverse generics for argument type?

[sighoya]

@Avi

By that definition, some P isn't existential. some P is shorthand, with the added benefit of hiding the real type from the caller. But that's all a compiler trick. some P is a stand-in from the perspective of the developer , never the compiler.

some P behaves like the following:

// magic modifier to hide the real type from everyone else.
private(type) typealias HiddenP = Int

public func f() -> HiddenP {}

One would not consider HiddenP to be an existential, and some P is nothing more than Swift's actual implementation of my made-up typealias modifier.

I give you right, here. We can actually consider some P in return type position as type aliases, but for the argument position, it seems not to working anymore.
The type aliases are created from other callees than the actual callee accepting the some P type, so we actually have to deal with different type aliases.