SE-0309: Unlock existential types for all protocols

[sighoya]

@Avi

This is a circular definition. What is an existential type?

In Swift a protocol type, whatever this means. Generally, a type which steal literals/values from other types, usually base types like integer, structs ... .

@Avi

Usage of some P in argument position would seem to make no sense. At that point, it's not hiding anything from the caller, and one can hide the concrete type from the callee today, by using generics.

The discussion about that has moved to reverse-generics-for-argument-type