Protocol as Dictionary Key or Set element

[xwu (Xiaodi Wu)]

@Danny

I think I understand conceptually why it doesn't work, in Swift, this was also not enough to help me understand why it does work in C# (maybe understanding the difference would help the OP with the Swift):

Why can't I use existential type for placeholder in generic type?

In C#, you would not be able to use the equivalent of FixedWidthInteger as a type. C# observes the invariant that any interface used as a type (i.e., existential) must satisfy itself as a constraint (i.e., conform to itself)—as Slava explained—and enforces this at the language level. (This does imply, unless I’m mistaken, that semantic guarantees not expressed in code cannot be enforced.) Until later versions, C# did not support static abstracts in interfaces (i.e., static requirements witho…