Have given this a thorough review, both the final proposal, and its development 
during the draft phases on the list, etc.

Unfortunately I have to give the final proposal as it stands a strong -1.

While I appreciate the future benefits of more generic existential types, the 
final proposal introduces syntax that is flat-out confusing and inconsistent 
with the rest of Swift 3.


In Swift, the & operator is not used for composition, it serves only as the 
“Bitwise AND" operator for integer operations.


OptionSetType gives a good example, where Swift has actively abandoned & for 
composition. Previously code was in an Obj-C style, e.g.

  NSOptionBox & NSOptionCarton

but now uses “,” for composition within an array context:

  [.box, .carton]


For this reason, I find that the pattern `<…, …, …>` is still a more Swift-y 
type composition syntax than the proposal.

Scott

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