That option should not be disallowed. Here is a simple example you might want 
to build at some point:

protocol ProtocolName {
    associatedtype AssociatedType

extension ProtocolName where AssociatedType == Int {
    struct InnerType {}

Adrian Zubarev
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Am 17. Oktober 2016 um 20:30:58, Karl via swift-evolution 
( schrieb:

Is your vision that each conforming type would have to provide its own nested 
type as specified by the protocol?

Or could the protocol itself define a nested type and anything could use it?

protocol FloatingPoint: … {
    enum RoundingRule {
        // Do I put an implementation here?

No, types which are defined inside the protocol are implemented there. 
Providing your own types to satisfy a conformance is what associated types are 

If you wanted something like that, you could do it with a nested protocol + 
associated type:

protocol FloatingPoint {

    protocol _RoundingRule { func round(_ : Super) -> Super }
    associatedType RoundingRule : _RoundingRule

struct Float : FloatingPoint {

    enum RoundingRule : _RoundingRule {
        func round(_ val: Float) -> Float {
            /* switch self, perform rounding… */ 

That brings up an interesting point, though - we would need a way to refer to 
the outer protocol (I used “Super” here).

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