[swift-evolution] protocol can only be used as a generic constraint because it has Self or associated type requirements

[Matthew Johnson]

>> 
>> It is definitely possible to solve this problem for associated types by binding or constraining them.  The ML module system has a feature that is roughly analogous to this.  
> 
> Keep in mind that a language feature like binding/constraining associated types is not a complete solution for real use-cases, so we would still need AnySequence<Element> in the library.  Otherwise it would be SequenceType<Generator: SomeGenerator>, which binds a type that you don't actually want to expose.
> 

Wouldn’t we get a lot of utility by binding / constraining Generator.Element?  Something like:

protocol<SequenceType where Generator.Element == Int>
or
protocol<SequenceType where Generator.Element: Equatable>

In that case we don’t know the specific binding of SequenceType’s Generator but we do know what constraints or binding its Element must have.

Matthew
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