This thread received very light, but positive feedback.  I would really like to 
see this feature added and am willing to draft and official proposal but am not 
able to implement it.  If anyone is interested in collaborating just let me 

> On Nov 24, 2017, at 5:03 PM, Matthew Johnson via swift-evolution 
> <> wrote:
> One of the most frequent frustrations I encounter when writing generic code 
> in Swift is the requirement that supertype constraints be concrete.  When I 
> mentioned this on Twitter 
> ( 
> <>) Doug Gregor 
> mentioned that this feature is smaller and mostly straightforward to design 
> and implement ( 
> <>).
> I currently have a PR open to add the high-level description of this feature 
> found below to the generics manifesto 
> ( 
> <>):
> Currently, supertype constraints may only be specified using a concrete class 
> or protocol type.  This prevents us from abstracting over the supertype.
> ```swift
> protocol P {
>   associatedtype Base
>   associatedtype Derived: Base
> }
> ```
> In the above example `Base` may be any type.  `Derived` may be the same as 
> `Base` or may be _any_ subtype of `Base`.  All subtype relationships 
> supported by Swift should be supported in this context including, but not 
> limited to, classes and subclasses, existentials and conforming concrete 
> types or refining existentials, `T?` and  `T`, `((Base) -> Void)` and 
> `((Derived) -> Void)`, etc.
> Generalized supertype constraints would be accepted in all syntactic 
> locations where generic constraints are accepted.
> I would like to see generalized supertype constraints make it into Swift 5 if 
> possible.  I am not an implementer so I will not be able to bring a proposal 
> forward alone but am interested in collaborating with anyone interested in 
> working on implementation.
> I am also interested in hearing general feedback on this feature from the 
> community at large.  Have you also found this limitation frustrating?  In 
> what contexts?  Does anyone have reservations about introducing this 
> capability?  If so, what are they?
> Matthew
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