[swift-evolution] [Review] SE-0174: Change `filter` to return an associated type

[Brent Royal-Gordon]

> On Apr 28, 2017, at 5:06 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:
> 
> What is your evaluation of the proposal?
I'm in favor of this proposal, and I don't agree with the previous reviewers who think this should be coupled with more advanced type system features. However, I have a few questions:

* How will associated type inference interact with this? If you write a type-specific `filter`, will it be automatically favored over the `Array` version, or will you have to manually specify a `Filtered` typealias?

* Do we want to constrain `Filtered` to `Sequence` or `Collection`? Do we want to require `where Filtered.Iterator.Element == Self.Iterator.Element`?

* Will `AnySequence`/`AnyCollection` have a `Filtered` of type `AnySequence`/`AnyCollection`, or one of type `Array`?

No time for a full review, but I thought it'd be helpful to note these things before the review period was up.

-- 
Brent Royal-Gordon
Architechies

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