[swift-evolution] PITCH: New :== operator for generic constraints

[Karl]

> On 17 Aug 2016, at 00:34, Charles Srstka via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> On Aug 16, 2016, at 5:30 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>> 
>> On Tue, Aug 16, 2016 at 5:19 PM, Charles Srstka via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> On Aug 16, 2016, at 5:13 PM, David Sweeris <davesweeris at mac.com <mailto:davesweeris at mac.com>> wrote:
>>> 
>>> Any proposal that expands the power of generic programming gets an almost automatic +1 from me.
>>> 
>>> I can't think of any circumstances in which I wouldn't want to use ":==" instead of ":". Are there any downsides to expanding ":" to mean what ":==" does?
>>> 
>>> Incidentally, I kinda thought things either already worked like this, or would work like this after generics were "completed", but I can't tell you why I thought that.
>> 
>> Me neither, but the last time I proposed that, people stated that there were some cases where this could not work. No concrete examples were given, but I assume it probably has something to do with associated type wackiness. :== seems like a workable compromise to me.
>> 
>> If an existential of a protocol P doesn't conform to itself, what can you do inside the body of a generic function that has a generic constraint specified with `:==`? In other words, what would we know about what's in common between such an existential of a protocol and types that conform to the protocol?
> 
> My proposal is that in such cases, using :== would lead to a compiler error.
> 
> Charles
> 
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I think the point is that existentials not conforming to their protocols is the underlying reason this doesn’t work already.

From the previous thread:
> There are a couple of reasons this is the case.  IIRC in some cases it actually isn't possible for the existential to conform to the protocol in a sound way.  And even when it is possible, I believe it has been said that it is more difficult to implement than you might think.  Hopefully the situation will improve in the future but I'm not aware of any specific plans at the moment.

It seems like a reasonably straightforward axiom. I would be interested to learn more about those edge-cases.

Karl
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