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

[Charles Srstka]

> On Aug 16, 2016, at 5:30 PM, Xiaodi Wu <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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