[swift-evolution] [Proposal] Associated Type and Generic One-to-One Mapping

[Xiaodi Wu]

For every Bar.T that is currently distinct from Foo.T, this would be
source-breaking.
On Sat, Jun 24, 2017 at 01:51 David Sweeris via swift-evolution <
swift-evolution at swift.org> wrote:

>
> > On Jun 23, 2017, at 5:28 PM, David Moore via swift-evolution <
> swift-evolution at swift.org> wrote:
> >
> > I do indeed have quite a few real examples of this, such prompted me to
> bring this up. I think this could be done without any impact to existing
> code, but it would require some type of keyword. Take the following as a
> possible prototype.
> >
> > protocol Foo {
> >     associatedtype T
> > }
> >
> > struct Bar<T> : Foo {
> >     keyword typealias T // Or really any other syntactical
> implementation.
> > }
> >
> > With an opt-in method we could implement this without affecting existing
> code, thereby making this more viable. I will send some examples later.
>
> At one point there was talk of just having generic parameters
> automatically becoming typealiases:
> struct Bar<T> : Foo {
>     // `T` is automatically an implicit typealias for, well, `T`
> }
>
> Dunno if that’s still the plan (or to what degree it ever was), but it’d
> work for me. I don’t even think it’d break source compatibility (though it
> may make a lot of typealiases unneeded.
>
> - Dave Sweeris
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