> On Feb 8, 2017, at 10:30 PM, Douglas Gregor <dgre...@apple.com> wrote: > > >> On Feb 8, 2017, at 10:21 PM, Slava Pestov <spes...@apple.com> wrote: >> >> Hah, Doug and I were just discussing this. >> >> In Swift 3.1, we generalized where clauses to allow them to add requirements >> on outer generic parameters. However we did not remove the diagnostic >> prohibiting a where clause from being attached to a non-generic method. In >> theory this can be made to work; the only slightly tricky thing is we will >> get a GenericParamList with zero parameters but non-zero requirements, which >> would require shuffling some things around to avoid assertions. >> >> This would be a good starter project for someone who wanted to learn more >> about the generics system. >> >> As for index(of:) and the specific details of the stdlib that are involved >> here, I have no idea — I’m just talking about the bogus diagnostic itself. > > Well, I think Brent is talking about doing this on a protocol requirement, > which is more interesting because not all conforming types would satisfy the > requirement…
Since there would be no way to invoke the requirement on such a type, could we leave the entry blank in the witness table or emit a fatalError() thunk or something? Slava > > - Doug > >> >> Slava >> >>> On Feb 8, 2017, at 9:57 PM, Brent Royal-Gordon via swift-evolution >>> <swift-evolution@swift.org> wrote: >>> >>> In an article on `Collection` today*, Ole Begemann points out that >>> `index(of:)`, along with other `Equatable`- and `Comparable`-constrained >>> `Collection` methods, cannot be overridden. Actually, it *can* be, but only >>> through a private mechanism—there's a `_customIndexOfEquatableElement(_:)` >>> method that's invisible in the generated interface. But that only proves >>> the need for a way to make methods like these overridable. >>> >>> The problem is that the `index(of:)` method should only be offered when the >>> element is `Equatable`—otherwise it simply won't work. But there's no way >>> to specify this rule in current Swift. In theory, we could describe such a >>> requirement with something like this: >>> >>> func index(of element: Iterator.Element) -> Index? where >>> Iterator.Element: Equatable >>> >>> But this is not permitted—you get an error indicating that `where` clauses >>> are only allowed on generic methods. Adding a spurious generic parameter >>> allows this code to compile, but with a deprecation warning indicating that >>> this is deprecated. I don't know if it would actually behave correctly, >>> however. >>> >>> Is this a feature we should add? Is this the way to add it? Would it have >>> non-additive ABI impact? (The private `index(of:)` override would certainly >>> go away, but that's why it's private, I suppose.) I don't seem to remember >>> seeing something like this in the generics manifesto, so I thought it was >>> worth bringing up. >>> >>> >>> >>> * https://oleb.net/blog/2017/02/sorted-array/ >>> >>> -- >>> Brent Royal-Gordon >>> Architechies >>> >>> _______________________________________________ >>> swift-evolution mailing list >>> swift-evolution@swift.org >>> https://lists.swift.org/mailman/listinfo/swift-evolution >> > _______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution
