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
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