[swift-evolution] Keyword for protocol conformance

[Xiaodi Wu]

On Tue, Aug 23, 2016 at 2:13 AM, Charles Srstka <cocoadev at charlessoft.com>
wrote:

> On Aug 23, 2016, at 12:11 AM, Charlie Monroe <charlie at charliemonroe.net>
> wrote:
>
>
> I don't see it as sub-par in this example (this actually happened to me):
>
> @objc protocol Foo {
> optional func bar()
> }
>
> class FooImpl: Foo {
> func bar() { ... }
> }
>
> Now imagine that bar() gets renamed in the protocol to baz(). You get no
> warnings, nothing - since the bar() was optional (or can have default
> implementation as Chalers mentioned). FooImpl still conforms to Foo and
> bar() can live on there happily.
>
>
> Had that happen to me a bunch of times, especially when I realize that the
> method needs to have one more argument and then forget to add it everywhere
> (which is harder to use search-and-replace for, as well).
>
> Here’s another case where this can bite you:
>
> In file P.swift:
>
> protocol P {
> func foo()
> }
>
> extension P {
> func foo()
> }
>
> In another file, S.swift:
>
> struct S: P {}
>
> Imagine I rename foo in the protocol, but forget to rename it in the
> extension. The method no longer has a default implementation, and we do
> indeed get an error, but the error’s in S.swift, in the wrong place. S
> complains that it doesn’t conform to the protocol. If there were a keyword
> on foo() in the extension, the compiler warning would be right where the
> problem is as soon as I renamed the method in the protocol, and it’d be a
> 2-second fix.
>

This is a great motivating example, I think. I'm convinced that we need
some way to improve this scenario.

That said, it'd probably have to be opt-in because major source-breaking
changes are going to be frowned upon for Swift 4, and changing the syntax
for every single protocol conformance would be hugely source-breaking. That
said, I think an opt-in level of safety, where you mark particular
declarations or even entire extensions as `@conforming` (or some similar
syntax) would give the user most or all of these benefits while maintaining
source compatibility. I would be +1 on a proposal like that.



> Charles
>
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