[swift-evolution] [Pitch/Reality Check] Allow instance members as parameter default values

[Nate Cook]

Hello all,

I was surprised to find that I can't use an instance member as the default value of a method parameter, only constants and the run-time calculated #file, #line, etc. Is it possible to remove this limitation?

I'd like to propose that we add an overload of the collection index(_:offsetBy:) methods that use the collection's startIndex as a default parameter, but we can't provide that without either this feature or an extra overload. If the declaration looks like this:

extension Collection {
    func index(_ i: Index = startIndex, offsetBy n: IndexDistance) -> Index {
        // ...
    }
}

then calling that method with an omitted first parameter would treat it as something like this:

extension Collection {
    func index(offsetBy n: IndexDistance) -> Index {	
        let i = startIndex
        // ...
    }
}

Is this just syntactic sugar, or am I missing something that makes this harder than it looks? I can see how more complex expressions could be useful there, too, and can't find an obvious reason we couldn't use any code that could be inserted at the top of the method body.

Thanks!
Nate

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