SE-0293: Extend Property Wrappers to Function and Closure Parameters

[hborla (Holly Borla)]

@Lantua

@hborla Would it be possible to use a pre-transformed ABI, and generate post-transformed boilerplate when being captured by a variable? So we'd have (Wrapper)->() ABI, and can still treat it as (Int)->() on the API level.

I do think it's possible to transform a closure or a function reference that has a property wrapper parameter so that it can be called with the wrapped type. @Jumhyn suggested such a transformation in the pitch thread:

@Jumhyn

Do you have thoughts on the tradeoffs of implementing a transformation for function references like I wrote above, where the problematic line in this example would become:

let bar = { (arg1: Int, arg2: Int) in foo(arg1: Box(wrappedValue: arg1), arg2: Box(wrappedValue: arg2)) }

?

However, the motivating use case for closures as laid out in the proposal is opting into property wrapper syntax for closures that accept property wrapper types.

EDIT: Remember that this is a general problem with parameter declaration attributes and other information like default arguments. I think it's worth solving the problem holistically, rather than specifically for property wrapper parameters.