Inconsistency in `Generic parameter 'T' could not be inferred `

Using Swift
1

Is shorthand the preferred way to write ifelse statements in Swift when working with generics?

protocol SomeExample {
    var id: String { get }
    init(id: String)
}

struct Object: SomeExample {
    var id: String
}

class Source {
    func get<T: SomeExample>(completion: ([T]) -> Void) {
        completion([T(id: "")])
    }
}

var outside: Object?
let source = Source()

// Error: Generic parameter 'T' could not be inferred
source.get(completion: {
    if let first: Object = $0.first {
        outside = first
    } else {
        fatalError()
    }
})

// I understand this is easier to infer
source.get(completion: {
    outside = $0.first
})

// But this should definitely be harder than the if
// statement that literally states the type but it
// runs fine
source.get(completion: { objects in
    objects.first != nil ? {
        outside = objects.first
    }() : {
        fatalError()
    }()
})
2

Swift's rule for type inference in closures is that if the closure contains multiple statements, the body is not used to determine its type. This matches the behavior outside of a closure, where later statements in a function do not affect the types of earlier statements in the function, but it does produce a discontinuity when you go from one statement to two.

3

This is a good motivation for expanding what can be done with expressions.

4

It's a motivation. I'm not sure it's a good one because (as frequently complained) massive expressions result in worse type-checker performance (something that may be possible to improve) and more opaque diagnostics (something that's probably inherent, at least to some degree).

5

Yes, it would require judicious use for sure. But I run across code regularly that would be improved by it. Simple closures with verbose return types that could be inferred are a relatively common example. I could be wrong, but I don’t think the cases I have in mind would run into significant type checker performance issues.