https://twitter.com/harlanhaskins/status/974700944377102336

snipe.swift

func ＝(_ lhs: UInt8, _ rhs: UInt8) -> UInt8 { return 0 }

struct WHelper {
    subscript(_ a: (UInt8, UInt8) -> (UInt8)) -> () -> Void {
        get { return { } }
    }
}

func w(_ a: () -> Void) -> WHelper {
    return WHelper()
}

struct X {
    subscript(_ a: (UInt8, UInt8) -> (UInt8)) -> (() -> Void) -> Void {
        get { return { _ in } }
    }
}
let x = X()

struct Y {
    subscript(_ a: (UInt8, UInt8) -> (UInt8), _ b: () -> Void) -> () -> Void {
        get { return { } }
    }
}
let y = Y()

struct z {
    subscript(_ a: (UInt8, UInt8) -> (UInt8), _ b: () -> Void) -> Void {
        get { }
    }
}

w{}[&]()
x[&](){}
y[&]{}()
z()[&]{}

w{}[＝]()
x[＝](){}
y[＝]{}()
z()[＝]{}