koher

struct Foo { var a: Int } struct Bar { var foo: Foo } let k1: KeyPath<Bar, Foo> = \.foo let k2: WritableKeyPath<Foo, Int> = \.a let k: WritableKeyPath<Bar, Int> = k1.appending(path: k2)

Version:

swift-4.1.1-RELEASE

Output:

Error:

/usercode/main.swift:11:39: error: cannot convert value of type 'KeyPath<Bar, Int>' to specified type 'WritableKeyPath<Bar, Int>' let k: WritableKeyPath<Bar, Int> = k1.appending(path: k2) ~~~^~~~~~~~~~~~~~~~~~~ as! WritableKeyPath<Bar, Int>