昨日の SE-0110 の件、 Core Team の Joe Groff がここで話してたのと同じこと言ってる。

Furthermore, this probably comes up most commonly with dictionaries, since they're a sequence of tuples. The element tuple for dictionaries has element labels (key: Key, value: Value), so instead of writing `{ tuple in let (key, value) = tuple; f(key, value) }`, you could use the implicit argument and write `{ f($0.key, $0.value) }`.

https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170522/036788.html