koher

この "Swift 6 language mode" はまだ存在しない架空のモードなのか。

this syntax should be required for existential types under the Swift 6 language mode

https://swiftfiddle.com/zxvu5klsmnd3lljuw5g2cumamu で compiler options に -swift-version 6 を指定したら動きました．nightly だと行けるようです．

SwiftFiddle is an online playground for creating, sharing and embedding Swift fiddles (little Swift programs that run directly in your browser).