Created
September 1, 2017 02:49
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| protocol P0 {} | |
| class Cat : P0 {} | |
| func f0(_ x: Cat?) { | |
| if case let p as P0 = x { | |
| print("yes P0") | |
| } else { | |
| print("not P0") | |
| } | |
| } | |
| f0(Cat()) // yes P0 |
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Optional型それ自体はプロトコルを満たしていなくても as で判定すると内側の型に基づいて判定されてしまう
以下のコードは CustomStringConvertible でそれが起きることを避けるためのロジック
https://github.com/apple/swift/blob/0ebf95b8ac7b231636b08f863091b3eb0a7926eb/stdlib/public/core/OutputStream.swift#L355