↑の defer の挙動の違いは Error Handling Rationale and Proposal に書かれてる。 Swift は意図的にスコープ単位に改変して導入した。

Overall, I think it is better to use a Go-style defer than a Java-style try ... finally. ... Unlike Go, I think this should be tied to scope-exit, not to function-exit.

https://github.com/apple/swift/blob/master/docs/ErrorHandlingRationale.rst#clean-up-actions-1

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