これはバグでしょうか？多分 [array] が再現のポイントです。それがなければ通ります。

func foo<R, T>( array: [T], operation: (Int) throws -> R ) rethrows -> R { return try array.withUnsafeBytes { [array] _ in return try operation(array.count) } }

error: repl.swift:5:16: error: call can throw, but the error is not handled; a function declared 'rethrows' may only throw if its parameter does return try array.withUnsafeBytes { [array] _ in ^ repl.swift:5:38: note: call is to 'rethrows' function, but argument function can throw return try array.withUnsafeBytes { [array] _ in ^

(edited)