is で true はバグな気がする。
1:56 AM
 8> let b: CustomStringConvertible = a error: repl.swift:8:34: error: value of optional type 'Int?' not unwrapped; did you mean to use '!' or '?'? let b: CustomStringConvertible = a ^ ! 8> let c: CustomDebugStringConvertible = a c: CustomDebugStringConvertible = { payload_data_0 = 0x000000000000002a payload_data_1 = 0x0000000000000000 payload_data_2 = 0x0000000000000000 instance_type = 0x0000000101416700 libswiftCore.dylib`InitialAllocationPool + 80 protocol_witness_0 = 0x00000001013d1b78 libswiftCore.dylib`protocol witness table for <A> Swift.Optional<A> : Swift.CustomDebugStringConvertible in Swift }