-frontend -replだとエラーになるね。
$ swift -frontend -repl -sdk $(xcrun --show-sdk-path --sdk macosx) *** You are running Swift's integrated REPL, *** *** intended for compiler and stdlib *** *** development and testing purposes only. *** *** The full REPL is built as part of LLDB. *** *** Type ':help' for assistance. *** (swift) let t = Int.self // t : Int.Type = Swift.Int (swift) let a: [t] = [2, 3, 5] <REPL Input>:1:9: error: use of undeclared type 't' let a: [t] = [2, 3, 5] ^ (swift)