1> let a = [""].map { $0 }.prefix(3) a: ArraySlice<String> = 1 value { [0] = "" } 2> let b = [""].prefix(3).map { $0 } error: repl.swift:2:9: error: ambiguous use of 'prefix' let b = [""].prefix(3).map { $0 } ^ 2> let b = [""].prefix(3) b: ArraySlice<String> = 1 value { [0] = "" } 3> b.map { $0 } $R0: [String] = 1 value { [0] = "" }