SE-0014 以前はどっちも普通に動いた。 sh-session $ /Library/Developer/Toolchains/swift-2.2-SNAPSHOT-2016-01-06-a.xctoolchain/usr/bin/swift Welcome to Apple Swift version 2.2-dev (LLVM 3ebdbb2c7e, Clang f66c5bb67b, Swift 54dcd16759). Type :help for assistance. 1> let a = [""].map { $0 }.prefix(3) 2. let b = [""].prefix(3).map { $0 } // Ambiguous use of 'prefix' a: ArraySlice<String> = 1 value { [0] = "" } b: [String] = 1 value { [0] = "" } 
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