[omochi@omochi-iMac swhash]$ export TOOLCHAINS=org.swift.4120171204a [omochi@omochi-iMac swhash]$ swift --version Apple Swift version 4.1-dev (LLVM cdc9836644, Clang f0e38d2044, Swift 7a0598c7dd) Target: x86_64-apple-darwin16.7.0 [omochi@omochi-iMac swhash]$ swift b.swift[omochi@omochi-iMac swhash]$ export TOOLCHAINS=org.swift.3020171204a [omochi@omochi-iMac swhash]$ swift --version Apple Swift version 4.1-dev (LLVM c698959a5c, Clang dff0a814ae, Swift 9fef06e59c) Target: x86_64-apple-darwin16.7.0 [omochi@omochi-iMac swhash]$ swift b.swift b.swift:6:1: error: conditional conformance of 'Box<T>' to 'Equatable' depends on an experimental feature (SE-0143) extension Box: Equatable where T: Equatable { ^$ swift --version だとほとんどわからんですね。