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The default Swift version leaves 4.0. Because it avoids build errors with Carthage on each Xcode version from the source compatibility between Swift 4.0, 4.2 and 5.0. With regard to CocoaPods, the pod spec is going to support `swift_versions` introduced in CocoaPods v1.7.0. For now, a user needs to override `SWIFT_VERSION` appropriately in Podfile.