Carthage update/bootstrap download Swift 3.1 RealmSwift module for swift 4

[liuxuan30]

I updated ChartsRealm(https://github.com/danielgindi/ChartsRealm) dependency finally, carthage update works fine, downloaded Realm 2.10.2 as expected. However when building ChartsRealm demo, Xcode9 complains

~/ChartsRealm/ChartsRealm/Classes/Data/RealmBarDataSet.swift:16:8: Module compiled with Swift 3.1 cannot be imported in Swift 4.0: ~/ChartsRealm/Carthage/Build/iOS/RealmSwift.framework/Modules/RealmSwift.swiftmodule/x86_64.swiftmodule

I then delete carthage folder and run carthage bootstrap again, showing build successful but still the same error. It looks like a wrong module is downloaded. How do we correct it? (build both for arm and x86 arch are the same error)

Carthage output log:

xuanliu:ChartsRealm xuanliu$ carthage bootstrap
*** Checking out Charts at "v3.0.4"
*** Downloading realm-cocoa.framework binary at "v2.10.2"
*** xcodebuild output can be found in /var/folders/8v/6dg7d6mx2850sv1gp8ts9thm0000gn/T/carthage-xcodebuild.HDBArl.log
*** Building scheme "Charts" in Charts.xcodeproj

xuanliu:ChartsRealm xuanliu$ 

Realm framework version: 2.10.2

Xcode version: 9

iOS/OSX version: 11

Dependency manager + version: carthage 0.17.2

[bdash]

The output you've shared indicates that you're using Xcode 9 to when building, but your currently active command-line tools are from an older version of Xcode (Xcode 8.3.x). You need to use the same Xcode version for both to ensure that Carthage gives you frameworks built for the appropriate Swift version.

You can run xcode-select -p to verify which command-line tools are active, and you can use sudo xcode-select -s /path/to/Xcode.app to switch the active tools.

[liuxuan30]

What? I only have Xcode9 installed..

xuanliu:script xuanliu$ xcode-select -p
/Applications/Xcode.app/Contents/Developer

This can also confirm. But shall I point to just Xcode.app or keep Xcode.app/Contents/Developer?

[bdash]

It’s also possible the issue is that the version of Carthage you’re using predates detecting that prebuilt binaries were built with a different Swift version. v0.17.2 is far behind the most recent version of Carthage, which I think is currently v0.26.2. Can you try updating to a recent version of Carthage?

[bdash]

v0.20.0 or newer of Carthage has the support for falling back to building from source if the prebuilt binaries are built with a different version of Xcode than you’re using locally.

[liuxuan30]

oh yeah, thanks for pointing this out! I don't realize it's a feature of carthage