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Short story: Xcode 10 is set to Swift 4, but it is using Swift 4.2.

Long story: Our project is not ready for Swift 4.2, since our dependent frameworks are not Swift 4.2, yet. But, I would like to, at least, use Xcode 10.

  • I have Xcode 9.4.1 and Xcode 10 installed.
  • I have Command Line Tools set to use Xcode 9.4.1:

enter image description here

  • In Xcode 10, I have the project setting, Swift Language Version, set to Swift 4.

enter image description here

  • I have verified on the command line the Swift version the shell is using is 4.1.2:

"swift --version Apple Swift version 4.1.2 (swiftlang-902.0.54 clang-902.0.39.2)"

  • I've run carthage update.
  • I've quit Xcode 10 and relaunched.
  • I have deep cleaned and deleted Derived Data.
  • I build and I get this error:

"Module compiled with Swift 4.1.2 cannot be imported by the Swift 4.2 compiler: .../App/Carthage/Build/iOS/RealmSwift.framework/Modules/RealmSwift.swiftmodule/arm64.swiftmodule"

To me, this indicates that I have my intended Swift version (4.1.2) set correctly and Xcode is still trying to use the wrong version of Swift (4.2).

Does anyone have suggestions on how to resolve this? Am I missing something? Xcode bug?

Heliotropix
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4 Answers4

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I'm not positive, but my guess is that this is the reason:

I have Command Line Tools set to use Xcode 9.4.1

You have to recompile your Swift 4 dependencies using Xcode 10, because Swift is not ABI-stable, i.e. binaries compiled with different compiler versions are not compatible. And even Swift 4.0 binaries compiled with different versions of Xcode may not be compatible.

I don't have much experience with Carthage, but my guess is that you should use Xcode 10 as a command-line tool, but you have to set a different Swift language version to be used by Xcode 10. It should probably be specified in the project (in this case RealmSwift), or using environment variables, but you'll need someone else to answer with more details, I'm afraid.

FreeNickname
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  • That makes sense. I think the "cannot be imported by the Swift 4.2 compiler" message is a red herring. I, initially, tried compiling with Xcode 10, but frameworks were built with Swift 4.2 and I couldn't find a way to have the command line use Swift 4. – Heliotropix Sep 22 '18 at 14:46
  • I can set my command line environment to use the Xcode 10 toolset, but how do I set it to use Swift 4 rather Swift 4.2? – Heliotropix Sep 22 '18 at 14:55
  • @Heliotropix, it's usually set using `SWIFT_VERSION` build setting in the Xcode project. It can also be passed as an environment variable to xcodebuild, if I'm not mistaken. Theoretically, Carthage should just take it from the project. I don't know if there's any way to force it to use a particular Swift version. What happens if you simply compile it with Xcode 10 toolchain? Does it fail? What error does it produce? In the RealmSwift project (`master` branch) looks like it's actually set to use Swift 3 :) – FreeNickname Sep 22 '18 at 21:35
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Using Realm with Xcode 10 requires building it from source. In Terminal write:

git clone --recursive https://github.com/realm/realm-cocoa.git

cd realm-cocoa

sh build.sh ios-swift
Saqib Saud
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Andreea Lavinia Ionescu
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    Thank you for your reply. I can also use "Carthage update --no-use-binaries", but with the Xcode 10 toolset, it uses Swift 4.2. How do I specify Swift 4. It's not a Carthage issue, its a shell environment issue. – Heliotropix Sep 22 '18 at 14:59
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you can use multiple swift toolchains. download url swift toolchain download after you install some version. restart your xcode, in xocde menu you can see toolchains option.

lingyfh
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  • I have just updated to Xcode 10.2 and an external library not working on the new SDK and I received the same build error for Swift 5. I downloaded the swift toolchain. Now build error gone but when running it on device/simulator causes crash with error "Reason: Incompatible library version: Instamojo requires version 1.0.0 or later, but libswiftCore.dylib provides version 0.0.0". Any idea? – Anand Apr 05 '19 at 14:17
  • you need update Instamojo version or build Instamojo source for your project. – lingyfh Apr 07 '19 at 14:50
0

I had the same problem and then I realised I had not selected Xcode 10 as my Xcode. Try:

sudo xcode-select -s /Applications/Xcode10.app

Also, if you will be switching between Xcode9 and 10, I recommend you the following: If you change from 9 to 10, run mv Carthage Carthage9 and then rebuild Carthage. Whenever you want to switch back, move Carthage to Carthage10 and do mv Carthage9 Carthage. This will save you lots of time building.

regina_fallangi
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  • Not all of my frameworks build right now with Swift 4.2. My issue is not building them with Xcode 10. I can do that. What I want to do is build them with Swift 4.1.2 and use them in my pre-Swift 4.2 project. When I do, Xcode is giving my the error saying they can't be imported by the Swift 4.2 compiler. How do I remedy that? – Heliotropix Sep 20 '18 at 13:42
  • Quoted from your question: " But, I would like to, at least, use Xcode 10.". That, for me, means you can't build Xcode 10. But in this comment you say you can? I suggest you reformulate your question and explain your situation better. – regina_fallangi Sep 20 '18 at 13:54