Apparently you are using Carthage. Then you need to recompile the modules using the Swift 4 compiler, even if it is in 3.2 mode.
First you must configure the compiler for the toolchain of Xcode 9, using the following command on the terminal:
sudo xcode-select --switch /Applications/Xcode-beta.app/Contents/Developer
Note that you must pass the path to where you saved the Xcode-Beta.
After that you can build the framework in question using:
carthage update ObjectMapper --platform iOS --no-use-binaries
This command will compile the framework using the toolchain defined in step 1. It may be a little slow, but after that your project should work smoothly.
Note
To return to toolchain standard execute command on terminal:
sudo xcode-select --switch /Applications/Xcode.app/Contents/Developer
Answer adapted from this in OS English.
This works if you are using
CocoaPods
to manage dependencies or is doing manuel integration, in the case ofCarthage
needs to be the way explained in my reply.– Otávio