When I switch from Xcode8 to Xcode9 and convert to Swift4, I get the following error. Here's how to make it disappear.

The use of Swift 3 @objc inference in Swift 4 mode is deprecated. Please address deprecated @objc inference warnings, test your code with “Use of deprecated Swift 3 @objc inference” logging enabled, and then disable inference by changing the "Swift 3 @objc Inference" build setting to "Default" for the "TargetName" target.

Select each target -> Build Settings -> Search for inference in the search window -> Set Swift @objc inference to Default　
If there are other targets (UITest and UnitTest), set them to Default too.
It should be gone now!