Possible bug in compiler? - 'optional' requirements are an Objective-C compatibility feature; add '@objc'

I am trying to build a react-native project and just facing this one error which is preventing the build from getting succeeded.
The project uses a library spokestack-ios. Unfortunately the library is unmaintained so couldn't ask questions there.

The impacted line is:

@objc optional func didTrace(_ trace: String) -> Void

And the error thrown is:
'optional' requirements are an Objective-C compatibility feature; add '@objc'

It seems the compiler is not picking up the @objc keyword in front. If I use the "insert fix" feature of XCode, it just adds an extra @objc which is weird.
The entire file is here
I couldn't see anyone else having the same issue. Any help or pointers would be appreciated.

Try commenting that line; the exact requirement is already stated in the Tracer protocol that SpokestackDelegate is refining.