Why am I getting undefined symbol _OBJC_CLASS_$_FIRInstanceID? - ios

Question

This is an iOS app that uses Firebase.
If I comment out code that gets Firebase's instanceID tokens, then I'm able to compile and link.
I installed Firebase with CocoaPods. No errors at all, no warnings. (Before pod install I did pod update. So I got the latest Firebase.)
The reference comes from AppDelegate.o. In AppDelegate.swift I have
import UserNotifications
import Firebase
import FirebaseInstanceID
import FirebaseMessaging
import GoogleUtilities
I know you'll ask to see a lot more about the environment. I'm not sure everything I should show you. I'll edit the question as you request more information.
UPDATE 1: Here's something I also notice in the output window.
<Warning>: 5.8.0 - [Firebase/Core][I-COR000022] Firebase Analytics is not available. To add it, include Firebase/Core in the Podfile or add FirebaseAnalytics.framework to the Link Build Phase
But my Podfile looks just fine. Here's my Podfile.
# Uncomment the next line to define a global platform for your project
# platform :ios, '9.0'
target 'XKCD' do
# Comment the next line if you're not using Swift and don't want to use dynamic frameworks
use_frameworks!
# Pods for XKCD
pod 'Firebase/Core'
pod 'Firebase/Messaging'
target 'XKCDTests' do
inherit! :search_paths
# Pods for testing
end
target 'XKCDUITests' do
inherit! :search_paths
# Pods for testing
end
end
UPDATE 2: Here's what seems as a pretty relevant event. I added Firebase/Database to my Podfile just to see if I could use the database. I couldn't even compile the project with more linking errors, so I totally gave up on the task, removed Firebase/Database from my Podfile and said pod install to my shell once again to reflect my change of mind. Here's what happened:
$ pod install
Analyzing dependencies
Removing FirebaseAuth
Removing FirebaseAuthInterop
Removing FirebaseDatabase
Removing GTMSessionFetcher
Removing leveldb-library
Downloading dependencies
Installing Firebase 5.11.0 (was 5.8.0)
Installing FirebaseAnalytics 5.3.0 (was 5.1.2 and source changed to `https://github.com/CocoaPods/Specs.git` from `https://github.com/cocoapods/specs.git`)
Installing FirebaseAnalyticsInterop (1.1.0)
Installing FirebaseCore 5.1.6 (was 5.1.3 and source changed to `https://github.com/CocoaPods/Specs.git` from `https://github.com/cocoapods/specs.git`)
Installing FirebaseInstanceID 3.3.0 (was 3.2.1 and source changed to `https://github.com/CocoaPods/Specs.git` from `https://github.com/cocoapods/specs.git`)
Installing FirebaseMessaging 3.2.1 (was 3.1.2 and source changed to `https://github.com/CocoaPods/Specs.git` from `https://github.com/cocoapods/specs.git`)
Installing GoogleAppMeasurement 5.3.0 (was 5.1.2 and source changed to `https://github.com/CocoaPods/Specs.git` from `https://github.com/cocoapods/specs.git`)
Installing GoogleUtilities 5.3.4 (was 5.2.3)
Using Protobuf (3.6.1)
Using nanopb (0.3.8)
Generating Pods project
Integrating client project
Sending stats
Pod installation complete! There are 2 dependencies from the Podfile and 10 total pods installed.
[!] Automatically assigning platform `ios` with version `8.0` on target `XKCD` because no platform was specified. Please specify a platform for this target in your Podfile. See `https://guides.cocoapods.org/syntax/podfile.html#platform`.
I make no conclusions for the moment. (Uncommenting the InstanceID code and trying to compile it again blows the same message about undefined symbol _OBJC_CLASS_$_FIRInstanceID. Commenting it out and recompiling runs the app just fine, though I still see that "Firebase Analytics is not available" message in the output window.)