Firebase Undefined symbols for architecture x86_64

Question

I am using firebase as part of a project I am working on in iOS, and I just stumbled upon the following linking error when I try to compile my code against an iPhone 6 or 7

Answer 1

It seems to be a glitch with derived data. I quitted out of Xcode, I removed all Derived Data in /Users/USER_NAME/Library/Developer/Xcode/PROJECT_NAME

Restarted Xcode, compiled and all works as expected, no need to change anything in Active Architecture Only or anything.