You're right.
Upon digging even more and deleting various cached files and removing extra source code, the real problem was that my 'ComponentSystem.h' was generated once, but didn't get regenerated again by the pre-processor:
// This file is generated by the pre-processor. Do not modify it.
// Generated on: Wed Jan 18 00:48:19 2023
However, my 'ComponentSystem.cpp' file did:
// This file is generated by the pre-processor. Do not modify it.
// Generated on: Wed Jan 18 10:31:21 2023
I realized this was the actual problem when I was getting compile errors for code that wasn't even in the project anymore.
Upon deleting 'ComponentSystem.h' and getting it re-generated, the error now goes away.
