`po` gives `error: <EXPR>:1:1: error: use of unresolved identifier`

So it turns out there was probably a bug in the past that when you were adding a Swift file it would add/ask a Bridging header, but it wouldn't add that line to your project

SWIFT_OPTIMIZATION_LEVEL = "-Onone";

which means you'd stay in this state

resulting in error: <EXPR>:1:1: error: use of unresolved identifier!

I could only find that out because I moved around files in my project and when I added ObjC files to the project, it asked me about a Bridging header (although I had one already!) and luckily added that SWIFT_OPTIMIZATION_LEVEL. One could consider this is a bug to consider the default value is fastest, but then again I guess this was only a bug in the past and got fixed now.

Still, I fixed now it might be a bug the other way around, if it add none in the release build. I can't test this right now because for testing this I only had a Debug build. I'll leave that as an exercise :) for Apple's Engineers.

I set Swift Complier to "-O"

SWIFT_OPTIMIZATION_LEVEL = "-O";

and set it back to "-Onone".

SWIFT_OPTIMIZATION_LEVEL = "-Onone";

Then it works.

Fixed after setting:

Optimization Level = None[-Onone] under Swift Compiler - Code Generation

Note that setting Optimization Level under Apple LLVM 8.0 - Code Generation has no effect