Swift blank Xcode Test doesn't compile but app does

Question

I have a default/blank test file and every other code file has the test listed in its targets. My app builds and runs fine, but the test just doesn't. I went to write a first test and this happened so I can't say at what stage this began.

public protocol A: CustomStringConvertible {
static func fz() -> Self
static func fw() -> Self
//adding similar dummy function doesn't give the same effect for it
//static func blah() -> Self
}

public protocol B: A, CustomDebugStringConvertible, Equatable {
    //unimportant code
}

public protocol C: B, ExpressibleByIntegerLiteral {}

public extension C {
    public static func fz() -> Self {
        return 0
    }
    public static func fw() -> Self {
        return 1
    }
    //public static func blah() -> Self {
    //  return 1
    //}
}
extension Int: C {//primary error band here
    public var debugDescription: String {
        return self.description
    }
    //other unimportant code
}

I get the errors

Type 'Int' does not conform to protocol 'A'

Multiple matching functions named 'fz()' with type '() -> Self'

Candidate exactly matches

Candidate exactly matches

and the same for fw

The candidates aren't clear, but the second (of 2) in both cases is the protocol extension. I can't find another it's pointing to.

I'm afraid due to the nature of this project I can't reveal more code except over private message.