Expose c-function for testing

Question

I have a c-function within a class:

@interface MyClass: NSObject
@end

@implementation MyClass 
static void MyFunction(void) {
// Do some stuff.
}
@end

And I'd like to expose my function for testing:

@interface MyClass (testing)
static void MyFunction(void);
@end

But this leads to error:

Function 'MyFunction' has internal linkage but is not definedclang(-Wundefined-internal)

Is there some way to expose this method?