Losing the magic

I think the original was (0 ? (*fn)(arg) : store_callback(fn, arg)), and using enough optimization that the direct call won't be generated. Once people demonstrated the benefit and possibility of doing this sort of testing, a later innovation was the sizeof trick to make the code really explicitly execute as "discard a small integer", which will be nothing following even unoptimized code generation (since even minimal register allocation will just discard the code).