Is there a way to make code below work as expected? Suppose first `if` is a `proc` macro and second is written plain in code; proc could be defined in a separate file which in turn may or may not be included by filtering it out with `match` directive. What is expected is that output file is 8 (instead of 0) bytes.

Both 8-byte and 0-byte are the correct solutions to this source, so you have to play some tricks with fasm to make it choose a different route while resolving. Since "used" has default value FALSE when there is not enough information to make prediction, and this happens in the first pass, it chooses the "skip-if" route in the first pass and this leads to finding the 0-byte solution. You can, however, use this trick to make the first "if" block be assembled in the first pass:
As "defined" in the first pass has default value of FALSE, the "if" block gets assembled in the first pass and thus assembler finds the 8-byte solution.