%assign does not support $

[jlawson-yasm@…]

The "$" operator (current assembly position) cannot be used in expressions for 
the %assign directive.

This was seen in YASM version 0.3.0
(Note that your Bugzilla version table dropdown does not list 0.3.0 yet)

For example this fails with "`$' not supported":

   %assign ual ($-startseg)
   mov eax,ual

But this succeeds:

   mov eax,($-startseg)

I've tried this in "-p nasm -r nasm" and also without any -p or -r options.

[jlawson-yasm@…]

Note that in the above example, "startseg:" is a label defined previously.

[peter@…]

0.3.0 uses the real NASM preprocessor... and this bug was a crash workaround
until I figured out a better way to handle this.  As a workaround, use EQU for
the test case given.

The major difficulty here is for the %assign .... %if situation (e.g. you
%assign with a $ expression and then test that asssignment).  Because YASM only
preprocesses and parses a file once before passing the parsed result onto the
later assembly stages, whereas NASM parsed a file once, ran it all the way
through the assembler to determine offsets of jump targets, then re-parsed (and
re-preprocessed) the file to generate the output, the YASM preprocessor cannot
know the output addresses (e.g. $) whereas the NASM preprocessor can.

Thus, while it may be not too hard to support this particular testcases' usage,
not all usages of $ in %assign will be able to be supported without significant
impact to the overall design of YASM: multiple parses, or the slightly more
preferable (but still difficult to implement) option of pushing back %if
decisions to the output stage.