[cfe-dev] Macro expansion weirdness (or bug?)

[Chris Lattner]

On Mar 25, 2010, at 10:51 AM, Abramo Bagnara wrote:

> 
> Expanding this snippet:
> 
> #define n(v) v
> #define l X m
> #define m l
> n(m)
> 
> clang preprocessor generate three MacroExpands callbacks in this order:
> 
> n, m, l
> 
> As it can be seen no other expansion are attempted (so to avoid
> recursivity) and everything is fine.
> 
> If I remove the X as in:
> 
> #define n(v) v
> #define l m
> #define m l
> n(m)
> 
> the generated MacroExpands callbacks are in order:
> 
> n, m, l, m, l
> 
> This seems very strange to me and I guess it a symptom of a bug
> somewhere in macro expansion machinery.

This is expected and is how macro expansion (which is a horrible part of the standard works).  However, it could be a problem where the callback isn't invoked as you'd expect it to *shrug*.

-Chris