Re: Sparse preprocessing bug with zero-arg variadic macros

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Aug 31, 2017 at 09:54:33PM +0100, Al Viro wrote:
> What a mess...  Note that for non-vararg it *is* the right interpretation
> (with #define A(x) [x] we will have A() interpreted as "empty token sequence
> as the only argument", not "no arguments given").  For vararg case we
> normally do not need to distinguish "not given" and "empty" - the only
> thing that cares is exactly the ,## kludge.  There with
> #define B(x,...) [x,##__VA_ARGS__]
> B(1) and B(1,) yield [1] and [1,] resp.  And for everything other than
> "just ..." we even get it right...
> 
> I see what's going on there; will post a fix in a few.


Fix macro argument parsing for (...) case

Nasty corner case for the sake of ,##__VA_ARGS__ perversion - for something
like #define A(x,...) [x,##__VA_ARGS] we want A(1) to expand to [1] and
A(1,) - to [1,].  In other words, "no vararg given" and "vararg empty" are
different and need to be distinguished.  Unfortunately, in case when there
was nothing but vararg we got it wrong - #define A(...) ,##__VA_ARGS ended
up with A() interpreted as "one empty argument" (as it would in non-vararg
case) rather than "zero arguments".

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
diff --git a/pre-process.c b/pre-process.c
index 74414df..8800dce 100644
--- a/pre-process.c
+++ b/pre-process.c
@@ -296,9 +296,11 @@ static int collect_arguments(struct token *start, struct token *arglist, struct
 		for (count = 0; count < wanted; count++) {
 			struct argcount *p = &arglist->next->count;
 			next = collect_arg(start, p->vararg, &what->pos, p->normal);
-			arglist = arglist->next->next;
 			if (eof_token(next))
 				goto Eclosing;
+			if (p->vararg && wanted == 1 && eof_token(start->next))
+				break;
+			arglist = arglist->next->next;
 			args[count].arg = start->next;
 			args[count].n_normal = p->normal;
 			args[count].n_quoted = p->quoted;