Re: preprocessor/4923: Concatenation appears to handle whitespace incorrectly

[Zack Weinberg <zack@codesourcery.com>]

The following reply was made to PR preprocessor/4923; it has been noted by GNATS.

From: Zack Weinberg <zack@codesourcery.com>
To: s0009525@chelt.ac.uk
Cc: gcc-gnats@gcc.gnu.org
Subject: Re: preprocessor/4923: Concatenation appears to handle whitespace incorrectly
Date: Wed, 21 Nov 2001 09:24:54 -0800

 On Wed, Nov 21, 2001 at 04:30:06PM -0000, s0009525@chelt.ac.uk wrote:
 > #define FOO(x) x
 > #define BAR    FOO(abc) ## def
 > BAR
 > 
 > /* cpp < test.h */
 > 
 > # 1 ""
 >  abc  def
 
 This behavior is correct.  The concatenation operator acts before the
 macro FOO is expanded.  At that point the tokens on either side of ##
 are ")" and "def".  Pasting them together produces an invalid token,
 ")def", which triggers undefined behavior - we choose to pretend the
 ## never happened.  Then "FOO ( abc )" gets expanded to produce "abc".
 Since "abc" and "def" were not concatenated, cpp has to put a space
 between them so they are interpreted sa separate tokens.
 
 gcc 3.x will warn you when this happens:
 
 $ cpp-3.0 test.c
 test.c:5:1: warning: pasting ")" and "def" does not give a valid 
 preprocessing token
 # 5 "test.c"
 abc def
 
 zw