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Re: Bogus Bison 3.8.2 warning: suspicious sequence in the output

From: Akim Demaille
Subject: Re: Bogus Bison 3.8.2 warning: suspicious sequence in the output
Date: Sat, 6 Nov 2021 20:09:45 +0100

Hi Marko,

> Le 27 oct. 2021 à 15:04, Marko Mäkelä <marko.makela@iki.fi> a écrit :
> Hi,
> Bison 3.8.2 is issuing the following warning for a grammar whose semantic 
> action includes the variable name my_charset_utf8mb4_bin:
> yy_mariadb.cc:35834: warning: suspicious sequence in the output: b4_bin 
> [-Wother]


> It seems to me that the regular expression in 
> http://git.savannah.gnu.org/cgit/bison.git/tree/src/scan-skel.l is too 
> greedy, or the check should be suppressed if the input contains such 
> identifiers.

Let's start with "too greedy".  Collecting all the identifiers in the input is 
too much effort for this warning.

> A quick search of the Bison Texinfo for b4_ or m4_ did not find anything. 
> Because those are not documented as reserved words or sequences, I think that 
> identifiers like that should be allowed in semantic actions without Bison 
> reporting any warnings.

The pattern was stupid and caught occurrences of b4_/m4_ anywhere.  The point 
was to rather to catch \bb4_ and \bm4_.  This is what the attached commit does. 
 Yes, it does mean that if someone uses b4_foo or m4_foo, she'll get a spurious 
warning.  If that's really a problem, I'll have to introduce a dedicated -W.  
But that would lessen the interest of the warning, if most people don't enable 

Hum...  Actually "escaping" the user's input to avoid the warning would be a 
better option.  I'll think about it.


commit c95d0dd5f54b98825e4cfe00a807db78a073e376
Author: Akim Demaille <akim.demaille@gmail.com>
Date:   Sat Nov 6 19:24:04 2021 +0100

    warnings: be less picky about occurrences of m4_/b4_ in the output
    Reported by Marko Mäkelä.
    * src/scan-skel.l: It is ok to have foob4_ or foom4_.
    * tests/skeletons.at (Suspicious sequences): New.

diff --git a/THANKS b/THANKS
index be743a23..14e678c0 100644
--- a/THANKS
+++ b/THANKS
@@ -117,6 +117,7 @@ Marc Autret               autret_m@epita.fr
 Marc Mendiola             mmendiol@usc.edu
 Marc Schönefeld           marc.schoenefeld@gmx.org
 Mark Boyall               wolfeinstein@gmail.com
+Marko Mäkelä              marko.makela@iki.fi
 Martin Blais              blais@furius.ca
 Martin Jacobs             martin.jacobs@arcor.de
 Martin Mokrejs            mmokrejs@natur.cuni.cz
diff --git a/src/scan-skel.l b/src/scan-skel.l
index 60232a2e..5a93ce03 100644
--- a/src/scan-skel.l
+++ b/src/scan-skel.l
@@ -54,7 +54,9 @@ static void output_mapped_file (char const *name);
 /* Identifiers of our M4 macros.  */
-macro    [bm]4_[a-zA-Z_0-9]*
+macro      [bm]4_[a-zA-Z_0-9]*
+/* Safe sequence of word-constituent characters.  */
+identifier [A-Za-z_0-9]+
@@ -90,7 +92,7 @@ macro    [bm]4_[a-zA-Z_0-9]*
   /* This pattern must not match more than the previous @ patterns. */
 @[^@{}''(\n]*   fail_for_invalid_at (yytext);
 \n              out_lineno++; ECHO;
-[^bm@\n]+       ECHO;
+[^a-z@\n]+      ECHO;
   /* If there are still identifiers that look like macros, such as
      b4_synbol, this probably an error, say a typo in M4, or
@@ -104,6 +106,7 @@ macro    [bm]4_[a-zA-Z_0-9]*
                             "suspicious sequence in the output: %s", yytext);
+{identifier}    ECHO;
 .               ECHO;
diff --git a/tests/skeletons.at b/tests/skeletons.at
index 0ed11027..30f25a61 100644
--- a/tests/skeletons.at
+++ b/tests/skeletons.at
@@ -324,3 +324,44 @@ AT_SETUP([[Fatal errors but M4 continues producing 
+## ---------------------- ##
+## Suspicious sequences.  ##
+## ---------------------- ##
+AT_SETUP([[Suspicious sequences]])
+[[%skeleton "./skel.c"
+  myb4_unevaluated
+  mym4_unevaluated
+start: ;
+AT_BISON_CHECK([[input1.y]], [], [],
+[[input1.tab.c:10: warning: suspicious sequence in the output: b4_unevaluated 
+input1.tab.c:11: warning: suspicious sequence in the output: 
m4@&t@_unevaluated [-Wother]

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