[binutils-gdb] gdb: silence unused-but-set-variable warning about yynerrs in cp-name-parser.y

[Simon Marchi <simark@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=e57f7fa070e16683c0e1ddd0d1307d243fad990d

commit e57f7fa070e16683c0e1ddd0d1307d243fad990d
Author: Simon Marchi <simon.marchi@efficios.com>
Date:   Wed Oct 12 13:32:56 2022 -0400

    gdb: silence unused-but-set-variable warning about yynerrs in cp-name-parser.y
    
    When building with clang 15 on Ubuntu 20.04, I get:
    
          CXX    cp-name-parser.o
        cp-name-parser.c.tmp:1777:9: error: variable 'cpnameyynerrs' set but not used [-Werror,-Wunused-but-set-variable]
            int yynerrs;
                ^
        /home/smarchi/src/binutils-gdb/gdb/yy-remap.h:58:18: note: expanded from macro 'yynerrs'
        #define yynerrs         GDB_YY_REMAP (yynerrs)
                                ^
        /home/smarchi/src/binutils-gdb/gdb/yy-remap.h:40:29: note: expanded from macro 'GDB_YY_REMAP'
        #define GDB_YY_REMAP(YYSYM) GDB_YY_REMAP_1 (GDB_YY_REMAP_PREFIX, YYSYM)
                                    ^
        /home/smarchi/src/binutils-gdb/gdb/yy-remap.h:39:39: note: expanded from macro 'GDB_YY_REMAP_1'
        #define GDB_YY_REMAP_1(PREFIX, YYSYM) GDB_YY_REMAP_2 (PREFIX, YYSYM)
                                              ^
        /home/smarchi/src/binutils-gdb/gdb/yy-remap.h:38:39: note: expanded from macro 'GDB_YY_REMAP_2'
        #define GDB_YY_REMAP_2(PREFIX, YYSYM) PREFIX ## YYSYM
                                              ^
        <scratch space>:45:1: note: expanded from here
        cpnameyynerrs
        ^
    
    This is because clang 15 warns for something like this:
    
        int n;
        n = 0;
        ++n;
    
    whereas previous versions do not.
    
    yynerrs is defined in yyparse and is there for actions to use.  Since
    the actions in cp-name-parser.y don't use it, we get a warning.  We see
    this problem on this particular .y file because it uses `%pure-parser`
    [1], which makes yynerrs a local rather than a global.
    
    I initially fixed this by using
    DIAGNOSTIC_IGNORE_UNUSED_BUT_SET_VARIABLE (like in commit f7aa1a5acc5
    ("gold: Suppress "unused" variable warning on Clang")), but then I
    realized we could suppress the warning in a more fine-grained way using
    this in a rule:
    
        (void) yynerrs;
    
    [1] https://www.gnu.org/software/bison/manual/html_node/Error-Reporting-Function.html
    
    Change-Id: I6cae7a4207c19fe1b719e2ac19be69122ebe3af1

Diff:
---
 gdb/cp-name-parser.y | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/gdb/cp-name-parser.y b/gdb/cp-name-parser.y
index 34c691ddabb..1108c787b7e 100644
--- a/gdb/cp-name-parser.y
+++ b/gdb/cp-name-parser.y
@@ -346,7 +346,12 @@ static void yyerror (cpname_state *, const char *);
 %%
 
 result		:	start
-			{ state->global_result = $1; }
+			{
+			  state->global_result = $1;
+
+			  /* Avoid warning about "yynerrs" being unused.  */
+			  (void) yynerrs;
+			}
 		;
 
 start		:	type