X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/8bb4c753e2d626ee90460e6d9f9cbaa2cd1f560d..668c5d192776326107790e8d622dfa7ddd015ea3:/NEWS

diff --git a/NEWS b/NEWS
index 22d182fa..2b7c1013 100644
--- a/NEWS
+++ b/NEWS
@@ -3,6 +3,32 @@ Bison News
 
 Changes in version 2.1a:
 
+* New warning: unused values
+  Typed right-hand side symbols whose value are not used are reported.
+  For instance:
+
+     exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
+	| exp "+" exp
+	;
+
+  will trigger a warning about $$ and $5 in the first rule, and $3 in
+  the second ($1 is copied to $$ by the default rule).  This example
+  most likely contains three errors, and should be rewritten as:
+
+     exp: exp "?" exp ":" exp { $$ = $1 ? $3 : $5; }
+	| exp "+" exp         { $$ = $1 + $3; }
+	;
+
+  However, if the original actions were really intended, the warnings
+  can be suppressed by letting Bison believe the values are used, e.g.:
+
+     exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
+	| exp "+" exp         { $$ = $1; (void) $3; }
+	;
+
+  The warning is intended to help catching lost values and memory leaks.
+  If a value is ignored, its associated memory typically is not reclaimed.
+
 * %destructor vs. YYABORT, YYACCEPT, and YYERROR.
   Destructors are now called when user code invokes YYABORT, YYACCEPT,
   and YYERROR, for all objects on the stack, other than objects