Regenerate.

[bison.git] / NEWS

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@@ -3,16 +3,47 @@ Bison News
 
 Changes in version 2.1a:
 
-* %destructor vs. YYACCEPT, YYERROR, and YYABORT.
-  When the parsing/action is cut by the user using one of these
-  special actions, the stack is freed except the right-hand side
-  symbols of the current rule.
-
-* GLR, C++ LALR(1) parsers.
-  These parser skeletons are now distributed with the same special
-  license exception that the C LALR(1) parser skeleton has had since
-  Bison 1.24.  Hence the FSF imposes no copyright restriction on the
-  output of Bison when it is generating such parsers.
+* 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; }
+       ;
+
+  If there are mid-rule actions, the warning is issued if no action
+  uses it.  The following triggers no warning: $1 and $3 are used.
+
+     exp: exp { push ($1); } '+' exp { push ($3); sum (); };
+
+  Mid-rule actions that use $$ cause the corresponding value to be
+  set, therefore the following action must use it.  The following rule
+  triggers a warning about $2.
+
+     exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
+
+  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
+  corresponding to the right-hand side of the current rule.
 
 * %expect, %expect-rr
   Incorrect numbers of expected conflicts are now actual errors,