POSIX: warn if %prec's token was not defined.

Reported by Florian Krohm at
<http://lists.gnu.org/archive/html/bug-bison/2009-12/msg00005.html>.
* NEWS (2.4.2): Document.
* src/reader.c (grammar_rule_check): Implement.
(grammar_current_rule_prec_set): Add comments explaining that we
here assume a %prec identifier is a token, but we still manage
to support POSIX.
* tests/input.at (%prec's token must be defined): New test
group.
(cherry picked from commit f1ceed0774)

ChangeLog

@@ -1,3 +1,16 @@
+2009-12-30  Joel E. Denny  <jdenny@clemson.edu>
+
+	POSIX: warn if %prec's token was not defined.
+	Reported by Florian Krohm at
+	<http://lists.gnu.org/archive/html/bug-bison/2009-12/msg00005.html>.
+	* NEWS (2.4.2): Document.
+	* src/reader.c (grammar_rule_check): Implement.
+	(grammar_current_rule_prec_set): Add comments explaining that we
+	here assume a %prec identifier is a token, but we still manage
+	to support POSIX.
+	* tests/input.at (%prec's token must be defined): New test
+	group.
+
 2009-12-31  Joel E. Denny  <jdenny@clemson.edu>
 
 	* HACKING (Release Procedure): Recommend a secure automake.

NEWS

@@ -182,6 +182,16 @@ Bison News
 
 * Changes in version 2.4.2 (????-??-??):
 
+** `%prec IDENTIFIER' requires IDENTIFIER to be defined separately.
+
+  POSIX specifies that an error be reported for any identifier that does
+  not appear on the LHS of a grammar rule and that is not defined by
+  %token, %left, %right, or %nonassoc.  Bison 2.3b and later lost this
+  error report for the case when an identifier appears only after a
+  %prec directive.  It is now restored.  However, for backward
+  compatibility with recent Bison releases, it is only a warning for
+  now.  In Bison 2.5 and later, it will return to being an error.
+
 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
 
 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,

src/reader.c

@@ -320,6 +320,15 @@ grammar_rule_check (const symbol_list *r)
 	    warn_at (r->location, _("unset value: $$"));
 	}
   }
+
+  /* See comments in grammar_current_rule_prec_set for how POSIX
+     mandates this complaint.  It's only for identifiers, so skip
+     it for char literals and strings, which are always tokens.  */
+  if (r->ruleprec
+      && r->ruleprec->tag[0] != '\'' && r->ruleprec->tag[0] != '"'
+      && !r->ruleprec->declared && !r->ruleprec->prec)
+    warn_at (r->location, _("token for %%prec is not defined: %s"),
+             r->ruleprec->tag);
 }
 
 
@@ -396,6 +405,16 @@ grammar_midrule_action (void)
 void
 grammar_current_rule_prec_set (symbol *precsym, location loc)
 {
+  /* POSIX says that any identifier is a nonterminal if it does not
+     appear on the LHS of a grammar rule and is not defined by %token
+     or by one of the directives that assigns precedence to a token.  We
+     ignore this here because the only kind of identifier that POSIX
+     allows to follow a %prec is a token and because assuming it's a
+     token now can produce more logical error messages.  Nevertheless,
+     grammar_rule_check does obey what we believe is the real intent of
+     POSIX here: that an error be reported for any identifier that
+     appears after %prec but that is not defined separately as a
+     token.  */
   symbol_class_set (precsym, token_sym, loc, false);
   if (current_rule->ruleprec)
     complain_at (loc, _("only one %s allowed per rule"), "%prec");

tests/input.at

@@ -799,6 +799,26 @@ AT_BISON_CHECK([input.y], [1], [],
 AT_CLEANUP
 
 
+## ------------------------------- ##
+## %prec's token must be defined.  ##
+## ------------------------------- ##
+
+AT_SETUP([[%prec's token must be defined]])
+
+# According to POSIX, a %prec token must be defined separately.
+
+AT_DATA([[input.y]],
+[[%%
+start: %prec PREC ;
+]])
+
+AT_BISON_CHECK([[input.y]], [[0]], [],
+[[input.y:2.8-17: warning: token for %prec is not defined: PREC
+]])
+
+AT_CLEANUP
+
+
 ## -------------------------------- ##
 ## Reject unused %code qualifiers.  ##
 ## -------------------------------- ##