yacc.c: make sure we properly propagated the user's number for error

* data/skeletons/yacc.c (YYERRCODE): Be truthful.
* tests/input.at (Redefining the error token): Check that.

TODO

@@ -315,13 +315,8 @@ It would be a very nice source of inspiration for the other languages.
 Valentin Tolmer is working on this.
 
 ** YYERRCODE
-Defined to 256, but not used, not documented.  Probably the token
-number for the error token, which POSIX wants to be 256, but which
-Bison might renumber if the user used number 256.  Keep fix and doc?
-Throw away?
-
-Also, why don't we output the token name of the error token in the
-output?  It is explicitly skipped:
+Why don't we output the token name of the error token in the output?  It is
+explicitly skipped:
 
       /* Skip error token and tokens without identifier.  */
       if (sym != errtoken && id)

data/skeletons/yacc.c

@@ -673,7 +673,7 @@ static const ]b4_int_type_for([b4_toknum])[ yytoknum[] =
 
 /* Error token number */
 #define YYTERROR        1
-#define YYERRCODE       256
+#define YYERRCODE       ]b4_symbol(1, user_number)[
 
 ]b4_locations_if([[
 ]b4_yylloc_default_define[

tests/input.at

@@ -326,6 +326,21 @@ input.y:2.14-16: error: redefining user token number of error
       |              ^~~
 ]])
 
+# While at it, make sure we properly used the user's number for
+# "error".
+AT_DATA([input.y],
+[[%token error 123
+%%
+exp:
+]])
+
+AT_BISON_CHECK([input.y])
+
+AT_CHECK([$EGREP -E '123|256' input.tab.c], [],
+[[       0,   123,   257
+#define YYERRCODE       123
+]])
+
 AT_CLEANUP