api.value.type: diagnose guaranteed failure with --yacc

Instead of generating invalid C code, generate an error when --yacc and
'%define api.value.type union' are used together.

* data/bison.m4: Issue an error in this case.
* tests/types.at (%yacc vs. %define api.value.type union): New, check this
error.
* doc/bison.texi (Type Generation): Document it.
* tests/output.at: Check that '-o y.tab.c' and '-y' behave equally
wrt generated file names.
* NEWS (Use of YACC='bison -y'): New.
Promote the use of 'bison -o y.tab.c'.

NEWS

@@ -23,6 +23,26 @@ GNU Bison NEWS
   Missing semicolons at the end of actions are no longer added (as announced
   in the release 2.5).
 
+*** Use of YACC='bison -y'
+
+  TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
+  Bison extensions.
+
+  Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
+  Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
+  'y.tab.h' and 'y.outout') to be generated from 'foo.y'.
+
+  To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
+  implementation of Yacc, was using Bison as 'bison -y'.  While it does
+  ensure compatible output file names, it also enables warnings for
+  incompatibilities with POSIX Yacc.  In other words, 'bison -y' triggers
+  warnings for Bison extensions.
+
+  Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
+  (which also generates 'y.tab.h' and 'y.output' when needed).
+  Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
+  flags (YFLAGS, or AM_YFLAGS with Automake).
+
 ** Bug fixes
 
 *** The epilogue is no longer affected by internal #defines (glr.c)
@@ -276,7 +296,8 @@ GNU Bison NEWS
 
   The %define variable api.value.type supports several special values.  The
   value "union" means that the user provides genuine types, not union member
-  names such as "ival" and "sval" above.
+  names such as "ival" and "sval" above (WARNING: will fail if
+  -y/--yacc/%yacc is enabled).
 
     %define api.value.type "union"
     %token <int> INT "integer"

data/bison.m4

@@ -960,7 +960,12 @@ m4_define_default([b4_parse_param], [])
 m4_define_default([b4_location_initial_column], [1])
 m4_define_default([b4_location_initial_line],   [1])
 
-# Sanity checks.
+
+## --------------- ##
+## Sanity checks.  ##
+## --------------- ##
+
+# api.prefix >< %name-prefix.
 b4_percent_define_ifdef([api.prefix],
 [m4_ifdef([b4_prefix],
 [b4_complain_at(b4_percent_define_get_loc([api.prefix]),
@@ -968,9 +973,19 @@ b4_percent_define_ifdef([api.prefix],
                 [%name-prefix],
                 [%define api.prefix])])])
 
+# api.value.type >< %union.
 b4_percent_define_ifdef([api.value.type],
 [m4_ifdef([b4_union_members],
 [b4_complain_at(b4_percent_define_get_loc([api.value.type]),
                 [['%s' and '%s' cannot be used together]],
                 [%union],
                 [%define api.value.type])])])
+
+# api.value.type=union >< %yacc.
+b4_percent_define_ifdef([api.value.type],
+[m4_if(b4_percent_define_get([api.value.type]), [union],
+[b4_yacc_if(dnl
+[b4_complain_at(b4_percent_define_get_loc([api.value.type]),
+                [['%s' and '%s' cannot be used together]],
+                [%yacc],
+                [%define api.value.type "union"])])])])

doc/bison.texi

@@ -3795,6 +3795,11 @@ yylval.TOK_INT = 42;
 return TOK_INT;
 @end example
 
+This Bison extension cannot work if @code{%yacc} (or
+@option{-y}/@option{--yacc}) is enabled, as POSIX mandates that Yacc
+generate tokens as macros (e.g., @samp{#define INT 258}, or @samp{#define
+TOK_INT 258}).
+
 This feature is new, and user feedback would be most welcome.
 
 A similar feature is provided for C++ that in addition overcomes C++

tests/output.at

@@ -62,8 +62,13 @@ AT_CHECK_OUTPUT([foo.y], [], [-dv -o foo.c],
                 [foo.c foo.h foo.output])
 AT_CHECK_OUTPUT([foo.y], [], [-dv -o foo.tab.c],
                 [foo.output foo.tab.c foo.tab.h])
-AT_CHECK_OUTPUT([foo.y], [], [-dv -y],
+
-                [y.output y.tab.c y.tab.h])
+AT_CHECK_OUTPUT([foo.y], [], [-dv -g --xml -y],
+                [y.dot y.output y.tab.c y.tab.h y.xml])
+# With '-o y.tab.c', we expect 'y.output' etc. (for compatility with Yacc).
+AT_CHECK_OUTPUT([foo.y], [], [-dv -g --xml -o y.tab.c],
+                [y.dot y.output y.tab.c y.tab.h y.xml])
+
 AT_CHECK_OUTPUT([foo.y], [], [-dv -b bar],
                 [bar.output bar.tab.c bar.tab.h])
 AT_CHECK_OUTPUT([foo.y], [], [-dv -g -o foo.c],

tests/types.at

@@ -37,6 +37,26 @@ AT_BISON_CHECK([[input.y]], [[1]], [[]],
 
 AT_CLEANUP
 
+## ---------------------------------------- ##
+## %yacc vs. %define api.value.type union.  ##
+## ---------------------------------------- ##
+
+AT_SETUP([[%yacc vs. %define api.value.type union]])
+
+AT_DATA([[input.y]],
+[[%yacc
+%define api.value.type "union"
+%%
+exp: %empty;
+]])
+
+AT_BISON_CHECK([[input.y]], [[1]], [[]],
+[[input.y:2.9-22: error: '%yacc' and '%define api.value.type "union"' cannot be used together
+]])
+
+AT_CLEANUP
+
+
 ## ---------------- ##
 ## api.value.type.  ##
 ## ---------------- ##