warnings: enable -Wuseless-cast, and eliminate warnings

Prompted by Frank Heckenbach.
https://lists.gnu.org/archive/html/bug-bison/2019-11/msg00016.html.

* configure.ac (warn_cxx): Add -Wuseless-cast.
* data/skeletons/c.m4 (b4_attribute_define): Define
YY_IGNORE_USELESS_CAST_BEGIN and YY_IGNORE_USELESS_CAST_END.
* data/skeletons/glr.c (YY_FPRINTF): New, replaces YYFPRINTF, wrapped
with YY_IGNORE_USELESS_CAST_BEGIN and YY_IGNORE_USELESS_CAST_END.
(YY_DPRINTF): Likewise.
* tests/actions.at: Remove useless cast.
* tests/headers.at: Adjust.

Author: akimd

TODO

@@ -1,4 +1,16 @@
 * Bison 3.5
+** Deprecate YYPRINT
+The doc shows it too much.
+
+** glr.c: parse.assert
+There are many assertions in glr.c that are there to check the skeleton
+itself, not the user code.  So it should be under the control of
+parse.assert.
+
+** java, d: error.verbose
+The code checks dynamically for error.verbose.  It should be controlled by
+M4.
+
 ** doc
 I feel its ugly to use the GNU style to declare functions in the doc.  It
 generates tons of white space in the page, and may contribute to bad page

configure.ac

@@ -151,8 +151,6 @@ if test "$enable_gcc_warnings" = yes; then
 
   # Warnings for the test suite, and maybe for bison if GCC is modern
   # enough.
-  gl_WARN_ADD([-Wmissing-declarations], [WARN_CFLAGS_TEST])
-  gl_WARN_ADD([-Wmissing-prototypes], [WARN_CFLAGS_TEST])
   test $lv_cv_gcc_pragma_push_works = yes &&
     AS_VAR_APPEND([WARN_CFLAGS], [" $WARN_CFLAGS_TEST"])
 
@@ -179,6 +177,14 @@ if test "$enable_gcc_warnings" = yes; then
               [[if (sizeof (long) < sizeof (int)) return 1;]])])
   gl_WARN_ADD([-Wzero-as-null-pointer-constant], [WARN_CXXFLAGS],
               [AC_LANG_PROGRAM([], [nullptr])])
+  # Before GCC6, the pragmas don't work well enough to neutralize
+  # this warning.
+  gl_WARN_ADD([-Wuseless-cast], [WARN_CXXFLAGS],
+              [AC_LANG_PROGRAM([], [
+              #if defined __GNUC__ && ! defined __ICC && ! defined __clang__ && __GNUC__ < 6
+              syntax error
+              #endif
+              ])])
   gl_WARN_ADD([-Werror], [WERROR_CXXFLAGS])
   # Warnings for the test suite only.
   for i in $warn_tests;

data/skeletons/c.m4

@@ -327,11 +327,11 @@ m4_define([b4_attribute_define],
 
 #if defined __GNUC__ && ! defined __ICC && 407 <= __GNUC__ * 100 + __GNUC_MINOR__
 /* Suppress an incorrect diagnostic about yylval being uninitialized.  */
-# define YY_IGNORE_MAYBE_UNINITIALIZED_BEGIN \
-    _Pragma ("GCC diagnostic push") \
-    _Pragma ("GCC diagnostic ignored \"-Wuninitialized\"")\
+# define YY_IGNORE_MAYBE_UNINITIALIZED_BEGIN                            \
+    _Pragma ("GCC diagnostic push")                                     \
+    _Pragma ("GCC diagnostic ignored \"-Wuninitialized\"")              \
     _Pragma ("GCC diagnostic ignored \"-Wmaybe-uninitialized\"")
-# define YY_IGNORE_MAYBE_UNINITIALIZED_END \
+# define YY_IGNORE_MAYBE_UNINITIALIZED_END      \
     _Pragma ("GCC diagnostic pop")
 #else
 # define YY_INITIAL_VALUE(Value) Value
@@ -343,6 +343,18 @@ m4_define([b4_attribute_define],
 #ifndef YY_INITIAL_VALUE
 # define YY_INITIAL_VALUE(Value) /* Nothing. */
 #endif
+
+#if defined __cplusplus && defined __GNUC__ && ! defined __ICC && 6 <= __GNUC__
+# define YY_IGNORE_USELESS_CAST_BEGIN                          \
+    _Pragma ("GCC diagnostic push")                            \
+    _Pragma ("GCC diagnostic ignored \"-Wuseless-cast\"")
+# define YY_IGNORE_USELESS_CAST_END            \
+    _Pragma ("GCC diagnostic pop")
+#endif
+#ifndef YY_IGNORE_USELESS_CAST_BEGIN
+# define YY_IGNORE_USELESS_CAST_BEGIN
+# define YY_IGNORE_USELESS_CAST_END
+#endif
 ]])