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

@@ -108,6 +108,7 @@ if test "$enable_gcc_warnings" = yes; then
     -Wwrite-strings'
   warn_c='-Wbad-function-cast -Wstrict-prototypes'
   warn_cxx='-Wextra-semi -Wnoexcept -Wold-style-cast -Wundefined-func-template
+    -Wuseless-cast
     -Wweak-vtables'
   # Warnings for the test suite only.
   #

data/skeletons/c.m4

@@ -327,19 +327,30 @@ 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")
+# if defined __cplusplus
+#  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
 #else
 # define YY_INITIAL_VALUE(Value) Value
 #endif
 #ifndef YY_IGNORE_MAYBE_UNINITIALIZED_BEGIN
 # define YY_IGNORE_MAYBE_UNINITIALIZED_BEGIN
 # define YY_IGNORE_MAYBE_UNINITIALIZED_END
 #endif
+#ifndef YY_IGNORE_USELESS_CAST_BEGIN
+# define YY_IGNORE_USELESS_CAST_BEGIN
+# define YY_IGNORE_USELESS_CAST_END
+#endif
 #ifndef YY_INITIAL_VALUE
 # define YY_INITIAL_VALUE(Value) /* Nothing. */
 #endif