guile: Add as_a_scm_t_subr

Building GDB in C++ mode on Fedora 20, the gdb/guile/ code shows ~280
errors like:

 src/gdb/guile/guile.c:515:1: error: invalid conversion from ‘scm_unused_struct* (*)(SCM, SCM) {aka scm_unused_struct* (*)(scm_unused_struct*, scm_unused_struct*)}’ to ‘scm_t_subr {aka void*}’ [-fpermissive]

This commit fixes them all.

gdb/ChangeLog:
2015-09-07  Pedro Alves  <palves@redhat.com>

	* guile/guile-internal.h (as_a_scm_t_subr): New.
	* guile/guile.c (misc_guile_functions): Use it.
	* guile/scm-arch.c (arch_functions): Use it.
	* guile/scm-block.c (block_functions, gdbscm_initialize_blocks):
	Use it.
	* guile/scm-breakpoint.c (breakpoint_functions): Use it.
	* guile/scm-cmd.c (command_functions): Use it.
	* guile/scm-disasm.c (disasm_functions): Use it.
	* guile/scm-exception.c (exception_functions)
	(private_exception_functions): Use it.
	* guile/scm-frame.c (frame_functions)
	* guile/scm-gsmob.c (gsmob_functions): Use it.
	* guile/scm-iterator.c (iterator_functions): Use it.
	* guile/scm-lazy-string.c (lazy_string_functions): Use it.
	* guile/scm-math.c (math_functions): Use it.
	* guile/scm-objfile.c (objfile_functions): Use it.
	* guile/scm-param.c (parameter_functions): Use it.
	* guile/scm-ports.c (port_functions, private_port_functions): Use
	it.
	* guile/scm-pretty-print.c (pretty_printer_functions): Use it.
	* guile/scm-progspace.c (pspace_functions): Use it.
	* guile/scm-string.c (string_functions): Use it.
	* guile/scm-symbol.c (symbol_functions): Use it.
	* guile/scm-symtab.c (symtab_functions): Use it.
	* guile/scm-type.c (type_functions, gdbscm_initialize_types): Use
	it.
	* guile/scm-value.c (value_functions): Use it.

1 files changed, 44 insertions, 0 deletions

diff --git a/gdb/guile/guile-internal.h b/gdb/guile/guile-internal.h
index 509120b..017309a 100644
--- a/gdb/guile/guile-internal.h
+++ b/gdb/guile/guile-internal.h
@@ -50,6 +50,50 @@ typedef struct
 
 #define END_VARIABLES { NULL, SCM_BOOL_F, NULL }
 
+#ifdef __cplusplus
+
+/* Although scm_t_subr is meant to hold a function pointer, at least
+   in some versions of guile, it is actually a typedef to "void *".
+   That means that in C++, an explicit cast is necessary to convert
+   function pointer to scm_t_subr.  But a cast also makes it possible
+   to pass function pointers with the wrong type by mistake.  So
+   instead of adding such casts throughout, we use 'as_a_scm_t_subr'
+   to do the conversion, which (only) has overloads for function
+   pointer types that are valid.
+
+   See https://lists.gnu.org/archive/html/guile-devel/2013-03/msg00001.html.
+*/
+
+static inline scm_t_subr
+as_a_scm_t_subr (SCM (*func) (void))
+{
+  return (scm_t_subr) func;
+}
+
+static inline scm_t_subr
+as_a_scm_t_subr (SCM (*func) (SCM))
+{
+  return (scm_t_subr) func;
+}
+
+static inline scm_t_subr
+as_a_scm_t_subr (SCM (*func) (SCM, SCM))
+{
+  return (scm_t_subr) func;
+}
+
+static inline scm_t_subr
+as_a_scm_t_subr (SCM (*func) (SCM, SCM, SCM))
+{
+  return (scm_t_subr) func;
+}
+
+#else
+
+/* In C, just do an implicit conversion.  */
+#define as_a_scm_t_subr(func) func
+
+#endif
 /* Scheme functions to define during initialization.  */
 
 typedef struct