typesafe_cb: show flaw in typesafe_cb with undefined structs.

ccan/typesafe_cb/test/compile_ok-typesafe_cb-undefined.c

+#include <ccan/typesafe_cb/typesafe_cb.h>
+#include <stdlib.h>
+
+/* const args in callbacks should be OK. */
+
+static void _register_callback(void (*cb)(void *arg), void *arg)
+{
+}
+
+#define register_callback(cb, arg)				\
+	_register_callback(typesafe_cb(void, (cb), (arg)), (arg))
+
+static void _register_callback_pre(void (*cb)(int x, void *arg), void *arg)
+{
+}
+
+#define register_callback_pre(cb, arg)					\
+	_register_callback_pre(typesafe_cb_preargs(void, (cb), (arg), int), (arg))
+
+static void _register_callback_post(void (*cb)(void *arg, int x), void *arg)
+{
+}
+
+#define register_callback_post(cb, arg)					\
+	_register_callback_post(typesafe_cb_postargs(void, (cb), (arg), int), (arg))
+
+struct undefined;
+
+static void my_callback(const struct undefined *undef)
+{
+}
+
+static void my_callback_pre(int x, const struct undefined *undef)
+{
+}
+
+static void my_callback_post(const struct undefined *undef, int x)
+{
+}
+
+int main(int argc, char *argv[])
+{
+	struct undefined *handle = NULL;
+
+	register_callback(my_callback, handle);
+	register_callback_pre(my_callback_pre, handle);
+	register_callback_post(my_callback_post, handle);
+	return 0;
+}