tcon: add test case for const members.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/tcon/tcon.h

@@ -321,6 +321,8 @@
  * tcon_container_of() evaluates to a pointer to the container
  * structure.  With sufficient compiler support, the pointer will be
  * correctly typed, and the type of @member_ptr will be verified.
+ * Note that const is discarded; a const @member_ptr still yields a
+ * non-const container (unless @canary is const).
  *
  * Returns NULL if @member_ptr is NULL.
  */

ccan/tcon/test/compile_ok-container.c

@@ -0,0 +1,35 @@
+#include <stdlib.h>
+
+#include <ccan/tcon/tcon.h>
+#include <ccan/build_assert/build_assert.h>
+#include <ccan/tap/tap.h>
+
+struct inner {
+	int inner_val;
+};
+
+struct outer {
+	int outer_val;
+	struct inner inner;
+};
+
+struct info_base {
+	char *infop;
+};
+
+struct info_tcon {
+	struct info_base base;
+	TCON(TCON_CONTAINER(concan, struct outer, inner));
+};
+
+int main(int argc, char *argv[])
+{
+	/* Const should work! */
+	const struct outer *ovar;
+	struct outer *o;
+	struct info_tcon info;
+
+	o = tcon_container_of(&info, fi, &ovar->inner);
+	return (o == ovar);
+}
+