Add protection for use of oplist within a container oplist

m-variant.h

@@ -609,7 +609,15 @@
 // deferred
 #define VARIANTI_OPLIST_P1(arg) VARIANTI_OPLIST_P2 arg
 
-#define VARIANTI_OPLIST_P2(name, ...)                                   \
+/* Validate the oplist before going further */
+#define VARIANTI_OPLIST_P2(name, ...)					\
+  M_IF(M_REDUCE(M_OPLIST_P, M_AND, __VA_ARGS__))(VARIANTI_OPLIST_P3, VARIANTI_OPLIST_FAILURE)(name, __VA_ARGS__)
+
+/* Prepare a clean compilation failure */
+#define VARIANTI_OPLIST_FAILURE(name, ...)				\
+  M_LIB_ERROR(ONE_ARGUMENT_OF_VARIANT_OPLIST_IS_NOT_AN_OPLIST, name, __VA_ARGS__)
+
+#define VARIANTI_OPLIST_P3(name, ...)                                   \
   (INIT(M_C(name,_init)),                                               \
    INIT_SET(M_C(name, _init_set)),                                      \
    SET(M_C(name,_set)),                                                 \