The problem is probably that State uses the parent pointer (and thus isn't constant if indexed from an object(_HMAC)). In this case it is indexed from the class, which renders a constant, but the value is invalid for cloning the class, so it is hidden from indexing in the runtime. However there are some special cases for this in the compiler, but they probably got broken when OPT_EXTERNAL_DEPEND was changed to no longer be regarded as constant.
Fixed by using a different approach in the compiler.
/grubba