There are some kinds of declarations that can appear in a derived class and hide names from a base class, but for which the syntax does not permit a [[hiding]] attribute. For example:

    struct B1 {
        int N;
        int M;
    };
    struct B2 {
        int M;
    };
    struct [[base_check]] D: B1, B2 {
        enum { N };    // hides B1::N but cannot take an attribute
        using B1::M;   // hides B2::M but cannot take an attribute
    };