*  @param t             The type to be checked.
     */
    Type checkClassType(DiagnosticPosition pos, Type t) {
        if (t.tag != CLASS && t.tag != ERROR)
            return typeTagError(pos,
                                diags.fragment("type.req.class"),
                                (t.tag == TYPEVAR)
                                ? diags.fragment("type.parameter", t)
                                : t);
        else
            return t;
    }