typedef struct {
  uint8_t fix;
  union {
      uint8_t param;
      void *ptr;
   };
}flex_struct_t;

flex_struct_t rec;

PROGMEM flex_struct_t fixed = {.fix=0, .param = 123};

PROGMEM flex_struct_t fixed = {0,  123};

PROGMEM flex_struct_t fixed = {.fix=0, {.param = 123}};

typedef struct {
    prog_char    *text;
    uint8_t        type;
    union{
        struct {
            union{
                uint8_t id;
                int        *num;
            };
            func_ptr    func;
        };
        struct {
            uint8_t        vcnt;
            uint8_t        *vid;
            prog_char    **var;
        };
        _tm    submenu;
    };
} menu_item_t;

#define simple_item(t,i,f)    {.text = t, .type = i_simple, {{{.id=i}, .func = f}}}
#define int_item(t,n,f)        {.text = t, .type = i_int, {{{.num = n}, .func = f}}}
#define var_item(t,c,vi,v)    {.text = t, .type = i_var,{ {.vcnt=0}} }
#define sub_item(t,s)        {.text = t, .type = i_sub, {.submenu = s}}

PROGMEM menu_t mainmenu = {
        .itemcount = 5,
        .items = {
                simple_item(m1,1,NULL),
                int_item(m2,&varvar,NULL),
                var_item(m3,3,&vi,&vars),
                simple_item(m4,1,NULL),
                simple_item(m5,2,NULL)
        }
};

union{
                uint8_t id;
                int        *num;
            };

union{
        struct {
            union{
                uint8_t id;
                int        *num;
            };
            func_ptr    func;
        };
        struct {
            uint8_t        vcnt;
            uint8_t        *vid;
            prog_char    **var;
        };

typedef struct {
    uint8_t itemcount;
    menu_item_t items[];
} menu_t;

typedef union {
    char c;
    double d;
} u_t;

u_t foo = { 5.0 };

u_t foo = { .c = (char)5.0 };

typedef struct {
    prog_char    *text;
    uint8_t        type;
    union{
        _tm    submenu;
        struct {
            union{
                uint8_t id;
                int        *num;
            };
            func_ptr    func;
        };
        struct {
            uint8_t        vcnt;
            uint8_t        *vid;
            prog_char    **var;
        };
    };
} menu_item_t;

struct ItemHeader
{
    prog_char    *text;
              uint8_t        type;
    void *content;
};

struct Type1Item
{
      union
    {
                   uint8_t id;
                   int *num;
    };
    void (*func);
};


struct Type2Item
{
    uint8_t        vcnt;
    uint8_t        *vid;
    prog_char    **var;
};

PROGMEM Type1Item Item1 =
{
    100, NULL
};

PROGMEM Type2Item Item2 =
{
    50, NULL, NULL
};

PROGMEM char m1[] = {"Hello"};
PROGMEM char m2[] = {"Hello2"};

PROGMEM ItemHeader mainmenu[] =
{
    {m1, 0, &Item1},
    {m2, 1, &Item2},
};

typedef union {
    char c;
    double d;
} u_t;

u_t foo = (u_t)5.0;

typedef struct {
    prog_char    *text;
    uint8_t        type;
    union outer {  /* -fms-extensions */
        struct simplei { /* -fms-extensions */
            union{
                uint8_t id;
                int        *num;
            };
            func_ptr    func;
        };
        struct vari { /* -fms-extensions */
            uint8_t        vcnt;
            uint8_t        *vid;
            prog_char    **var;
        };
        _tm    submenu;
    };
} menu_item_t;

typedef struct {
    uint8_t itemcount;
    menu_item_t items[];
} menu_t;

enum { i_simple, i_int, i_var, i_sub };

#define simple_item(t,i,f)    {.text = t, .type = i_simple, (union outer)(struct simplei){{.id=i}, .func = f} }
#define int_item(t,n,f)        {.text = t, .type = i_int, (union outer)(struct simplei){{.num = n}, .func = f} }
#define var_item(t,c,vi,v)    {.text = t, .type = i_var, (union outer)(struct vari){.vcnt=0} }
#define sub_item(t,s)        {.text = t, .type = i_sub, {.submenu = s}}

int i1 = 5;
int i2 = i1; // error: initializer element is not constant

typedef struct {
    union{
        uint8_t id;
        int        *num;
    };
    func_ptr    func;
} simple_t;

typedef struct {
    uint8_t        vcnt;
    uint8_t        *vid;
    prog_char    **var;
} var_t;

typedef struct {
    prog_char    *text;
    uint8_t        type;
    union {
        simple_t;  // это неименованные для работы
        var_t;
        _tm    submenu;
        // вспомогательные для инициализации
        simple_t simple_helper; // первая не нужна, но для однообразия пусть будет
        var_t var_helper;
    };
} menu_item_t;