typedef struct
{
    short s;
    int i;

    struct Ss
    {
        short s;
    };
} S;

error C2020: 's' : 'struct' member redefinition

typedef struct
{
    short s;
    union U
    {
        int uu;
    };

    struct S
    {
        short ss;
    };

    union
    {
        int i;
        double d;
    };

} A;

A a = {1, 2, 3, 4};
printf("A.s=%d A.uu=%d A.ss=%d A.i=%d n", a.s, a.uu, a.ss, a.i);

A.s=1 A.uu=2 A.ss=3 A.i=4