Are identically named members of unnamed structs in a union an error or a GCC bug?

The below code can be compiled successfully in Visual C++. I like it and it is sweet!

#include <stdio.h>

#ifdef _MSC_VER
    #pragma warning(push)
    #pragma warning(disable:4201)
    #pragma pack(push,1)
    #define PACKED
#else
    #define PACKED __attribute__ ((__packed__))
#endif

union A {
    struct {
        int a:1;
        int b:2;
        int c1:29;
    }PACKED;
    struct {
        int a:1;
        int b:2;
        int c2:28;
        int d:1;
    }PACKED;
    int val;
}PACKED;

#ifdef _MSC_VER
    #pragma pack(pop)
    #pragma warning(pop)
#endif
#undef PACKED

int main(){
    A test;
    test.val = 0x1078FFF7;
    printf("sizeof(A): %d, test.a: %d.\n", sizeof(A), test.a);
    return -1;
}

Output with the file built with MSC:

sizeof(A): 4, test.a: -1.

But in GCC, including the latest gcc-7, it failed to be compiled, :(

struct.cpp:13:15: error: redeclaration of ‘signed char:1 A::<unnamed struct>::a’
     int a:1;
           ^
struct.cpp:7:15: note: previous declaration ‘signed char:1 A::<unnamed struct>::a’
     int a:1;
           ^
struct.cpp:14:15: error: redeclaration of ‘signed char:2 A::<unnamed struct>::b’
     int b:2;
           ^
struct.cpp:8:15: note: previous declaration ‘signed char:2 A::<unnamed struct>::b’
     int b:2;
           ^

Is it a bug in GCC?

---

_{Thanks for your comments, I just understood this question may be invalid for C; But for C++ part, I still have the concern. Personally I like Visual C++ compile behavior, it can save tons of code in my scenario}

It has been discussed in P.P.'s answer and the comments, that what you want isn't correct and GCC behaves correctly. But an easy workaround which fulfills your need might be that you rename a and b in the second struct:

union A {
    struct {
        int a:1;
        int b:2;
        int c1:29;
    }PACKED;
    struct {
        int unused_name_a:1;
        int unused_name_b:2;
        int c2:28;
        int d:1;
    }PACKED;
    int val;
}PACKED;

This worked for me in GCC and Clang and should still allow your trick to work nicely.