[Issue 24193] New: A size of unions with struct and bitfields is incompatible with C

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24193

          Issue ID: 24193
           Summary: A size of unions with struct and bitfields is
                    incompatible with C
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: ttanjo at gmail.com

Here is a C code that declares a union with a struct and bitfields:

```c
#include <stdio.h>

typedef union {
    struct {
        int type:8;
        int magic32;
    } data;

    int type:8;
} U;

int main(void)
{
        printf("%ld\n", sizeof(U));
        return 0;
}
```

```console
$ gcc --version
gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ gcc sample.c -o sample
$ ./sample
8
```

I expect that the size of the following equivalent D union `U` is same as the
result of C (i.e., 8).

```d
union U {
    struct S {
        int type:8;
        int magic32;
    }
    S data;

    int type:8;
}

pragma(msg, U.sizeof);

void main() {}
```

I checked the size with the following command:
```console
$ dmd --version
DMD64 D Compiler v2.105.2
Copyright (C) 1999-2023 by The D Language Foundation, All Rights Reserved
written by Walter Bright
$ dmd -preview=bitfields sample.d
```

Expected result:
```console
$ dmd -preview=bitfields sample.d
8LU
```

Actual result:
```console
$ dmd -preview=bitfields sample.d
4LU
```

It causes problems when using C libraries in D including with importC.

Related: https://github.com/ldc-developers/ldc/issues/4477

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