Bug 24193 – Incorrect size of unions with bit fields

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

The following: ``` union U { struct S { int a,b; } S data; int type:8; } pragma(msg, U.S.sizeof); pragma(msg, U.sizeof); ``` prints: 8LU 4LU Which is, of course, wrong. Curiously, if `int a,b;` is replaced with `long x;`, it prints: 8LU 8LU

Not specific to C, it equally affects D.

@WalterBright created dlang/dmd pull request #15750 "fix Issue 24193 - Incorrect size of unions with bit fields" fixing this issue: - fix Issue 24193 - Incorrect size of unions with bit fields https://github.com/dlang/dmd/pull/15750

dlang/dmd pull request #15750 "fix Issue 24193 - Incorrect size of unions with bit fields" was merged into master: - 0f96f556335ae162673f4c0e11be0365594ba7a8 by Walter Bright: fix Issue 24193 - Incorrect size of unions with bit fields https://github.com/dlang/dmd/pull/15750