| Issue |
202387
|
| Summary |
Incorrect __builtin_offsetof behavior of enum constants used as index
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
olson-dan
|
This simple, reduced program fails:
```cpp
#include <stddef.h>
#include <stdint.h>
enum E : uint8_t { X = 127, Y = 229 };
struct S
{
int arr[256];
};
static_assert(
offsetof(S, arr[X]) ==
offsetof(S, arr[127]));
static_assert(
offsetof(S, arr[Y]) ==
offsetof(S, arr[229]));
```
godbolt here: https://gcc.godbolt.org/z/P4ea35TKW against ("clang trunk") which has this version output:
```
clang version 23.0.0git (https://github.com/llvm/llvm-project.git 47ef7495ad781b742a0c4435ca72590d297ba8bf)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /cefs/9c/9c7999660da1c2af18e75609_clang-trunk-20260608/bin
Internal compiler ID: clang_trunk
```
Internally we saw this on a version 19 clang.
The compiler's output (from goldbolt) is
```
<source>:15:5: error: static assertion failed due to requirement '__builtin_offsetof(S, arr[Y]) == __builtin_offsetof(S, arr[229])'
15 | offsetof(S, arr[Y]) ==
| ^~~~~~~~~~~~~~~~~~~~~~
16 | offsetof(S, arr[229]));
| ~~~~~~~~~~~~~~~~~~~~~
/cefs/9c/9c7999660da1c2af18e75609_clang-trunk-20260608/lib/clang/23/include/__stddef_offsetof.h:16:24: note: expanded from macro 'offsetof'
16 | #define offsetof(t, d) __builtin_offsetof(t, d)
| ^
<source>:15:25: note: _expression_ evaluates to '18446744073709551508 == 916'
15 | offsetof(S, arr[Y]) ==
| ~~~~~~~~~~~~~~~~~~~~^~
16 | offsetof(S, arr[229]));
| ~~~~~~~~~~~~~~~~~~~~~
1 error generated.
Compiler returned: 1
```
it is treating the index as if it were an `int8_t` rather than a `uint8_t`.
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