https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109096
Bug ID: 109096
Summary: __has_unique_object_representations does not account
for unnamed bitfield
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: gcc at jonathanmueller dot dev
Target Milestone: ---
__has_unique_object_representations should return false for types that contain
an unnamed bit field of non-zero bit-width, since that introduces padding bits;
yet it doesn't:
```
#include <cassert>
#include <cstring>
struct foo
{
int member : 8;
// We explitly introduce 24 padding bits here.
int : 24;
bool operator==(const foo&) const = default;
};
// Yet this assertion passes?!
static_assert(__has_unique_object_representations(foo));
// But we clearly don't have unique object representations:
int main()
{
// Create two objects with different object representations.
foo a, b;
std::memset(&a, 0xFF, sizeof(foo));
std::memset(&b, 0x99, sizeof(foo));
// Make all members identical to give them the same value representation.
a.member = 0;
b.member = 0;
// This passes; they have the same value representation.
assert(a == b);
// But this fails; they don't have unique object representations!
assert(std::memcmp(&a, &b, sizeof(foo)) == 0);
}
```
https://godbolt.org/z/Y4GzqYE8s
