https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108775
Bug ID: 108775
Summary: Move construction clobbers data through
[[no_unique_address]]
Product: gcc
Version: 12.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: danakj at orodu dot net
Target Milestone: ---
Clang also clobbers data in the same way:
https://github.com/llvm/llvm-project/issues/60711
(MSVC does not)
```
#include <stdint.h>
#include <iostream>
#include <string>
#include <vector>
struct S2 {
uint64_t a;
};
struct S : public S2 {
S(uint8_t b, uint64_t a) : S2(a), b(b) {}
uint8_t b;
// 3 bytes padding.
};
static_assert(sizeof(S) == 16);
union U {
U() {}
[[no_unique_address]] S s;
};
static_assert(sizeof(U) == 16);
struct D {
D(uint8_t i) : i(i) {}
[[no_unique_address]] U u;
uint8_t i;
};
static_assert(sizeof(D) == 16);
int main() {
D d(1u);
std::cout << (int)d.i << "\n";
// Regular construction into `D::u` doesn't touch `D::i`. Good.
new (&d.u.s) S(2, 3);
std::cout << (int)d.i << "\n";
S s(4, 5);
// Move construction into `D::u` overwrites `D::i`. Bad.
new (&d.u.s) S(std::move(s));
std::cout << (int)d.i << "\n";
return d.i;
}
```
Godbolt: https://godbolt.org/z/YzrM59E5a
