[libcxx-commits] [libcxx] [libc++][ranges][abi-break] Fix `movable_box` overwriting memory of data that lives in the tail padding (PR #71314)

[via libcxx-commits]

huixie90 wrote:

> Is an empty object allowed to access it's padding? If yes, we have to remove `[[no_unique_address]]` in all cases.

I don't know the answer, and if the following code is valid, then yes we cannot use EBO or `no_unqiue_address`

```
struct e {
    e(){
        std::memset(this, 0x00, sizeof(e));
    }
};

struct x :e {
    bool b = true;

    void foo() {
        std::destroy_at(std::addressof(static_cast<e&>(*this)));
        std::construct_at(std::addressof(static_cast<e&>(*this)));
    }
};

struct y {
    [[no_unique_address]] e e1;
    bool b= true;

    void foo() {
        std::destroy_at(std::addressof(e1));
        std::construct_at(std::addressof(e1));
    }
};

static_assert(sizeof(x) == sizeof(bool));
static_assert(sizeof(y) == sizeof(bool));

int main(){
    x x1;
    assert(x1.b);
    x1.foo();
    assert(x1.b); // boom

    y y1;
    assert(y1.b);
    y1.foo();
    assert(y1.b); // boom
}
```

https://github.com/llvm/llvm-project/pull/71314