<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/67367>67367</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            std::vector is changed when copy-assignment in vector::insert(pos, const_value) throws
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          leni536
      </td>
    </tr>
</table>

<pre>
    version: clang 17.0.1
flags: `-stdlib=libc++ -std=c++20 -O2`

## Observed behavior

```cpp
#include <vector>
#include <iostream>

struct S {
    S() = default;
 S(const S&) = default;
    S(S&&) = default;
    S& operator=(const S&) {
        throw 42;
    }
    S& operator=(S&&) = default;
};

int main() {
    std::vector<S> v;
 v.reserve(4);
    v.emplace_back();
    const S s;

    std::cout << v.size() << '\n';
    try {
        v.insert(v.begin(), s);
 } catch (...) {
        std::cout << "exception\n";
    }
 std::cout << v.size() << '\n';
}
```

This program prints:
```
1
exception
2
```

https://godbolt.org/z/4hqsPr137

## Expected behavior

https://timsong-cpp.github.io/cppwp/n4868/container.requirements.general#11.1
> if an exception is thrown by an `insert()` or `emplace()` function while inserting a single element, that function has no effects.

Therefore the size of the vector shouldn't change either.

## Notes

This is caused by moving the elements in the vector before copy assigning the value to be inserted to the moved-from element at the requested position.

https://github.com/llvm/llvm-project/blob/62f5636838ac80ebe8db9c31cdf9c5566db8f0c9/libcxx/include/vector#L1763
</pre>
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