Current implementations of vector assignment operations, including the copy, move, initializer_list-assignment operators, as well as all overloads of the assign functions and the C++23 assign_range function, are unsafe during reallocations triggered by assignments. This unsafety can lead to a broken vector with all elements erased, as reported in issue #5106.

The issue arises because all the assignment functions ultimately execute the following code during reallocations on assignments:

_Clear_and_reserve_geometric(_Newsize);
if constexpr (_Nothrow_construct) {
    _Mylast = _Uninitialized_{copy,fill,move}_n(_Myfirst, _Newsize, _Val, _Al);
    _ASAN_VECTOR_CREATE;
} else {
    _ASAN_VECTOR_CREATE_GUARD;
    _Mylast = _Uninitialized_{copy,fill,move}_n(_Myfirst, _Newsize, _Val, _Al);
}

This sequence is unsafe because the destruction of the old vector occurs strictly before the construction of the new vector elements. If any exception is raised during the subsequent construction of new elements, the original vector is already cleared and there is no way to recover it.

This patch fixes #5106 using a copy-and-swap idiom, ensuring that the vector remains intact in the event of exceptions during vector reallocations in assignments. In other words, this patch enhances the copy/initializer_list-assignment operators, the assign overloads, and C++23 assign_range of std::vector to provide strong exception guarantees during vector reallocations. However, it should be noted that in general, these assignment operations cannot always guarantee strong exception safety.