Does std::vector::reserve guarantee that the implementation will not invalidate iterators in this case?

Here is a function which intends to:

a) accept a vector of int

b) for each int in the input vector, append the inverse of this int

preconditions: none

postconditions: returned vector's size() is exacly 2 * the input vector's size.

Note that the vector is modified in-place.

Question:

Is this function strictly defined behaviour vis-a-vis iterator invalidation during the transform?

Bonus:

Is there a better/more succinct/robust way to write it?

Code:

std::vector<int> append_negatives(std::vector<int> v)
{
    v.reserve(v.size() * 2);
    std::transform(begin(v), end(v), 
                   back_inserter(v), 
                   [](auto&& x) { return -x; });
    return v;
}