It's not because of aliasing, it's because of the push_back call inside the inner loop, which may potentially have to allocate (in this case we know it doesn't,

It's not because of aliasing, it's because of the push_back call inside the inner loop, which may potentially have to allocate (in this case we know it doesn't, but the compiler does not). If you replace squareVec6 with vector<int> squareVec6(const vector<int>& v) { vector<int> result(v.size()); // preallocate // result.reserve(v.size()); transform(begin(v), end(v), begin(result), [](int i) { return i*i; }); return result; } the compiler will be happy to vectorize the loop.