Компилятор заменяет самописную merge на std::merge

// algorithm_impl.cpp
 
namespace FL::Algorithm
{
    template <typename RandAccessIt>
    void mergeSort(RandAccessIt start, RandAccessIt end, 
                   std::function<bool(typename std::iterator_traits <RandAccessIt>::value_type, 
                                      typename std::iterator_traits <RandAccessIt>::value_type)> const& comparator)
    {
        if(std::distance(start, end) > 1)
        {
            RandAccessIt mid = start;
            std::advance(mid, std::distance(start, end) / 2);
 
            mergeSort(start, mid);
            mergeSort(mid, end);
            merge(start, mid, end, comparator);
        }
    }
 
    template <typename RandAccessIt>
    void merge(RandAccessIt start, RandAccessIt mid, RandAccessIt end, 
               std::function<bool(typename std::iterator_traits <RandAccessIt>::value_type, 
               typename std::iterator_traits <RandAccessIt>::value_type)> const& comparator));
 
}

// host.cpp
 
#include <vector>
#include <iostream>
#include "algorithm.h"
 
int main()
{
    std::vector vec { 1, 2, 3, -2, 0, 0, 1, -2, -3, 4 };
 
    FL::Algortihm::mergeSort(vec.begin(), vec.end(), [](int l, int r) { return l >= r; })
 
    // ...
 
    return 0;
}

    template <typename RandAccessIt>
    void merge(RandAccessIt start, RandAccessIt mid, RandAccessIt end, 
               std::function<bool(typename std::iterator_traits <RandAccessIt>::value_type, 
               typename std::iterator_traits <RandAccessIt>::value_type)> const& comparator);

::FL::Algorithm::merge(start, mid, end, comparator);

(merge)(start, mid, end, comparator);