std::is_sorted return false when lambda return always true

Please explain why this code is crashing?

#include <vector>
#include <cassert>
#include <algorithm>

int main() {
    std::vector<int> vt = {1,2,3,4};
    assert(std::is_sorted(vt.begin(), vt.end(), [](const auto& a, const auto& b){ return true;}));
}

https://godbolt.org/

>Solution :

The comparison function (the lambda expression) is incorrect. It returns always true for vt[i] < vt[j] and for vt[j] < vt[i]. As a result the call of the algorithm has undefined behavior.

From the C++ Standard:

For algorithms other than those described in 25.4.3 to work correctly,
comp has to induce a strict weak ordering on the values.

For example the algorithm can be implemented the following way

template<class ForwardIterator, class Compare>
bool is_sorted( ForwardIterator first, 
                ForwardIterator last,
                Compare comp )
{
    auto prev = first;

    while ( first != last && ++first != last && !comp( *first, *prev ) )
    {
        ++prev;
    }

    return first == last;
} 

As you can see the expression !comp( *first, *prev ) will at once return false for a sorted sequence.