[SOLVED] The concept code works for operator "<" but doesn't work for operator ">"

Issue

So we were getting introduced to "concepts" in C++20 in our Advanced Programming class where we were trying the compiling the following code. We try to define concepts to restrict use of templates for types that satisfy them. The code compiles fine using gcc 10.0 with -std=c++20.

#include <iostream>
#include <vector>

template<typename T>
concept Cute = requires(T m){
    m.cuteness();
};

template<typename T>
concept Sortable = requires(T m, T n)
{
    m.operator<(n);
};

template<typename T>
requires Cute<T> && Sortable<T>
void animal_sort(std::vector<T>& arr)
{
    std::sort(arr.begin(), arr.end());
    std::cout << "{ ";
    for(const auto& elem : arr)
    {
        std::cout << elem << " ";
    }
    std::cout << "}\n";
}

class Penguin
{
private:
    int _height{};
    int _weight{};
    int _cuteness{};

public:
    Penguin() = default;
    Penguin( int height, int weight, int cuteness) 
    : _height(height), _weight(weight), _cuteness(cuteness)  
    {}

    int cuteness()
    {
        return _cuteness;
    }

    bool operator<(const Penguin& from)
    {
        return _cuteness < from._cuteness;
    }

    friend std::ostream& operator<<(std::ostream& os, const Penguin& penguin)
    {
        std::cout << "{ " <<  penguin._cuteness << " }";
        return os;
    }

};

int main()
{
    std::vector<Penguin> penguins;
    penguins.push_back(Penguin(10,10,5)); 
    penguins.push_back(Penguin(7,13,1));
    penguins.push_back(Penguin(9,15,3));

    animal_sort<Penguin>(penguins);
} 

However when I do a slight change and use operator ">" in the concept code:

#include <iostream>
#include <vector>

template<typename T>
concept Cute = requires(T m){
    m.cuteness();
};

template<typename T>
concept Sortable = requires(T m, T n)
{
    m.operator>(n);
};

template<typename T>
requires Cute<T> && Sortable<T>
void animal_sort(std::vector<T>& arr)
{
    std::sort(arr.begin(), arr.end());
    std::cout << "{ ";
    for(const auto& elem : arr)
    {
        std::cout << elem << " ";
    }
    std::cout << "}\n";
}

class Penguin
{
private:
    int _height{};
    int _weight{};
    int _cuteness{};

public:
    Penguin() = default;
    Penguin( int height, int weight, int cuteness) 
    : _height(height), _weight(weight), _cuteness(cuteness)  
    {}

    int cuteness()
    {
        return _cuteness;
    }

    bool operator>(const Penguin& from)
    {
        return _cuteness > from._cuteness;
    }

    friend std::ostream& operator<<(std::ostream& os, const Penguin& penguin)
    {
        std::cout << "{ " <<  penguin._cuteness << " }";
        return os;
    }

};

int main()
{
    std::vector<Penguin> penguins;
    penguins.push_back(Penguin(10,10,5)); 
    penguins.push_back(Penguin(7,13,1));
    penguins.push_back(Penguin(9,15,3));

    animal_sort<Penguin>(penguins);
}

I get the following compilation error:

n file included from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algobase.h:71,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/char_traits.h:39,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ios:40,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ostream:38,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/iostream:39,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h: In instantiation of 'constexpr bool __gnu_cxx::__ops::_Iter_less_iter::operator()(_Iterator1, _Iterator2) const [with _Iterator1 = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Iterator2 = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >]':
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1826:14:   required from 'constexpr void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1866:25:   required from 'constexpr void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1957:31:   required from 'constexpr void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:4842:18:   required from 'constexpr void std::sort(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >]'
<source>:19:14:   required from 'void animal_sort(std::vector<T>&) [with T = Penguin]'
<source>:66:25:   required from here
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h:45:23: error: no match for 'operator<' (operand types are 'Penguin' and 'Penguin')
   45 |       { return *__it1 < *__it2; }
      |                ~~~~~~~^~~~~~~~
In file included from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algobase.h:67,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/char_traits.h:39,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ios:40,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ostream:38,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/iostream:39,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_iterator.h:1112:5: note: candidate: 'template<class _IteratorL, class _IteratorR, class _Container> constexpr std::__detail::__synth3way_t<_IteratorR, _IteratorL> __gnu_cxx::operator<=>(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)' (reversed)
 1112 |     operator<=>(const __normal_iterator<_IteratorL, _Container>& __lhs,
      |     ^~~~~~~~
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_iterator.h:1112:5: note:   template argument deduction/substitution failed:
In file included from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algobase.h:71,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/char_traits.h:39,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ios:40,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ostream:38,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/iostream:39,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h:45:23: note:   'Penguin' is not derived from 'const __gnu_cxx::__normal_iterator<_IteratorL, _Container>'
   45 |       { return *__it1 < *__it2; }
      |                ~~~~~~~^~~~~~~~
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h: In instantiation of 'constexpr bool __gnu_cxx::__ops::_Val_less_iter::operator()(_Value&, _Iterator) const [with _Value = Penguin; _Iterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >]':
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1806:20:   required from 'constexpr void std::__unguarded_linear_insert(_RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Val_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1834:36:   required from 'constexpr void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1866:25:   required from 'constexpr void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1957:31:   required from 'constexpr void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:4842:18:   required from 'constexpr void std::sort(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >]'
<source>:19:14:   required from 'void animal_sort(std::vector<T>&) [with T = Penguin]'
<source>:66:25:   required from here
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h:98:22: error: no match for 'operator<' (operand types are 'Penguin' and 'Penguin')
   98 |       { return __val < *__it; }
      |                ~~~~~~^~~~~~~
In file included from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algobase.h:67,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/char_traits.h:39,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ios:40,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ostream:38,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/iostream:39,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_iterator.h:1112:5: note: candidate: 'template<class _IteratorL, class _IteratorR, class _Container> constexpr std::__detail::__synth3way_t<_IteratorR, _IteratorL> __gnu_cxx::operator<=>(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)' (reversed)
 1112 |     operator<=>(const __normal_iterator<_IteratorL, _Container>& __lhs,
      |     ^~~~~~~~
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_iterator.h:1112:5: note:   template argument deduction/substitution failed:
In file included from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algobase.h:71,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/char_traits.h:39,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ios:40,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ostream:38,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/iostream:39,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h:98:22: note:   'Penguin' is not derived from 'const __gnu_cxx::__normal_iterator<_IteratorL, _Container>'
   98 |       { return __val < *__it; }
      |                ~~~~~~^~~~~~~
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h: In instantiation of 'constexpr bool __gnu_cxx::__ops::_Iter_less_val::operator()(_Iterator, _Value&) const [with _Iterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Value = Penguin]':
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_heap.h:139:48:   required from 'constexpr void std::__push_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare&) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Distance = long int; _Tp = Penguin; _Compare = __gnu_cxx::__ops::_Iter_less_val]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_heap.h:246:23:   required from 'constexpr void std::__adjust_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Distance = long int; _Tp = Penguin; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_heap.h:355:22:   required from 'constexpr void std::__make_heap(_RandomAccessIterator, _RandomAccessIterator, _Compare&) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1646:23:   required from 'constexpr void std::__heap_select(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1917:25:   required from 'constexpr void std::__partial_sort(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1933:27:   required from 'constexpr void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Size = long int; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1954:25:   required from 'constexpr void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:4842:18:   required from 'constexpr void std::sort(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >]'
<source>:19:14:   required from 'void animal_sort(std::vector<T>&) [with T = Penguin]'
<source>:66:25:   required from here
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h:69:22: error: no match for 'operator<' (operand types are 'Penguin' and 'Penguin')
   69 |       { return *__it < __val; }
      |                ~~~~~~^~~~~~~
In file included from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algobase.h:67,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/char_traits.h:39,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ios:40,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ostream:38,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/iostream:39,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_iterator.h:1112:5: note: candidate: 'template<class _IteratorL, class _IteratorR, class _Container> constexpr std::__detail::__synth3way_t<_IteratorR, _IteratorL> __gnu_cxx::operator<=>(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)' (reversed)
 1112 |     operator<=>(const __normal_iterator<_IteratorL, _Container>& __lhs,
      |     ^~~~~~~~
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_iterator.h:1112:5: note:   template argument deduction/substitution failed:
In file included from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algobase.h:71,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/char_traits.h:39,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ios:40,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ostream:38,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/iostream:39,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h:69:22: note:   'Penguin' is not derived from 'const __gnu_cxx::__normal_iterator<_IteratorL, _Container>'
   69 |       { return *__it < __val; }
      |                ~~~~~~^~~~~~~
ASM generation compiler returned: 1
In file included from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algobase.h:71,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/char_traits.h:39,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ios:40,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ostream:38,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/iostream:39,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h: In instantiation of 'constexpr bool __gnu_cxx::__ops::_Iter_less_iter::operator()(_Iterator1, _Iterator2) const [with _Iterator1 = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Iterator2 = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >]':
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1826:14:   required from 'constexpr void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1866:25:   required from 'constexpr void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1957:31:   required from 'constexpr void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:4842:18:   required from 'constexpr void std::sort(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >]'
<source>:19:14:   required from 'void animal_sort(std::vector<T>&) [with T = Penguin]'
<source>:66:25:   required from here
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h:45:23: error: no match for 'operator<' (operand types are 'Penguin' and 'Penguin')
   45 |       { return *__it1 < *__it2; }
      |                ~~~~~~~^~~~~~~~
In file included from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algobase.h:67,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/char_traits.h:39,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ios:40,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ostream:38,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/iostream:39,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_iterator.h:1112:5: note: candidate: 'template<class _IteratorL, class _IteratorR, class _Container> constexpr std::__detail::__synth3way_t<_IteratorR, _IteratorL> __gnu_cxx::operator<=>(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)' (reversed)
 1112 |     operator<=>(const __normal_iterator<_IteratorL, _Container>& __lhs,
      |     ^~~~~~~~
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_iterator.h:1112:5: note:   template argument deduction/substitution failed:
In file included from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algobase.h:71,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/char_traits.h:39,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ios:40,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ostream:38,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/iostream:39,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h:45:23: note:   'Penguin' is not derived from 'const __gnu_cxx::__normal_iterator<_IteratorL, _Container>'
   45 |       { return *__it1 < *__it2; }
      |                ~~~~~~~^~~~~~~~
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h: In instantiation of 'constexpr bool __gnu_cxx::__ops::_Val_less_iter::operator()(_Value&, _Iterator) const [with _Value = Penguin; _Iterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >]':
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1806:20:   required from 'constexpr void std::__unguarded_linear_insert(_RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Val_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1834:36:   required from 'constexpr void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1866:25:   required from 'constexpr void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1957:31:   required from 'constexpr void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:4842:18:   required from 'constexpr void std::sort(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >]'
<source>:19:14:   required from 'void animal_sort(std::vector<T>&) [with T = Penguin]'
<source>:66:25:   required from here
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h:98:22: error: no match for 'operator<' (operand types are 'Penguin' and 'Penguin')
   98 |       { return __val < *__it; }
      |                ~~~~~~^~~~~~~
In file included from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algobase.h:67,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/char_traits.h:39,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ios:40,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ostream:38,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/iostream:39,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_iterator.h:1112:5: note: candidate: 'template<class _IteratorL, class _IteratorR, class _Container> constexpr std::__detail::__synth3way_t<_IteratorR, _IteratorL> __gnu_cxx::operator<=>(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)' (reversed)
 1112 |     operator<=>(const __normal_iterator<_IteratorL, _Container>& __lhs,
      |     ^~~~~~~~
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_iterator.h:1112:5: note:   template argument deduction/substitution failed:
In file included from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algobase.h:71,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/char_traits.h:39,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ios:40,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/ostream:38,
                 from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/iostream:39,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h:98:22: note:   'Penguin' is not derived from 'const __gnu_cxx::__normal_iterator<_IteratorL, _Container>'
   98 |       { return __val < *__it; }
      |                ~~~~~~^~~~~~~
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h: In instantiation of 'constexpr bool __gnu_cxx::__ops::_Iter_less_val::operator()(_Iterator, _Value&) const [with _Iterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Value = Penguin]':
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_heap.h:139:48:   required from 'constexpr void std::__push_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare&) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Distance = long int; _Tp = Penguin; _Compare = __gnu_cxx::__ops::_Iter_less_val]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_heap.h:246:23:   required from 'constexpr void std::__adjust_heap(_RandomAccessIterator, _Distance, _Distance, _Tp, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Distance = long int; _Tp = Penguin; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_heap.h:355:22:   required from 'constexpr void std::__make_heap(_RandomAccessIterator, _RandomAccessIterator, _Compare&) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1646:23:   required from 'constexpr void std::__heap_select(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1917:25:   required from 'constexpr void std::__partial_sort(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1933:27:   required from 'constexpr void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Size = long int; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:1954:25:   required from 'constexpr void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_algo.h:4842:18:   required from 'constexpr void std::sort(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__normal_iterator<Penguin*, std::vector<Penguin> >]'
<source>:19:14:   required from 'void animal_sort(std::vector<T>&) [with T = Penguin]'
<source>:66:25:   required from here
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/predefined_ops.h:69:22: error: no match for 'operator<' (operand types are 'Penguin' and 'Penguin')
   69 |       { return *__it < __val; }

What seems to be the problem?

Solution

The concept code is not having any problem with your < -> > change.

What has a problem with it is std::sort.

std::sort by default assumes that the elements are to be ordered by < comparison. This doesn't work since your change < -> > made it so that the class has no operator< anymore.

std::sort is complaining about this.

Essentially your new concept of Sortable is not the concept of Sortable that std::sort requires for the element type.

Answered By - user17732522

Answer Checked By - Senaida (WPSolving Volunteer)