So, I have different behaviour for range and non-range versions of less functor. That’s a minimal code example to reproduce the behaviour:

#include <vector>
#include <algorithm>
#include <functional>

class Int
{
public:
   Int(int d) : m_data(d) {}

   auto operator<=>(const Int& other) const {
      return this->m_data <=> other.m_data;
   }

private:
   int m_data;
};

int main()
{
   Int x(15);
   Int y(17);

   std::invoke(std::less(), x, y);         // OK, compiles well
   std::invoke(std::ranges::less(), x, y); // ERROR, does not compile
}

If you replace Int class with a primitive type like int, everything will be fine and will work as expected.

I’ve got a comiler error at the std::invoke(std::ranges::less(), x, y); line.

Error for Microsoft Visual Studio 2022 17.9.6

    Failed to specialize function template 'unknown-type std::invoke(_Callable &&,_Ty1 &&,_Types2 &&...) noexcept(<expr>)'
        C:UsersC-401Desktoptestheap_testheap_test.cpp(24,9):
        With the following template arguments:
            C:UsersC-401Desktoptestheap_testheap_test.cpp(24,9):
            '_Callable=std::ranges::less'
            C:UsersC-401Desktoptestheap_testheap_test.cpp(24,9):
            '_Ty1=Int &'
            C:UsersC-401Desktoptestheap_testheap_test.cpp(24,9):
            '_Types2={Int &}'
        C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includetype_traits(1736,50):
        'std::_Invoker_functor::_Call': no matching overloaded function found
            C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includetype_traits(1633,27):
            could be 'unknown-type std::_Invoker_functor::_Call(_Callable &&,_Types &&...) noexcept(<expr>)'
C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.39.33519includetype_traits(1727,19):
or       'unknown-type std::invoke(_Callable &&) noexcept(<expr>)'
    C:UsersC-401Desktoptestheap_testheap_test.cpp(24,9):
    'unknown-type std::invoke(_Callable &&) noexcept(<expr>)': expects 1 arguments - 3 provided

Error for gcc (looked via cppinsights . io service)

/usr/bin/../lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/functional:110:5: note: candidate template ignored: substitution failure [with _Callable = std::ranges::less, _Args = <Int &, Int &>]: no type named 'type' in 'std::invoke_result<std::ranges::less, Int &, Int &>'
  110 |     invoke(_Callable&& __fn, _Args&&... __args)
      |     ^

Some background info

Originally I faced that problem when I noticed that I am not able to use std::ranges::make_heap(), std::ranges::push_heap() and std::ranges::pop_heap() for my std::vector<T> where T type is quite like the Int in the code above – it contains several members and type should be comparable only by of these members, so I have the same operator<=> there. I’ve dug deep the template errors I’ve got and discovered that originally error is caused by the less function, but now I’m kinda stuck to explain the cause of the error