This is a follow-up question to Why std::stable_sort does not compile when std::sort does?

The problem is the same, except the simplify is now called get_const and returns a smart pointer to the const X.

The bigger picture is that I want to have a container of X, that can be cast (without copying) to a container of const X. Currently I implemented it as a base class ContainerOfConstX that holds std::vector<std::experimental::propagate_const<std::shared_ptr<X>>> _m and two views: one _const = _m | std::views::transform(get_const); and the other as _mutable = _m | std::views::transform(simplify);. There is a Derived class ContainerOfX that serves functions based on _mutable.

All two containers expose the natural functions to their names, such as begin(), end(), front(), back(), push_back(std::shared_ptr<[const] X>), etc. with the expected semantics.

#include <ranges>
#include <vector>
#include <experimental/propagate_const>
#include <memory>
#include <algorithm>

struct X {
   X(int x) : mx(x) {}
   int mx;
};

std::shared_ptr<const X>& get_const(std::experimental::propagate_const<std::shared_ptr<X>>& i) {
   return get_underlying(i); // error: invalid initialization of reference of type ‘std::shared_ptr<const X>&’ from expression of type ‘std::shared_ptr<X>’
}

int operator<=>(const std::shared_ptr<X>& lhs, const std::shared_ptr<X>& rhs) {
   return 0;
}

int main() {
   std::vector<std::experimental::propagate_const<std::shared_ptr<X>>> m; //originally a member variable of a class that works as custom container of X.
   m.push_back(std::make_shared<X>(4));
   m.push_back(std::make_shared<X>(2));
   m.push_back(std::make_shared<X>(3));
   m.push_back(std::make_shared<X>(1));
   
   // Some code that works with vector of non-const Xs.

   decltype(m | std::views::transform(get_const)) r = m | std::views::transform(get_const);

   // Some code that works with vector of const X. In particular:

   std::sort(r.begin(), r.end()); 
   std::stable_sort(r.begin(), r.end());
}

Maybe the issue is with the std::experimental::propagate_const? I would very much prefer not to use it at all, because in the end I need to target msvc as well.

Please note, that making the argument to get_const const gets the code compile (not on clang, though), with the warning message: warning: returning reference to temporary: return get_underlying(i)

Godbolt link