How to Declar Member Function with both `requires` and `-> return_type`

Question

I want to write a class template, which:

• A<float> have just a member variable val;
• A<int> have a member variable val and a member function make_unsigned (return unsigned).

I want to explicitly state the return value of make_unsigned. Here is my code.

#include <concepts>

template<typename T> struct A
{
    T val;
    std::make_unsigned_t<T> make_unsigned() requires std::signed_integral<T>
    {
        return static_cast<std::make_unsigned<T>::type>(val);
    }
};


int main()
{
    A<float> val;
    return 0;
}

Also I have tried auto make_unsigned() requires std::signed_integral<T> -> std::make_unsigned_t<T> and auto make_unsigned() -> std::make_unsigned_t<T> requires std::signed_integral<T>, but all of them cannot be compiled. It complains:

~ LANG=C g++ main.cpp -std=c++20 -o main
In file included from /usr/include/c++/10.2.0/bits/move.h:57,
                 from /usr/include/c++/10.2.0/bits/stl_pair.h:59,
                 from /usr/include/c++/10.2.0/utility:70,
                 from /usr/include/c++/10.2.0/array:38,
                 from main.cpp:1:
/usr/include/c++/10.2.0/type_traits: In instantiation of ‘struct std::make_unsigned<float>’:
/usr/include/c++/10.2.0/type_traits:1965:11:   required by substitution of ‘template<class _Tp> using make_unsigned_t = typename std::make_unsigned::type [with _Tp = float]’
main.cpp:8:29:   required from ‘struct A<float>’
main.cpp:17:14:   required from here
/usr/include/c++/10.2.0/type_traits:1826:62: error: invalid use of incomplete type ‘class std::__make_unsigned_selector<float, false, false>’
 1826 |     { typedef typename __make_unsigned_selector<_Tp>::__type type; };
      |                                                              ^~~~
/usr/include/c++/10.2.0/type_traits:1733:11: note: declaration of ‘class std::__make_unsigned_selector<float, false, false>’
 1733 |     class __make_unsigned_selector;

Or:

~ LANG=C g++ main.cpp -std=c++20 -o main
main.cpp:8:59: error: base operand of ‘->’ is not a pointer
    8 |     auto make_unsigned() requires std::signed_integral<T> -> std::make_unsigned_t<T>
      |                                                           ^~
main.cpp:8:62: error: invalid use of ‘using make_unsigned_t = typename std::make_unsigned<_Tp>::type’
    8 |     auto make_unsigned() requires std::signed_integral<T> -> std::make_unsigned_t<T>
      |                                                              ^~~
cc1plus: error: expression must be enclosed in parenthese

Or:

~ LANG=C g++ main.cpp -std=c++20 -o main
In file included from /usr/include/c++/10.2.0/bits/move.h:57,
                 from /usr/include/c++/10.2.0/bits/stl_pair.h:59,
                 from /usr/include/c++/10.2.0/utility:70,
                 from /usr/include/c++/10.2.0/array:38,
                 from main.cpp:1:
/usr/include/c++/10.2.0/type_traits: In instantiation of ‘struct std::make_unsigned<float>’:
/usr/include/c++/10.2.0/type_traits:1965:11:   required by substitution of ‘template<class _Tp> using make_unsigned_t = typename std::make_unsigned::type [with _Tp = float]’
main.cpp:8:10:   required from ‘struct A<float>’
main.cpp:17:14:   required from here
/usr/include/c++/10.2.0/type_traits:1826:62: error: invalid use of incomplete type ‘class std::__make_unsigned_selector<float, false, false>’
 1826 |     { typedef typename __make_unsigned_selector<_Tp>::__type type; };
      |                                                              ^~~~
/usr/include/c++/10.2.0/type_traits:1733:11: note: declaration of ‘class std::__make_unsigned_selector<float, false, false>’
 1733 |     class __make_unsigned_selector;

How can I do it?

Answer 1

std::make_unsigned is not SFINAE friendly, so you have to

• don't be explicit on return type, use auto/decltype(auto):

  auto make_unsigned() requires std::signed_integral<T>
  {
      return static_cast<std::make_unsigned<T>::type>(val);
  }
  

• add a "friendly" trait (a trait which doesn't make the program ill-formed with invalid type), and additionally use a return type (as function is not template, no SFINAE here):

  template <typename T>
  struct make_unsigned_or_self
  {
      using type = T;    
  };
  template <typename T>
  using make_unsigned_or_self_t = typename make_unsigned_or_self<T>::type;
  
  template <std::signed_integral T>
  struct make_unsigned_or_self<T>
  {
       using type = std::make_unsigned_t<T>;
  };
  

  and then

  make_unsigned_or_self_t<T> make_unsigned() requires std::signed_integral<T>
  {
      return static_cast<std::make_unsigned<T>::type>(val);
  }
  

  Demo