How to write a C++ concept which checks if a type is supported by numeric_limits or not

I want to write a C++ concept which checks if a given type is supported by the numeric_limits library or not. I wrote the following code to do this:

// test.cc

#include <limits>

template<typename T>
concept UsableType = requires(T v)
{
        {std::numeric_limits<T>::min()} -> T;
        {std::numeric_limits<T>::max()} -> T;
};

template<UsableType T>
class MyClass
{
        private:
                T val;
        public:
                void set(T v)
                {
                        val = v;
                }
};

int main()
{
        MyClass<int> obj;
        obj.set(1);
        return 0;
}

However, when I try to compile this using g++ -std=c++23 test.cc I get

test.cc:6:44: error: return-type-requirement is not a type-constraint
    6 |         {std::numeric_limits<T>::min()} -> T;
      |                                            ^
test.cc:7:44: error: return-type-requirement is not a type-constraint
    7 |         {std::numeric_limits<T>::max()} -> T;
      |                                            ^

Why do I get this error when numeric_limits supports int?

>Solution :

The error is due to the fact that on the right of -> you need a concept, not a type. This should compile:

#include <limits>
#include <concepts>

template<typename T>
concept UsableType = requires(T v)
{
        {std::numeric_limits<T>::min()} -> std::same_as<T>;
        {std::numeric_limits<T>::max()} -> std::same_as<T>;
};

However, std::numeric_limits<T> (and it’s member functions min() and max()) will exist whether or not there is any meaningful specialisation and they will always return T() (see example online). Use std::numeric_limits<T>::is_specialised to verify if it actually has any meaning.

template <typename T>
concept UsableType = std::numeric_limits<T>::is_specialized; 

See it online (no error for int, error for std::string)