Why is a class not assignable?

Why is func_getter class provided below is not assignable?

#include <type_traits>
#include <string>

template <class T, class ReturnType>
using FuncPtr = ReturnType(T::*)() const;

template <class T, class ReturnType>
class func_getter
{
public:

    using MyFuncPtr = FuncPtr<T, ReturnType>;

    constexpr func_getter(MyFuncPtr p) : m_p(p) {}

    func_getter(const func_getter&) = default;
    func_getter(func_getter&&) = default;
    func_getter& operator = (const func_getter&) = default;
    func_getter& operator = (func_getter&&) = default;

    constexpr ReturnType operator() (const T& val) const
    {
        return (val.*m_p)();
    }

private:

    MyFuncPtr m_p;
};

struct X
{
    int a;
    std::string b;

    int GetA() const
    {
        return a;
    }

    int GetA1() const
    {
        return a;
    }
};

using GetAPtr = FuncPtr<X, int>;

static_assert(std::is_copy_constructible_v<GetAPtr>);
static_assert(std::is_copy_assignable_v<GetAPtr>);
static_assert(std::is_move_constructible_v<GetAPtr>);
static_assert(std::is_move_assignable_v<GetAPtr>);

inline constexpr auto a_getter = func_getter(&X::GetA);

static_assert(std::is_copy_constructible_v<decltype(a_getter)>);
static_assert(std::is_move_constructible_v<decltype(a_getter)>);

all the above static_asserts do not fail, but the following fail (at least with MSVC2022):

static_assert(std::is_copy_assignable_v<decltype(a_getter)>);
static_assert(std::is_move_assignable_v<decltype(a_getter)>);

why?

>Solution :

decltype(a_getter) is not just GetAPtr. a_getter is constexpr, hence const, and cannot be assigned to.

If you want to infer GetAptr from a_getter you can use std::remove_const_t or std::decay_t (https://godbolt.org/z/7qvhaqxeW).