[Bug c++/90858] Pointer to member is treated as non-constexpr

[m.cencora at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90858

--- Comment #2 from m.cencora at gmail dot com ---
This bug exists since the beginning of C++11 constexpr support (gcc 4.7).

Here is a reproducer for C++11:
struct Base
{
    int a;

    template <typename F>
    static constexpr int for_all_data_members(F&& func)
    {
        return func(&Base::a); 
    }
};

struct MyStruct
{
    int a;
    int b;
    Base c;

    template <typename F>
    static constexpr int for_all_data_members(F&& func)
    {
        return func(
            &MyStruct::a
          , &MyStruct::b
#ifndef FIX_COMPILATION
          , &MyStruct::c
#endif
        ); 
    }
};

constexpr int serialize(int val)
{
    return 0;
}

template <typename Class>
struct SerializeAll
{
    const Class & object;

    template <typename ...Args>
    constexpr int operator()(Args ...args)
    {
        using LtrExpander = int[];
        return (LtrExpander{ serialize(object.*args)... }, 0);
    }
};

template <typename T>
constexpr int serialize(const T& obj)
{
    return T::for_all_data_members(SerializeAll<T>{obj});
}

constexpr bool test(const MyStruct& v)
{
    return (serialize(v), true);
}

static_assert(test(MyStruct{}), "");