decltype as a return type in class member function

Question

I got error compiling below code.

struct B{
    double operator()(){
        return 1.0;
    }
};

struct A {
    auto func() -> decltype(b())
    {
             


        

Answer 1

You can also get it to work like this:

struct B
{
    double operator()()
    {
        return 1.0;
    }
};

// my implementation does not have std::declval
template < typename T > T&& declval();

struct A
{
    B b;
    auto func() -> decltype(declval().operator()())
    {
        return b();
    }
};

edit: or since B is in scope already anyway no need for auto, -> decltype and declval

struct A
{
    B b;
    decltype(Q()()) func()
    {
        return b();
    }
};