How to initialize variables in classes that require mentions of other classes for which circular declarations would be necessary?

In the following example the forward declaration the struct Y foward declaration is not enough. If you comment out X::b, it compiles fine, since Y has a full struct declaration to work with, but X only has the forward declaration.

#include <functional>
#include <iostream>

struct Y;

struct X
{
    std::function<bool(Y&)> b{[] (auto& y_) { return y_.a; }};

    bool a{false};
};

struct Y
{
    std::function<bool(X&)> b{[] (auto& x_) { return x_.a; }};

    bool a{true};
};

int main()
{
    return 0;
}

Ideone

The following is the fix I could come up with:

#include <functional>
#include <iostream>

struct Y;

struct X
{
    X();
    std::function<bool(Y&)> b;

    bool a{false};
};

struct Y
{
    Y();
    std::function<bool(X&)> b{[] (auto& x_) { return x_.a; }};

    bool a{true};
};

X::X() : b([] (auto& y_) { return y_.a; }) {}
Y::Y() : b([] (auto& x_) { return x_.a; }) {}

int main()
{
    return 0;
}

Ideone

And while it works in this example, if the classes were polymorphic, this would require using X::X; or using Y::Y; in children of those classes respectively.

Is there a way to do this in the header files themselves?