Explicitly defaulted constructors and initialisation of member variables

I'm slightly confused about what happens when a ctor is explicitly defaulted.

Are the two code samples below equivalent?

Are there any constraints on Y to be able to use the first option?

class X
{
public:
    X() = default;

private:
    Y m_y;
};

class X
{
public:
    X() : m_y() {}

private:
    Y m_y;
};