Why does this virtual destructor trigger an unresolved external?

Consider the following:

In X.h:

class X
{
    X();
    virtual ~X();
};

X.cpp:

#include "X.h"

X::X()
{}

Try to build this (I'm using a .dll target to avoid an error on the missing main, and I'm using Visual Studio 2010):

Error 1 error LNK2001: unresolved external symbol "private: virtual __thiscall X::~X(void)" (??1X@@EAE@XZ)

Small modifications result in a successful build, however:

X.h:

class X
{
    inline X(); // Now inlined, and everything builds
    virtual ~X();
};

or

X.h:

class X
{
    X();
    ~X(); // No longer virtual, and everything builds
};

What causes the unresolved external in the linker when the .dtor is virtual or when the .ctor isn't inlined?

EDIT:

Or, perhaps more interestingly, why do I not get an unresolved external if I make the destructor non-virtual, or if I inline the constructor?