Script:
local function test()
    return 1, 2, nil, nil
end

function DebugSpellCast(event, player, spell, skipCheck)
    print(player:GetLocation())
    print(spell:GetTargetDest())
    print(spell:GetTargetDest4())
    print(test())
end
RegisterPlayerEvent(5, DebugSpellCast)

Output:
-9976.318359375    603.54681396484    39.692855834961    5.5698990821838
-9974.3896484375
-9974.3896484375   598.3837890625     39.303981781006
1    2    nil    nil


Implementation of the functions:
    int GetTargetDest(Eluna* E, Spell* spell)
    {
#ifndef TRINITY
        if (!(spell->m_targets.m_targetMask & TARGET_FLAG_DEST_LOCATION))
            return 3;
        float x, y, z;
        spell->m_targets.getDestination(x, y, z);
#else
        if (!spell->m_targets.HasDst())
            return 3;
        float x, y, z;
        spell->m_targets.GetDstPos()->GetPosition(x, y, z);
#endif
        Eluna::Push(E->L, x);
        Eluna::Push(E->L, y);
        Eluna::Push(E->L, z);
        return 3;
    }

    int GetTargetDest4(Eluna* E, Spell* spell)
    {
#ifndef TRINITY
        if (!(spell->m_targets.m_targetMask & TARGET_FLAG_DEST_LOCATION))
            return 3;
        float x, y, z;
        spell->m_targets.getDestination(x, y, z);
#else
        if (!spell->m_targets.HasDst())
            return 3;
        float x, y, z;
        spell->m_targets.GetDstPos()->GetPosition(x, y, z);
#endif
        Eluna::Push(E->L, x);
        Eluna::Push(E->L, y);
        Eluna::Push(E->L, z);
        return 3;
    }