unknown override specifier and missing type specifier - Visual Studio 2017

Question

I've been getting a strange error in Visual Studio 2017. The error is

Error C3646 'collisionSys': unknown override specifier
Error C4430 missing type specifier - int assumed. Note: C++ does not support default-int

The strange thing about it is that when I build the solution, I get this error but when I delete the declaration of collisionSys and build, then re-declare collisionSys and build again, it succeeds. Though it happens again when I make some changes in CollisionSystem.hpp
My code is as follows:

CollisionSystem.hpp

 #ifndef COLLISION_SYSTEM__HPP
    #define COLLISION_SYSTEM__HPP

    #include <iostream>
    #include <vector>
    #include <SFML/Graphics.hpp>
    #include "../EntityManager.hpp"
    #include "../Entity.hpp"
    #include "../Components/Display.hpp"
    #include "../Components/Flags.hpp"
    #include "../Components/Movement.hpp"
    #include <Level.hpp>

    class Level;

    class CollisionSystem
    {
    public:
        CollisionSystem();
        void run(Level&, CameraSystem*, EntityManager*);
    private:
        static std::vector<std::shared_ptr<Entity>> m_Ents;
    };
#endif

CollisionSystem.cpp

#include <Systems/CollisionSystem.hpp>

std::vector<std::shared_ptr<Entity>> CollisionSystem::m_Ents;

CollisionSystem::CollisionSystem()
{

}

void CollisionSystem::run(Level& level, CameraSystem* cameraSys, 
EntityManager* EntMan)
{
    // Some collision detection here
}

LocalWorld.hpp

#ifndef LOCAL_WORLD__HPP
#define LOCAL_WORLD__HPP

#include <iostream>
#include <map>
#include <conio.h>
#include "GameState.hpp"
#include "StateMgr.hpp"
#include "Game.hpp" 
#include "TileMap.hpp"
#include "Level.hpp"
#include "ECS/Entity.hpp"
#include "ECS/Components/IncludeAllComponents.hpp"
#include "ECS/ResourceManager.hpp"
#include "ECS/EntityManager.hpp"
#include "ECS/Systems/IncludeAllSystems.hpp"

// Map Names
enum class MapName
{
    Map1
};

// Forward declaration
class Level;

class LocalWorld : public GameState
{
public:
    LocalWorld();
    ~LocalWorld();

    virtual void Pause();
    virtual void Resume();

    virtual void HandleEvents(sf::Event& eType);
    virtual void Update();
    virtual void Draw();
private:
    std::map<MapName, Level*> Maps;
    Level* activeLevel;

    ResourceManager ResMan;
    EntityManager EntMan;

    CameraSystem cameraSys;
    MovementSystem movementSys;
    ControlSystem controlSys;
    RenderSystem renderSys;
    CollisionSystem collisionSys;
};

#endif

The error is on line CollisionSystem collisionSys; in LocalWorld.hpp

IncludeAllSystems.hpp

#ifndef INCLUDE_ALL_SYSTEMS__HPP
#define INCLUDE_ALL_SYSTEMS__HPP

#include "CameraSystem.hpp"
#include "ControlSystem.hpp"
#include "MovementSystem.hpp"
#include "RenderSystem.hpp"
#include "CollisionSystem.hpp"

#endif

Here, this file includes all the Systems with CollisionSystem.hpp I looked at other questions with the same error but still can't fix it.

EDIT:

The error is fixed by moving #include <Level.hpp> from CollisionSystem.hpp to CollisionSystem.cpp. Can anyone explain why is that so?

on what lines are the errors? did you enable c++11? I dont see any `override` btw — 463035818_is_not_an_ai, Jan 31 '18 at 17:46
Seems you might use more forward declaration... Maybe a circular dependency ? — Jarod42, Jan 31 '18 at 17:47
@tobi303 The error is on line CollisionSystem collisionSys; in LocalWorld.hpp — Se7eN, Jan 31 '18 at 17:47
@Jarod42 I tried declaring CollisionSystem in LocalWorld.hpp but it says undefined class — Se7eN, Jan 31 '18 at 17:48
@Jarod42 for simplicity, I didn't post the code in the run function of CollisionSystem, so other headers are used in this function — Se7eN, Jan 31 '18 at 18:09
@Ashar7: all those headers should go in cpp not in header. That's my point. — Jarod42, Jan 31 '18 at 18:13
@tobi303 that's MSVC's way of barfing when it encounters an undeclared identifier. Well, one of them anyway. — Quentin, Jan 31 '18 at 18:28
Using PH is a technique to avoid compiling the same header time and time. No changes in the header, no recompilation. But sometimes, changes in your code (about including some header) make the compiler to mess up. The solution is to delete the precompiled headers and re-compile the whole project again. — Ripi2, Feb 01 '18 at 12:41
@Ripi2 I added CollisionSystem.hpp in include ditectories, does that treat it as a PH? — Se7eN, Feb 01 '18 at 13:15
@Ripi2 Well, I don't need PH as I'm always editing those files. — Se7eN, Feb 01 '18 at 16:19
@Ripi2 I created a new project and put everything there, but still the error isn't solved — Se7eN, Feb 01 '18 at 16:19
Does `GameState` class has a member called `collisionSys`? If so, you are trying to redefine it without being virtual. — Ripi2, Feb 01 '18 at 16:34
Oh and the error is fixed now by moving `#include ` to `CollisionSystem.cpp` from `CollisionSystem.hpp`. Thanks to @Jarod42 — Se7eN, Feb 01 '18 at 17:08