“使用未定义类型”错误即使我已经预先声明了类型

我正在制作一个国际象棋游戏，我有一些大致如下所示的课程。

struct MovementValidator;
struct Piece
{
    Piece(MovementValidator* movementValidator) :
        movementValidator{ movementValidator }
    {
    }

    virtual ~Piece() { delete movementValidator;  }

    enum class Type
    {
        PAWN
    };

    MovementValidator* movementValidator;
    virtual Type GetType() = 0;
};

struct PawnMovementValidator;
struct Pawn : public Piece
{
    Pawn() :
        Piece{ new PawnMovementValidator{} }
    {
    }

    Type GetType() override
    {
        return Type::PAWN;
    }
};

struct MovementValidator
{
    virtual void Validate(const Piece& piece) = 0;
};

struct PawnMovementValidator : public MovementValidator
{
    void Validate(const Piece& piece) override
    {

    }
};


int main()
{
    Piece* piece = new Pawn{};
}

我Piece{ new PawnMovementValidator{} }在 Pawns 构造函数中遇到在线错误。错误是：

Use of undefined type 'PawnMovementValidator'

为什么在使用PawnMovementValidator类型之前预先声明类型会发生这种情况？