问从类型'A‘的右值初始化类型'A&’的非常数引用无效

EN

#include<iostream>

class A {

    public:
        void fun() { fun2(); }
        ~A() = default;
    protected:
        virtual void fun2() = 0 ;
};

class B : public A {
    protected:
        virtual void fun2() { std::cout<<"Hiiii";} 
};

class X {
    public:
        X(A& a) {
           a.fun();
        }  
};

class Y {

    public:
        Y( int x ) : val(x), x(B())
        {}
    private:
        int val;
        X x;
 };


 int main() {

    Y y(5);
 }