Visual Studio Code – Why can this C++ child class be constructed by objects of parent class type

class AAA
{
    int m_Int;
public:
    AAA() : m_Int{12} {}
};

class BBB
{
    int m_Int1;
public:
    BBB() : m_Int1{12} {}
};

class CCC : public AAA, public BBB {};
AAA a;
BBB b;

CCC c{ a, b };

Why can object c be constructed by parent class object?

I tried to find out which standard support this syntax. I wrote the code with Visual Studio, and I found C++ 14 does not support this, but C++17 does. I also found that the construct process of c call AAA and BBB‘s copy constructor.

I want to know what the syntax is and where to find the item.