C++ explicit ctor

#include <iostream>

class A {
public:
    explicit A(int n): n(n) {
        std::cout << "A(int) ctor: " << n << std::endl;
    }
private:
    int n;
};

class B {
public:
    B(int n): n(n) {}
    operator int() {return n;} // implicit conversion operator
private:
    int n;
};

int main()
{
    A x(5);      // OK
    A y(B(15));  // why compiles?
    return 0;
}