static_cast<Derived>(base)

[Slazer]

Why does the following code fails to compile?

#include <iostream>
using namespace std;

class Base{
};

class Derived:public Base{
};

int main() {
    
    Base base;
    static_cast<Derived>(base);

    return 0;
}

[Peter87]

Because there is no valid conversion from Base to Derived.

For this to work you would have to declare a Derived constructor taking a Base as argument
Derived::Derived(const Base&) or declare a conversion operator Base::operator Derived().