In the following case, when using assignment on an optional vector of enum values I cannot explain the behavior of the code. Why does assignment of ... = {C} compile and create a vector of size 2, while ... = {2} does not compile?

enum Type
{
    A, B, C
};


int main(int argc, char** argv)
{
    std::optional<std::vector<Type>> types;

    types = { A };
    std::cout << types->size() << std::endl; // => 0

    types = { C };
    std::cout << types->size() << std::endl; // => 2

    // Conclusion, this uses the vector size constructor as the enum can be converted to integer (sicne it is not defined as enum class).
    // So i should be able to just provde the size as an integer instead of a `Type`? but:
    // types = { 7 };
    // does not compile.
    // Then what is allowing the previous lines to use the vector size constructor?
}

And this works as expected, which gives the behavior I expected for the first example aswel.

enum class TypeClass
{
    A, B, C
};

int main(int argc, char** argv)
{
    std::optional<std::vector<TypeClass>> types;
    types = { TypeClass::A };
    types = { TypeClass::C };
}