Why std::optional::value_or dont have a specialization for default ctor types?

Unfortunately this value_or doesn’t have this option and I agree with with Max Langhof’s comment that it should be a separate function value_or_default.

Nevertheless I’ve put together a little workaround for your case:

struct default_constructor_t {
  explicit default_constructor_t() = default;
};

inline constexpr default_constructor_t default_constructor{};

class Class {
...
  Class(default_constructor_t) : Class{} {}
...
};

int GetMember(std::optional<Class> Object) {
  return Object.value_or(default_constructor).GetMember();
}

In this solution, you need to add an additional implicit constructor to your class. This solution is also lazy. It will call default constructor only if optional doesn’t have a value.