std::monostate is a "unit type"[1]; there's only one value with with type monostate (the value is std::monostate{}), so all monostate values are equivalent. In

std::monostate is a "unit type"[1]; there's only one value with with type monostate (the value is std::monostate{}), so all monostate values are equivalent. Infalible is a "empty type"[2]; there are no values with type Infalible, so a value cannot be constructed, so Optional<Infalible> is always None, never Some(infallible). Importantly, the compiler knows this and can use it to reason about the correctness of code. C++ has no empty types. Void is close, but it's sometimes used where a unit type would be used, and anyway it's not a first class type. For example, you can't use std::optional<void>. Even if it were possible to make an empty type in C++, it wouldn't give you anything, because the compiler isn't equipped to reason about them. BTW, the rust equivalent to std::optional<std::monostate> is Optional<()>. The empty tuple is Rust's idomatic unit type. [1]: https://en.wikipedia.org/wiki/Unit_type https://en.wikipedia.org/wiki/Unit_type [2]: https://en.wikipedia.org/wiki/Empty_type https://en.wikipedia.org/wiki/Empty_type