When people talk about the "expression problem", what they're describing is the fact that, (in your example) if you add a new method to the trait, you have to go around and implement that new method on every type that implements the trait.
This is in contrast to if you had used an enum (sum type) instead, wherein adding a new operation is easy and can be done in a single place. But then in exchange, adding new variants requires going around and updating every existing pattern match to support the new variant.
This is in contrast to if you had used an enum (sum type) instead, wherein adding a new operation is easy and can be done in a single place. But then in exchange, adding new variants requires going around and updating every existing pattern match to support the new variant.