E4078

Cannot derive trait for type.

MoonBit allows you to derive implementation of some traits for your types. However, not all types can be automatically derived. For example, you cannot derive a trait for an abstract type.

Erroneous example

type T derive(Hash) // Error: Cannot derive trait Hash for type T: target type is abstract

Suggestion

You can implement the trait manually:

type T

impl Hash for T with hash_combine(self : T, hasher: Hasher) {
  // ...
}