E4225

Compiler diagnostic name: constr_any_pattern_not_suberror.

The wildcard constructor pattern Type::_ does not name a usable suberror type.

This pattern stands for all visible constructors of one suberror type. The named type must therefore be a suberror with at least one constructor, and its constructors must be visible at the pattern's use site. It cannot be an ordinary enum, struct, built-in type, empty suberror, or opaque suberror whose constructors are unavailable.

Erroneous example

///|
enum Status {
  Ready
  Busy
}

///|
pub fn is_status(error : Error) -> Bool {
  match error {
    Status::_ => true
    _ => false
  }
}

Suggestion

Name a visible, non-empty suberror type. If the value is not an error, use that type's ordinary patterns instead.

///|
priv suberror StatusError {
  NotReady
  Busy
}

///|
pub fn is_status_error(error : Error) -> Bool {
  match error {
    StatusError::_ => true
    _ => false
  }
}