E4226

Compiler diagnostic name: constr_any_pattern_scrutinee_not_error.

The wildcard constructor pattern Type::_ is being matched against a value whose type is not Error.

Type::_ selects every constructor belonging to one suberror type while matching the open Error type. It is not an alternative spelling for an ordinary exhaustive match on a value already typed as that suberror.

Erroneous example

///|
priv suberror ParseError {
  InvalidSyntax
}

///|
pub fn is_parse_error(error : ParseError) -> Bool {
  match error {
    ParseError::_ => true
  }
}

Suggestion

Use Type::_ when matching an Error, and retain a final _ branch for other suberror types. When the matched value already has the concrete suberror type, list its constructors normally.

///|
priv suberror ParseError {
  InvalidSyntax
}

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