E4167

Default implementation is not marked with = _.

Erroneous example

///|
pub(open) trait HasDefaultImpl {
  foo() -> Unit // It has default implementation but is not marked
}

///|
impl HasDefaultImpl with foo() {
  println("OK")
}

Suggestion

Add = _ to the methods that have default implementations.

///|
pub(open) trait HasDefaultImpl {
  foo() -> Unit = _
}