E4213

Compiler diagnostic name: invalid_constr_self_type.

Custom constructors cannot be defined for trait object types such as &Trait.

MoonBit supports custom constructors for user-defined concrete types, including structs, enums, and newtypes. A trait object is an erased runtime view rather than a concrete type that can own a constructor, so fn &Trait::Trait(...) is rejected.

Erroneous example

///|
pub(open) trait Renderable {
  fn render(Self) -> String
}

///|
pub fn[X : Renderable] &Renderable::Renderable(value : X) -> &Renderable {
  value as &Renderable
}

The erroneous example tries to make &Renderable::Renderable a constructor for the trait object type.

Suggestion

Use a normal function to convert a value to the trait object, or define a custom constructor on a concrete wrapper type.

///|
pub(open) trait Renderable {
  fn render(Self) -> String
}

///|
pub fn[X : Renderable] as_renderable(value : X) -> &Renderable {
  value as &Renderable
}