E4060

Method type mismatch.

We require the type of implementation to have the exactly the same type as the one defined in the trait. Notably, the following types are not considered the same.

• Labelled arguments with different order: (a~ : Int, b~ : Int) -> Unit and (b~ : Int, a~ : Int) -> Unit.

• Optional arguments: (a : Int, b? : Int) -> Unit and (a : Int) -> Unit.

错误示例

trait A {
  f(Self) -> Int
}

type T Int

impl A for T with f(self : T) -> Int {
  //              ^
  // Error: Method f of trait A is expected to have type (T) -> Int,
  //        it cannot be implemented with type (T) -> Unit
  ignore(self)
}

建议

Make sure the method type matches the trait method type.

impl A for T with f(self : T) -> Int {
  ignore(self)
}