trait Eq[-A] {
def eq(a: A, b: A): Boolean
}
object Eq {
implicit object IntEq extends Eq[Int] {
def eq(a: Int, b: Int) = a == b
}
}
trait Supertrait[+A]
object Supertrait {
implicit def Eq[A: Eq]: Eq[Supertrait[A]] = ???
}
trait Subtrait[+A] extends Supertrait[A]
object Subtrait {
implicit def Eq[A: Eq]: Eq[Subtrait[A]] = ???
}
def f[A](x: Subtrait[A])(implicit ev: Eq[Subtrait[A]]) = ???
f(new Subtrait[Int] {})
When compiling this code, the following error occurs:
Error:(32, 4) ambiguous implicit values:
both method Eq in object Supertrait of type [A](implicit evidence$1: Eq[A])Eq[Supertrait[A]]
and method Eq in object Subtrait of type [A](implicit evidence$2: Eq[A])Eq[Subtrait[A]]
match expected type Eq[Subtrait[Int]]
f(new Subtrait[Int] {})
^
Why doesn't the implicit def
in the Subtrait
companion object has higher precedence than the one in Supertrait
?
I'd like that implicit def
s in the companion object of subtraits have higher precedence than those in supertraits.
UPDATE
The LowPriorityImplicits
trick does not work either. See Enforcing precedence in implicit instances in Scala.