Question

This is a much simpler version of my earlier post ambiguous implicit conversion errors

Here is a code snippet from the post How can I chain implicits in Scala?

class A(val n: Int)
class B(val m: Int, val n: Int)
class C(val m: Int, val n: Int, val o: Int) {
  def total = m + n + o
}

object T2 {
  implicit def toA(n: Int): A = new A(n)
  implicit def aToB[A1 <% A](a: A1): B = new B(a.n, a.n)
  implicit def bToC[B1 <% B](b: B1): C = new C(b.m, b.n, b.m + b.n)

  // works
  println(5.total)
  println(new A(5).total)
  println(new B(5, 5).total)
  println(new C(5, 5, 10).total)
}

Now if I add a class D and implicit def dToC as follows, I get errors at locations shown in the snippet.

class A(val n: Int)
class B(val m: Int, val n: Int)
class D(val m: Int, val n: Int) //Added by me
class C(val m: Int, val n: Int, val o: Int) {
    def total = m + n + o
}

object T2 {
  implicit def toA(n: Int): A = new A(n)
  implicit def aToB[A1 <% A](a: A1): B = new B(a.n, a.n)
  implicit def bToC[B1 <% B](b: B1): C = new C(b.m, b.n, b.m + b.n)
  implicit def dToC[D1 <% D](d: D1): C = new C(d.m, d.n, d.m + d.n)  //Added by me

  println(5.total) //Ambiguous implicit conversion error
  println(new A(5).total) //Ambiguous implicit conversion error
  println(new B(5, 5).total) //Ambiguous implicit conversion error
  println(new C(5, 5, 10).total) 
}

I don’t understand how both bToC and dToC are possible conversion functions for say println(new B(5, 5).total).

Answer 1

View bounds obviously get translated to an extra implicit function parameter. So both bToC and dToC get translated to [T](x: T)(implicit f: T => Something): C

The issue being that the compiler disregards the presence/absence of implicit arguments in the ambiguity check.

case class Foo[A](value: A)
implicit def foo[A](x: A)(implicit ev: Any =:= String) = Foo(x)
implicit def bar[A](x: A)(implicit ev: Any =:= Int) = Foo(x)
1.value //Ambiguous though none are applicable