View bounds and ambiguous implicit conversion error

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).

You can simplify it even further by removing class A, def toA and def aToB, the ambiguity remains (but I also don't see, why). — Malte Schwerhoff
It seems the second parameter list is not resolved while searching for an implicit if it contains a function. With a "normal" implicit parameter in the second parameter list the code works fine. I can't find any hint if this is a bug or a feature. — kiritsuku
@mhs Yes, removing class A makes sense as an independent post. But I kept class A so that people can correlate with the nice explanation in the How can I chain implicits in Scala? post. — dips
I guess the problem is caused by your typebounds or better the context bounds. The error message says: ... both method bToC in object $iw of type [B1](b: B1)(implicit evidence$2: B1 => B)C and methode dToC in object $iw of type [D1](d: D1)(implicit evidence$1: D1 => D)C ... The types of the method are not B and D but B1 and D1. These are context bounds and not view bounds anymore. That is what the evidence says us. Unfortunately I don't know how to fix this problem. — tgr

Kaito Kaito · Accepted Answer · 2012-07-30T14:00:29

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

View bounds and ambiguous implicit conversion error

2 Answers