It is possible for an implicit view to to itself require an implicit parameter. For example:
scala> implicit def listToStringList[A](as: List[A])(implicit f: A => String) = as map f listToStringList: [A](as: List[A])(implicit f: (A) => String)List[String] scala> implicit def i2s(i: Int) = i.toString i2s: (i: Int)java.lang.String scala> val l = List(1) l: List[Int] = List(1) scala> l: List[String] res0: List[String] = List(1)
What happens in your case? Well, let's look behind the curtains with scala -Ytyper-debug -Xlog-implicits with this script:
implicit def listToList[A, B](as: List[A])(implicit f: A => B): List[B] = as map f implicit def i2s(i: Int): String = i.toString val l = List(1) l: List[String]
The compiler then explains:
typing (l: List[String]), pt = ?, undetparams = List(), implicits-enabled = true, silent = true typing List[String], pt = ?, undetparams = List(), implicits-enabled = true, silent = true typing scala.package, pt = ?, undetparams = List(), implicits-enabled = true, silent = true typing scala, pt = ?, undetparams = List(), implicits-enabled = true, silent = true typed scala:type with underlying package scala, undetparams = List(), pt = ? adapted scala:package scala to ?, List() typed scala.package:type with underlying object package, undetparams = List(), pt = ? adapted scala.package:object package to ?, List() typing String, pt = ?, undetparams = List(), implicits-enabled = true, silent = true typed scala.this.Predef.String:String, undetparams = List(), pt = ? adapted scala.this.Predef.String:String to ?, List() typed List[String]:List[String], undetparams = List(), pt = ? adapted List[String]:List[String] to ?, List() typing l, pt = List[String], undetparams = List(), implicits-enabled = true, silent = true typed $anon.this.l:=> List[Int], undetparams = List(), pt = List[String] Beginning implicit search for $anon.this.l expecting (List[Int]) => List[String] looking for a view begin implicit search: ($anon.this.l,(List[Int]) => List[String],true,List()) typed impl for (List[Int]) => List[String]? listToList:(as: List[?])(implicit f: (?) => ?)List[?] orig info= [A,B](as: List[A])(implicit f: (A) => B)List[B]/List()/true/true/this.type/true typedImplicit0 typing$anon.this.listToList with wildpt = (List[Int]) => List[String] from implicit listToList:[A,B](as: List[A])(implicit f: (A) => B)List[B] typing $anon.this.listToList, pt = ?, undetparams = List(), implicits-enabled = false, silent = false typing $anon.this, pt = ?, undetparams = List(), implicits-enabled = false, silent = false typed $anon.this:this.type with underlying java.lang.Object{}, undetparams = List(), pt = ? adapted $anon.this:java.lang.Object{} to ?, List() typed $anon.this.listToList:[A,B](as: List[A])(implicit f: (A) => B)List[B], undetparams = List(), pt = ? adapted $anon.this.listToList:[A,B](as: List[A])(implicit f: (A) => B)List[B] to ?, List(type A, type B) typing <argument>, pt = List[?], undetparams = List(), implicits-enabled = false, silent = false typed <argument>:List[Int], undetparams = List(), pt = List[?] adapted <argument>:List[Int] to List[?], List() typing <argument>, pt = List[Int], undetparams = List(), implicits-enabled = false, silent = false typed <argument>:List[Int], undetparams = List(), pt = List[Int] adapted <argument>:List[Int] to List[Int], List() typed implicit $anon.this.listToList[Int, B](<argument>):(implicit f: (Int) => B)List[B], pt = (List[Int]) => List[String] adapted implicit method listToList:(as: List[Int])(implicit f: (Int) => B)List[B] to (List[Int]) => List[String] incompatible: (as: List[Int])(implicit f: (Int) => B)List[B] does not match (List[Int]) => List[String] Implicit search yielded: SearchResult(<empty>, TreeTypeSubstituter(List(),List()))
I'm not quite sure if this is a bug or feature. But maybe this output will help someone else illuminate the matter further.
