Implicit conversion for generic type?

Question

Considering this function:

def justTrue[T, S](seq: S)(implicit ev: S <:< Seq[T]) = true
justTrue(List(1,2,3))
>> true

It works. But why can't the same signature be used as a implicit conversion?

implicit class TruthTeller[T, S](seq: S)(implicit ev: S <:< Seq[T]) {
  def justTrue = true
}
List(1,2,3).justTrue
>> error: Cannot prove that List[Int] <:< Seq[T].

Isn't implicit conversion just a function?

Answer 1

Your are completely right, this should work just as well in an implicit def/class.

This is a bug where a type parameter accidentally survives type inference from a parameter of an implicit view:

SI-7944: type variables escape into the wild

It is now fixed as of 2.11.0-M7:

Welcome to Scala version 2.11.0-M7 (OpenJDK 64-Bit Server VM, Java 1.7.0_45).
Type in expressions to have them evaluated.
Type :help for more information.

scala> :pa
// Entering paste mode (ctrl-D to finish)
implicit class TruthTeller[T, S](seq: S)(implicit ev: S <:< Seq[T]) {
  def justTrue = true
}
List(1,2,3).justTrue
// Exiting paste mode, now interpreting.

defined class TruthTeller
res0: Boolean = true

As for workarounds, there are many, you can use higher-kinded as in your answer, or for example force the inference of T from the seq parameter:

// the implicit ev isn't even needed here anymore
// but I am assuming the real use case is more complex
implicit class TruthTeller[T, S](seq: S with Seq[T])(implicit ev: S <:< Seq[T]) {
  def justTrue = true
}
// S will be inferred as List[Int], and T as Int
List(1,2,3).justTrue

Answer 2

The best solution I have found yet:

import scala.language.higherKinds
implicit class TruthTeller[T, S[T] <: Seq[T]](seq: S[T]) {
  def justTrue = true
}
List(1,2,3).justTrue
>> true

But I really want to know why the original code doesn't work.