How to create a typeclass instance for any subclass of Traversable in Scala -

i've created toy example illustrate compiler error don't understand. shouldn't implicit conversion c[_] <: traversable[t] safe[t] safe[c[t]] apply?

import scala.language.{implicitconversions, higherkinds}  class toyexample {    implicit val stringsafe = new safe[string] {     override def stringify(value: string): string = value   }    def main(args: array[string]): unit = {     val a: string = "c"     val b: seq[string] = seq("1", "2", "3")     safe(a)     safe(b)  // why compiler error?   }    def safe[t](value: t)(implicit safe: safe[t]): string = safe stringify value }  object safe {    implicit def safetraversable[c[_] <: traversable[t], t](implicit safe: safe[t]): safe[c[t]] =     new safe[c[t]] {       override def stringify(value: c[t]): string = value.map(safe.stringify).tostring()     } }  trait safe[t] {    def stringify(value: t): string } 

ok, figured out how code compile, don't understand reason.

by modifying implicit conversion remove higher-kinded placeholder, able compile:

implicit def safetraversable[c <: traversable[t], t](implicit safe: safe[t]): safe[c] =     new safe[c] {       override def stringify(value: c): string = value.map(safe.stringify).tostring()     } 

does understand why higher kinded type invalid?