Type bounds <: vs <:<

type-bounds.scala

//> using scala "2.13"

sealed trait SampleTrait
case class SampleValue(v: String) extends SampleTrait
object SampleSingleton extends SampleTrait

def f[X <: SampleTrait](x: X): Unit =
  println(x)

def g[X](x: X)(implicit ev: X <:< SampleTrait): Unit =
  println(x)

f(SampleSingleton)
f(SampleValue("hello"))

g(SampleSingleton)
g(SampleValue("hello"))

// case class C(v: String)
// f(C("hello")) // Doesn't compile
// g(C("hello")) // Doesn't compile

trait Algebra[A] {
  def op(a: A, b: A): A
}

implicit val algebraForA: Algebra[SampleTrait] = {
  case (SampleValue(v1), SampleValue(v2)) => SampleValue(v1 + " " + v2)
  case _                                  => SampleSingleton
}

implicit class Test[A](private val a: A) {
  def worksForAnyType(): Unit = println("worksForAnyType")

  def worksForSampleTraitSubtype(b: A)(implicit ev: A <:< SampleTrait): Unit = println(
    s"worksForSampleTraitSubtype ${implicitly[Algebra[SampleTrait]].op(a, b)}"
  )

}

10.worksForAnyType()
// 10.worksForSampleTraitSubtype(10) this doesn't compile
SampleValue("hello").worksForAnyType()
SampleValue("hello").worksForSampleTraitSubtype(SampleValue("world"))