Scala A More General Sum Using Tail Recursion

My solution is:

  def mySum[T: Numeric](l: List[T]): T = {
    val s = implicitly[Numeric[T]].zero

    @tailrec
    def inner[T: Numeric](l: List[T], s: T): T = l match {
      case Nil => s
      case h :: t => inner(t, implicitly[Numeric[T]].plus(s, h))
    }

    inner(l, s)
  }

This works perfect and accepts any Numeric type for sum.

However, the following code can't compile, can you figure out why? Hint: Desugar the code first.

  @tailrec
  def mySum[T: Numeric](l: List[T], s: T = implicitly[Numeric[T]].zero): T = l match {
    case Nil => s
    case h :: t => mySum(t, implicitly[Numeric[T]].plus(s, h))
  }

Notes: This is only for practice, the Scala version of sum which use foldLeft is safe to use.

yingy4/Scala A More General Sum Using Tail Recursion.md

Scala A More General Sum Using Tail Recursion