No ClassTag available despite ClassTag defined

Question

I have the following code:

trait MyTrait[B] {
  def func()
}

class A[B: ClassTag : Zero](val x: SparseVector[B]) extends MyTrait[B] {
  def this(x: Seq[(Int,B)]) = this({
    val (index, vals) = x.unzip
    vals.toArray
    new SparseVector[B](index.toArray, vals.toArray, vals.length)
  })

  def func(): Unit = {}
}

And get a error: No ClassTag available for B despite the fact that a ClassTag has been added to the generic parameter. What am I missing?

Above SparseVector is a member of the breeze linear algebra package (breeze.linalg)

The code is working fine (removing the unknown `extends MyTrait[B]`), is the error raised in the trait code? — Taig, Jan 20 '16 at 16:30
I oversimplified my example. Please see updated version above. — user1094206, Jan 20 '16 at 16:48

score 2 · Accepted Answer · answered Jan 20 '16 at 20:32

the problem is that context bounds (which are just desugared as implicit parameters), aren't applied to auxiliary constructors. You'll need to explicitly (hah) declare the parameter list:

def this(x: Seq[(Int,B)])(implicit z: Zero[B], ct: ClassTag[B]) = ...

This feels like a bug in the compiler, but I could see it argued both ways.

No ClassTag available despite ClassTag defined

1 Answers1