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trait Ring[T <: Ring[T]] {
def +(that: T): T
def *(that: T): T
}
class A extends Ring[A] {
def +(that: A) = new A
def *(that: A) = new A
}
class Poly[C <: Ring[C]](val c: C) extends Ring[Poly[C]] {
def +(that: Poly[C]) = new Poly(this.c + that.c)
def *(that: Poly[C]) = new Poly(this.c*that.c)
}
object Test extends App {
implicit def coef2poly[CI <: Ring[CI]](c: CI): Poly[CI] = new Poly(c)
val a = new A
val x = new Poly(new A)
println(x + a) // works
println(a + x) // works
val y = new Poly(new Poly(new A))
println(x + y*x) // works
println(x*y + x) // works
println(y*x + x) // works
println(x + x*y) // failed before, first with type error, after that was fixed with "orphan poly parameter CI".
}
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