Why does Scala's type inferencer fail with this set of implicit arguments involving parameterized types? -

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i define method parameterized type t has behavior dependent on implicit argument can found of type box[t]. following code has method defined foo. when called foo[int] or foo[string] without issue return 1 or "two" expected.

where things weird method bar. defined returning int, instead of foo[int] have foo. hope compiler infer t must of type int. not , instead fails:

bash $ scalac code.scala  types.scala:15: error: ambiguous implicit values:  both value 1 in object main of type => main.box[int]  , value 2 in object main of type => main.box[java.lang.string]  match expected type main.box[t]  def bar: int = foo              ^ 1 error found

what causing error? replacing foo foo[int] compiles fine. simpler situation there no box[t] type compiles fine. example below , uses argle , bargle instead of foo , bar.

object main extends application {    case class box[t](value: t)    implicit val 1 = box(1)   implicit val 2 = box("two")    def foo[t](implicit x: box[t]): t = {     x.value   }    // not compile:   // def bar: int = foo    // compile   def bar: int = foo[int]    println(bar)   // prints 1    // simpler situation there no box type    implicit val 3 = 3   implicit val 4 = "four"    def argle[t](implicit x: t): t = x   def bargle: string = argle    println(bargle)   // prints "four"  }

what going on in snippet causes behavior? interaction of implicit arguments, type inference, , erasure causing problems? there way modify code such line def foo: int = bar works?

someone else have explain why type inference mechanism cannot handle case, if looking cleanup code this:

object test extends app {    case class box[t](value: t)    implicit val one: box[int] = box(1)   implicit val two: box[string] = box("two")    def foo[t : box]: t = implicitly[box[t]].value     val bar = foo[int]   }

note that:

  1. i removed type annotation bar indicating type once (just in different spot wanted)
  2. i using app instead of deprecated application
  3. using context bound in type signature of foo

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