type variance & toMap

gistfile1.scala

scala> trait A
defined trait A

scala> 

scala> class B extends A
defined class B

scala> class C extends A
defined class C

//toMap a mix of B's & C's works fine
scala> (1 to 100).map{i => i -> {
     |   if (i % 2 == 0) new B() else new C()
     | }}.toMap
res1: scala.collection.immutable.Map[Int,ScalaObject with A] = ...


//However, we have problems if we try to toMap something that is templated w/ a mix of B's & C's
scala> class D[T <: A]
defined class D

scala> (1 to 100).map{i => i -> {
     |   if (i % 2 == 0) new D[B]() else new D[C]()}
     | }.toMap
<console>:13: error: Cannot prove that (Int, D[_1]) forSome { type _1 >: B with C <: ScalaObject with A } <:< (T, U).
                if (i % 2 == 0) new D[B]() else new D[C]()}}.toMap



//But if we make the templated class covariant in T, then its happy again
scala> class E[+T <: A]
defined class E

scala> (1 to 100).map{i => i -> {
     |   if (i % 2 == 0) new E[B]() else new E[C]()}
     | }.toMap
res3: scala.collection.immutable.Map[Int,E[ScalaObject with A]] = ...