Method in trait not a legal implementation in inheriting class (V2)

[allanrenucci]

package collection

trait Map[K, +V] extends MapOps[K, V, Map]

trait MapOps[K, +V, +CC[_, _]] {
  def collect[K2, V2](pf: PartialFunction[(K, V), (K2, V2)]): CC[K2, V2] = ???
}

trait SortedMapOps[K, +V, +CC[X, Y] <: Map[X, Y] with SortedMapOps[X, Y, CC]]
  extends MapOps[K, V, Map]

trait StrictOptimizedMapOps[K, +V, +CC[_, _]] extends MapOps[K, V, CC] {

  override def collect[K2, V2](pf: PartialFunction[(K, V), (K2, V2)]): CC[K2, V2] = ???
}

trait StrictOptimizedSortedMapOps[K, +V, +CC[X, Y] <: Map[X, Y] with SortedMapOps[X, Y, CC]]
  extends SortedMapOps[K, V, CC] with StrictOptimizedMapOps[K, V, Map]


package immutable {
  trait Map[K, +V] extends collection.Map[K, V] with MapOps[K, V, Map]

  trait MapOps[K, +V, +CC[X, +Y] <: MapOps[X, Y, CC]] extends collection.MapOps[K, V, CC]

  trait SortedMapOps[K, +V, +CC[X, +Y] <: Map[X, Y] with SortedMapOps[X, Y, CC]]
    extends MapOps[K, V, Map] with collection.SortedMapOps[K, V, CC]

  class TreeMap[K, +V] extends Map[K, V] with SortedMapOps[K, V, TreeMap]
    with StrictOptimizedSortedMapOps[K, V, TreeMap]
}

-- Error: tests/allan/Test.scala:86:8 ------------------------------------------
86 |  class TreeMap[K, +V] extends Map[K, V] with SortedMapOps[K, V, TreeMap]
   |        ^
   |method collect in trait StrictOptimizedMapOps is not a legal implementation of `collect` in class TreeMap
   |  its type             [K2, V2](pf: PartialFunction[(K, V), (K2, V2)]): collection.Map[K2, V2]
   |  does not conform to  [K2, V2]
   |  (pf: PartialFunction[(K, V), (K2, V2)]): collection.immutable.Map[K2, V2]
one error found

[allanrenucci]

I am not sure it is a bug in Dotty. This can be fixed with:

trait StrictOptimizedSortedMapOps[K, +V, +CC[X, Y] <: Map[X, Y] with SortedMapOps[X, Y, CC]]
-  extends SortedMapOps[K, V, CC] with StrictOptimizedMapOps[K, V, Map]
+  extends SortedMapOps[K, V, CC] with StrictOptimizedMapOps[K, V, CC]

cc/ @julienrf

[odersky]

I have the impression this is a bug in the library, not in dotty. @allanrenucci can you do a PR against the library?

[Blaisorblade]

It'd be also a bug in Scalac, no?
EDIT: I confirmed that TreeMap.collect forwards to StrictOptimizedSortedMapOps.collect, which is not sound, because StrictOptimizedSortedMapOps.collect gives a collection.Map instead of a collection.immutable.Map. But Scalac doesn't notice because the forwarder is generated after erasure, when the return types are Object everywhere.

[smarter]

Opened scala/bug#11042 to track a related soundness bug in scalac.

[Blaisorblade]

Closing as the bug was not in Dotty and the library was fixed — @allanrenucci correct me if I'm wrong.