infinite loop in implicit search when using collection-strawman

[smarter]

The offending code (adapted from the tests in https://github.com/scala/collection-strawman/blob/master/collections-contrib/src/test/scala/strawman/collection/decorators/MapDecoratorTest.scala, which apparently have never been run with Dotty):

import strawman.collection.decorators._

object Test {
  def foo(map: strawman.collection.Map[Int, Int]): Unit = {
    map.zipByKeyWith(map)((a, b) => a)
  }
}

% dotc -classpath "$(coursier fetch -p ch.epfl.scala:collections-contrib_2.12:0.5.0)" map.scala

[odersky]

The culprit is this beauty (from BuildFrom.scala)

implicit def buildFromSortedMapOps[CC[X, Y] <: SortedMap[X, Y] with SortedMapOps[X, Y, CC, _], K0, V0, K : Ordering, V]: BuildFrom[CC[K0, V0], (K, V), CC[K, V]] = new BuildFrom[CC[K0, V0], (K, V), CC[K, V]] {
  def newBuilder(from: CC[K0, V0]): Builder[(K, V), CC[K, V]] = from.sortedMapFactory.newBuilder[K, V]()
  def fromSpecificIterable(from: CC[K0, V0])(it: Iterable[(K, V)]): CC[K, V] = from.sortedMapFactory.from(it)
}

What happens is that gets called without a known instance of the parameter K. This means implicit search goes out an enumerates all ways a type (any type) can have an Ordering. There are a lot of such types, so it dies with an GC limit exceeded exception after going on for some time.

The problem is that the implicit def is typed with an expected type of

strawman.collection.BuildFrom[
  strawman.collection.Map[Int, Int](map).(strawman.collection.decorators.MapDecorator[Int, Int{this: strawman.collection.Map[Int, Int](map)}#this), 
  (Int, Int), 
 C]

but somehow it does not take into account that K should be Int.