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): ```scala import strawman.collection.decorators._ object Test { def foo(map: strawman.collection.Map[Int, Int]): Unit = { map.zipByKeyWith(map)((a, b) => a) } } ``` ```scala % dotc -classpath "$(coursier fetch -p ch.epfl.scala:collections-contrib_2.12:0.5.0)" map.scala ```