Failure in overload resolution with expected result type

[szeiger]

Discovered in scala/collection-strawman#117 (comment): The following simplified version compiles on Scala 2.13.0-M1 but fails on Dotty master:

  trait Iterable2[A] {
    def flatMap[B](f: A => B): Iterable2[B] = ???
  }
  trait SortedMap2[K, V] extends Iterable2[(K, V)] {
    def flatMap[K2, V2](f: ((K, V)) => (K2, V2))(implicit ordering: Ordering[K2]): SortedMap2[K2, V2] = ???
    def foo: SortedMap2[Int, Int] = flatMap { kv => (0, 0) }
  }

The failure is:

scala>   trait SortedMap2[K, V] extends Iterable2[(K, V)] {
             def flatMap[K2, V2](f: ((K, V)) => (K2, V2))(implicit ordering: Ordering[K2]): SortedMap2[K2, V2] = ???
             def foo: SortedMap2[Int, Int] = flatMap { kv => (0, 0) }
           }
-- [E080] Syntax Error: <console>:7:48 -----------------------------------------
7 |      def foo: SortedMap2[Int, Int] = flatMap { kv => (0, 0) }
  |                                                ^^
  |                     missing parameter type for parameter kv, expected = ?