Incomplete implicit dictionaries should be rejected

[milessabin]

This is Dotty's equivalent of scala/bug#11591. Here, rather than crashing the compiler, it compiles incorrectly yielding a null. The correct behaviour is to report an implicit search failure.

object Test {
  class A
  class B

  implicit def mkA(implicit b: => B): A = ???
  implicit def mkB(implicit a: A, i: Int): B = ???

  implicitly[A]
}

PR incoming ...