 Solution

The expression constructs a valid iterator object; it does not itself return the concatenated numeric or character values. The element types have a common supertype AnyVal, and covariance lets Iterator[Int] and Iterator[Char] serve as the two inputs to ConcatIterator[AnyVal]. In the ordinary Scala 2 typing of this expression, the common element type is AnyVal. One can make the intended widening explicit:Successive calls return 1, 2, 3, 'a', 'b', 'c', then exhaustion. Characters remain characters: viewing an existing iterator at a wider type does not convert its contents to integer character codes. No next() is invoked merely by constructing this adapter. The common-type argument uses subtype polymorphism and covariant type parameters, rather than requiring the two concrete input types to be equal.