Why does this Scala implicit conversion fail when explicit conversion works?

Question

Why doesn\'t the following implicit conversion work even though explicitly calling the function works?

scala> implicit def view[A, C](xs: C)(implicit ev:          


        

Answer 1

Simply put, as the error message suggests, it cannot infer A. There's no information the compiler can use to infer what A must be, so it must assume it can be anything.

You might retort that it can infer A from C <:< Iterable[A], but that would turn the problem on its head: instead of using <:< to check constraints, it would use the constraints to infer the type of A! That logic is not sound.