object test { sealed trait A[T] case class O[T](e: A[T]) extends A[Option[T]] def opt[U](v: Option[U], a: A[U]) = ??? def f[T](a: A[T], t: T) = { a match { case O(i) => opt(t, i) } } }

-- [E007] Type Mismatch Error: match_opt.scala:10:23 --------------------------- 10 | case O(i) => opt(t, i) | ^ | Found: (t : T) | Required: Option[T$1] | | where: T is a type in method f which is an alias of Option[T$1] | | longer explanation available when compiling with `-explain` 1 error found