Improve exhaustivity section of pattern matching

_tour/pattern-matching.md

@@ -134,7 +134,7 @@ println(showNotification(someVoiceRecording))  // prints You received a Voice Re
 The function `showNotification` takes as a parameter the abstract type `Notification` and matches on the type of `Notification` (i.e. it figures out whether it's an `Email`, `SMS`, or `VoiceRecording`). In the `case Email(sender, title, _)` the fields `sender` and `title` are used in the return value but the `body` field is ignored with `_`.
 
 ## Pattern guards
-Pattern guards are simply boolean expressions which are used to make cases more specific. Just add `if <boolean expression>` after the pattern.
+Pattern guards are boolean expressions which are used to make cases more specific. Just add `if <boolean expression>` after the pattern.
 
 {% tabs pattern-matching-5 class=tabs-scala-version %}
 {% tab 'Scala 2' for=pattern-matching-5 %}
@@ -231,36 +231,62 @@ def goIdle(device: Device): String = device match
 
 `def goIdle` has a different behavior depending on the type of `Device`. This is useful when the case needs to call a method on the pattern. It is a convention to use the first letter of the type as the case identifier (`p` and `c` in this case).
 
-## Sealed classes
-Traits and classes can be marked `sealed` which means all subtypes must be declared in the same file. This assures that all subtypes are known.
+## Sealed types
+
+You may have noticed that in the examples above the base types are qualified
+with the keyword `sealed`. This provides extra safety because the compiler
+checks that the `cases` of a `match` expression are exhaustive when the base
+type is `sealed`.
+
+For instance, in the method `showNotification` defined above, if we forget
+one case, say, `VoiceRecording`, the compiler emits a warning:
 
 {% tabs pattern-matching-7 class=tabs-scala-version %}
 {% tab 'Scala 2' for=pattern-matching-7 %}
-```scala mdoc
-sealed trait Furniture
-case class Couch(descriptor: String) extends Furniture
-case class Chair(descriptor: String) extends Furniture
-
-def findPlaceToSit(piece: Furniture): String = piece match {
-  case Couch(descriptor) => s"Lie on the $descriptor couch"
-  case Chair(descriptor) => s"Sit on the $descriptor chair"
+```scala
+def showNotification(notification: Notification): String = {
+  notification match {
+    case Email(sender, title, _) =>
+      s"You got an email from $sender with title: $title"
+    case SMS(number, message) =>
+      s"You got an SMS from $number! Message: $message"
+  }
 }
 ```
 {% endtab %}
 {% tab 'Scala 3' for=pattern-matching-7 %}
 ```scala
-sealed trait Furniture
-case class Couch(descriptor: String) extends Furniture
-case class Chair(descriptor: String) extends Furniture
-
-def findPlaceToSit(piece: Furniture): String = piece match
-  case Couch(descriptor) => s"Lie on the $descriptor couch"
-  case Chair(descriptor) => s"Sit on the $descriptor chair"
+def showNotification(notification: Notification): String =
+  notification match
+    case Email(sender, title, _) =>
+      s"You got an email from $sender with title: $title"
+    case SMS(number, message) =>
+      s"You got an SMS from $number! Message: $message"
 ```
 {% endtab %}
 {% endtabs %}
 
-This is useful for pattern matching because we don't need a "catch all" case.
+This definition produces the following warning:
+
+~~~
+match may not be exhaustive.
+
+It would fail on pattern case: VoiceRecording(_, _)
+~~~
+
+The compiler even provides examples of input that would fail!
+
+On the flip side, exhaustivity checking requires you to define all the subtypes
+of the base type in the same file as the base type (otherwise, the compiler
+would not know what are all the possible cases). For instance, if you try
+to define a new type of `Notification` outside of the file that defines
+the `sealed trait Notfication`, it will produce a compilation error:
+
+~~~
+case class Telepathy(message: String) extends Notification
+           ^
+        Cannot extend sealed trait Notification in a different source file
+~~~
 
 ## Notes