Add code tabs for _tour/lower-type-bounds (#2549)

Author: flomebul

_tour/lower-type-bounds.md

@@ -15,6 +15,8 @@ While [upper type bounds](upper-type-bounds.html) limit a type to a subtype of a
 
 Here is an example where this is useful:
 
+{% tabs upper-type-bounds_1 class=tabs-scala-version %}
+{% tab 'Scala 2' for=upper-type-bounds_1 %}
 ```scala mdoc:fail
 trait List[+A] {
   def prepend(elem: A): NonEmptyList[A] = NonEmptyList(elem, this)
@@ -24,13 +26,27 @@ case class NonEmptyList[+A](head: A, tail: List[A]) extends List[A]
 
 object Nil extends List[Nothing]
 ```
+{% endtab %}
+{% tab 'Scala 3' for=upper-type-bounds_1 %}
+```scala
+trait List[+A]:
+  def prepend(elem: A): NonEmptyList[A] = NonEmptyList(elem, this)
+
+case class NonEmptyList[+A](head: A, tail: List[A]) extends List[A]
+
+object Nil extends List[Nothing]
+```
+{% endtab %}
+{% endtabs %}
 
 This program implements a singly-linked list. `Nil` represents an empty list with no elements. `class NonEmptyList` is a node which contains an element of type `A` (`head`) and a reference to the rest of the list (`tail`). The `trait List` and its subtypes are covariant because we have `+A`.
 
 However, this program does _not_ compile because the parameter `elem` in `prepend` is of type `A`, which we declared *co*variant. This doesn't work because functions are *contra*variant in their parameter types and *co*variant in their result types.
 
 To fix this, we need to flip the variance of the type of the parameter `elem` in `prepend`. We do this by introducing a new type parameter `B` that has `A` as a lower type bound.
 
+{% tabs upper-type-bounds_2 class=tabs-scala-version %}
+{% tab 'Scala 2' for=upper-type-bounds_2 %}
 ```scala mdoc
 trait List[+A] {
   def prepend[B >: A](elem: B): NonEmptyList[B] = NonEmptyList(elem, this)
@@ -40,7 +56,23 @@ case class NonEmptyList[+A](head: A, tail: List[A]) extends List[A]
 
 object Nil extends List[Nothing]
 ```
+{% endtab %}
+{% tab 'Scala 3' for=upper-type-bounds_2 %}
+```scala
+trait List[+A]:
+  def prepend[B >: A](elem: B): NonEmptyList[B] = NonEmptyList(elem, this)
+
+case class NonEmptyList[+A](head: A, tail: List[A]) extends List[A]
+
+object Nil extends List[Nothing]
+```
+{% endtab %}
+{% endtabs %}
+
 Now we can do the following:
+
+{% tabs upper-type-bounds_3 %}
+{% tab 'Scala 2 and 3' for=upper-type-bounds_3 %}
 ```scala mdoc
 trait Bird
 case class AfricanSwallow() extends Bird
@@ -65,6 +97,9 @@ val allBirds = africanSwallows.prepend(EuropeanSwallow())
 // but this is a mistake! adding a list of birds widens the type arg too much. -Xlint will warn!
 val error = moreBirds.prepend(swallowsFromAntarctica)    // List[Object]
 ```
+{% endtab %}
+{% endtabs %}
+
 The covariant type parameter allows `birds` to get the value of `africanSwallows`.
 
 The type bound on the type parameter for `prepend` allows adding different varieties of swallows and getting a wider type: instead of `List[AfricanSwallow]`, we get a `List[Bird]`.