Add code tabs for _tour/variances (#2548)

Author: flomebul

_tour/upper-type-bounds.md

@@ -13,6 +13,8 @@ redirect_from: "/tutorials/tour/upper-type-bounds.html"
 In Scala, [type parameters](generic-classes.html) and [abstract type members](abstract-type-members.html) may be constrained by a type bound. Such type bounds limit the concrete values of the type variables and possibly reveal more information about the members of such types. An _upper type bound_ `T <: A` declares that type variable `T` refers to a subtype of type `A`.
 Here is an example that demonstrates upper type bound for a type parameter of class `PetContainer`:
 
+{% tabs upper-type-bounds class=tabs-scala-version %}
+{% tab 'Scala 2' for=upper-type-bounds %}
 ```scala mdoc
 abstract class Animal {
  def name: String
@@ -39,11 +41,47 @@ class PetContainer[P <: Pet](p: P) {
 val dogContainer = new PetContainer[Dog](new Dog)
 val catContainer = new PetContainer[Cat](new Cat)
 ```
+{% endtab %}
+{% tab 'Scala 3' for=upper-type-bounds %}
+```scala
+abstract class Animal:
+  def name: String
 
+abstract class Pet extends Animal
+
+class Cat extends Pet:
+  override def name: String = "Cat"
+
+class Dog extends Pet:
+  override def name: String = "Dog"
+
+class Lion extends Animal:
+  override def name: String = "Lion"
+
+class PetContainer[P <: Pet](p: P):
+  def pet: P = p
+
+val dogContainer = PetContainer[Dog](Dog())
+val catContainer = PetContainer[Cat](Cat())
+```
+{% endtab %}
+{% endtabs %}
+
+{% tabs upper-type-bounds_error class=tabs-scala-version %}
+{% tab 'Scala 2' for=upper-type-bounds_error %}
 ```scala mdoc:fail
 // this would not compile
 val lionContainer = new PetContainer[Lion](new Lion)
 ```
+{% endtab %}
+{% tab 'Scala 3' for=upper-type-bounds_error %}
+```scala
+// this would not compile
+val lionContainer = PetContainer[Lion](Lion())
+```
+{% endtab %}
+{% endtabs %}
+
 The `class PetContainer` takes a type parameter `P` which must be a subtype of `Pet`. `Dog` and `Cat` are subtypes of `Pet` so we can create a new `PetContainer[Dog]` and `PetContainer[Cat]`. However, if we tried to create a `PetContainer[Lion]`, we would get the following Error:
 
 `type arguments [Lion] do not conform to class PetContainer's type parameter bounds [P <: Pet]`