Merge pull request #2630 from bishabosha/tour-implicit-conversions

update tour/implicit-conversions for Scala 3

Author: bishabosha

_tour/implicit-conversions.md

@@ -10,46 +10,33 @@ previous-page: implicit-parameters
 redirect_from: "/tutorials/tour/implicit-conversions.html"
 ---
 
-An implicit conversion from type `S` to type `T` is defined by an implicit value which has function type `S => T`, or by an implicit method convertible to a value of that type.
+{% tabs implicit-conversion-defn class=tabs-scala-version %}
+{% tab 'Scala 2' %}
+In Scala 2, an implicit conversion from type `S` to type `T` is defined by an [implicit value]({% link _tour/implicit-parameters.md %}) which has function type `S => T`, or by an implicit method convertible to a value of that type.
+{% endtab %}
+{% tab 'Scala 3' %}
+In Scala 3, an implicit conversion from type `S` to type `T` is defined by a [given instance]({% link _tour/implicit-parameters.md %}) which has type `scala.Conversion[S, T]`. For compatibility with Scala 2, they can also be defined by an implicit method (read more in the Scala 2 tab).
+{% endtab %}
+{% endtabs %}
 
 Implicit conversions are applied in two situations:
 
-* If an expression `e` is of type `S`, and `S` does not conform to the expression's expected type `T`.
-* In a selection `e.m` with `e` of type `S`, if the selector `m` does not denote a member of `S`.
+1. If an expression `e` is of type `S`, and `S` does not conform to the expression's expected type `T`.
+2. In a selection `e.m` with `e` of type `S`, if the selector `m` does not denote a member of `S`.
 
-In the first case, a conversion `c` is searched for which is applicable to `e` and whose result type conforms to `T`.
-In the second case, a conversion `c` is searched for which is applicable to `e` and whose result contains a member named `m`.
+In the first case, a conversion `c` is searched for, which is applicable to `e` and whose result type conforms to `T`.
 
-If an implicit method `List[A] => Ordered[List[A]]` is in scope, as well as an implicit method `Int => Ordered[Int]`, the following operation on the two lists of type `List[Int]` is legal:
+An example is to pass a `scala.Int`, e.g. `x`, to a method that expects `scala.Long`. In this case, the implicit conversion `Int.int2long(x)` is inserted.
 
-```
-List(1, 2, 3) <= List(4, 5)
-```
 
-An implicit method `Int => Ordered[Int]` is provided automatically through `scala.Predef.intWrapper`. An example of an implicit method `List[A] => Ordered[List[A]]` is provided below.
+In the second case, a conversion `c` is searched for, which is applicable to `e` and whose result contains a member named `m`.
 
-```scala mdoc
-import scala.language.implicitConversions
+An example is to compare two strings `"foo" < "bar"`. In this case, `String` has no member `<`, so the implicit conversion `Predef.augmentString("foo") < "bar"` is inserted. (`scala.Predef` is automatically imported into all Scala programs.)
 
-implicit def list2ordered[A](x: List[A])
-    (implicit elem2ordered: A => Ordered[A]): Ordered[List[A]] =
-  new Ordered[List[A]] { 
-    //replace with a more useful implementation
-    def compare(that: List[A]): Int = 1
-  }
-```
-
-The implicitly imported object `scala.Predef` declares several aliases to frequently used types (e.g. `scala.collection.immutable.Map` is aliased to `Map`) and methods (e.g. `assert`) but also several implicit conversions.
-
-For example, when calling a Java method that expects a `java.lang.Integer`, you are free to pass it a `scala.Int` instead. That's because Predef includes the following implicit conversions:
-
-```scala mdoc
-import scala.language.implicitConversions
-
-implicit def int2Integer(x: Int) =
-  java.lang.Integer.valueOf(x)
-```
+**Beware the power of implicit conversions:**
 
+{% tabs implicit-conversion-warning class=tabs-scala-version %}
+{% tab 'Scala 2' %}
 Because implicit conversions can have pitfalls if used indiscriminately the compiler warns when compiling the implicit conversion definition.
 
 To turn off the warnings take either of these actions:
@@ -58,3 +45,17 @@ To turn off the warnings take either of these actions:
 * Invoke the compiler with `-language:implicitConversions`
 
 No warning is emitted when the conversion is applied by the compiler.
+{% endtab %}
+{% tab 'Scala 3' %}
+Because implicit conversions can have pitfalls if used indiscriminately the compiler warns in two situations:
+- when compiling a Scala 2 style implicit conversion definition.
+- at the call site where a given instance of `scala.Conversion` is inserted as a conversion.
+
+To turn off the warnings take either of these actions:
+
+- Import `scala.language.implicitConversions` into the scope of:
+  - a Scala 2 style implicit conversion definition
+  - call sites where a given instance of `scala.Conversion` is inserted as a conversion.
+- Invoke the compiler with `-language:implicitConversions`
+{% endtab %}
+{% endtabs %}