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Split doc about Implicit Conversions 
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‎docs/docs/reference/changed/implicit-conversions-spec.md‎

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---
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layout: doc-page
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title: "Implicit Conversions - More Details"
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---
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## Implementation
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An implicit conversion, or _view_, from type `S` to type `T` is
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defined by either:
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- An `implicit def` which has type `S => T` or `(=> S) => T`
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- An implicit value which has type `ImplicitConverter[S, T]`
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The standard library defines an abstract class `ImplicitConverter`:
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```scala
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abstract class ImplicitConverter[-T, +U] extends Function1[T, U]
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```
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Function literals are automatically converted to `ImplicitConverter`
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values.
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Views are applied in three situations:
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1. If an expression `e` is of type `T`, and `T` does not conform to
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the expression's expected type `pt`. In this case, an implicit `v`
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which is applicable to `e` and whose result type conforms to `pt`
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is searched. The search proceeds as in the case of implicit
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parameters, where the implicit scope is the one of `T => pt`. If
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such a view is found, the expression `e` is converted to `v(e)`.
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1. In a selection `e.m` with `e` of type `T`, if the selector `m` does
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not denote an accessible member of `T`. In this case, a view `v`
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which is applicable to `e` and whose result contains an accessible
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member named `m` is searched. The search proceeds as in the case of
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implicit parameters, where the implicit scope is the one of `T`. If
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such a view is found, the selection `e.m` is converted to `v(e).m`.
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1. In an application `e.m(args)` with `e` of type `T`, if the selector
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`m` denotes some accessible member(s) of `T`, but none of these
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members is applicable to the arguments `args`. In this case, a view
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`v` which is applicable to `e` and whose result contains a method
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`m` which is applicable to `args` is searched. The search proceeds
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as in the case of implicit parameters, where the implicit scope is
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the one of `T`. If such a view is found, the application
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`e.m(args)` is converted to `v(e).m(args)`.
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# Differences with Scala 2 implicit conversions
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In Scala 2, views whose parameters are passed by-value take precedence
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over views whose parameters are passed by-name. This is no longer the
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case in Scala 3. A type error reporting the ambiguous conversions will
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be emitted in cases where this rule would be applied in Scala 2.
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In Scala 2, implicit values of a function type would be considered as
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potential views. In Scala 3, these implicit value need to have type
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`ImplicitConverter`:
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```scala
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// Scala 2:
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def foo(x: Int)(implicit conv: Int => String): String = x
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// Becomes with Scala 3:
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def foo(x: Int)(implicit conv: ImplicitConverter[Int, String]): String = x
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// Call site is unchanged:
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foo(4)(_.toString)
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// Scala 2:
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implicit val myConverter: Int => String = _.toString
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// Becomes with Scala 3:
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implicit val myConverter: ImplicitConverter[Int, String] = _.toString
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```
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Note that implicit conversions are also affected by the [changes to
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implicit resolution](implicit-resolution.html) between Scala 2 and
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Scala 3.
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## Motivation for the changes
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The introduction of `ImplicitConverter` in Scala 3 and the decision to
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restrict implicit values of this type to be considered as potential
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views comes from the desire to remove surprising behavior from the
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language:
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```scala
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implicit val m: Map[Int, String] = Map(1 -> "abc")
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val x: String = 1 // scalac: assigns "abc" to x
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// Dotty: type error
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```
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This snippet contains a type error. The right hand side of `val x`
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does not conform to type `String`. In Scala 2, the compiler will use
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`m` as an implicit conversion from `Int` to `String`, whereas Scala 3
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will report a type error, because Map isn't an instance of
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`ImplicitConverter`.
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## Migration path
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Implicit values that are used as views should see their type changed
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to `ImplicitConverter`.
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For the migration of implicit conversions that are affected by the
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changes to implicit resolution, refer to the [Changes in Implicit
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Resolution](implicit-resolution.html) for more information.
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## Reference
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For more information about implicit resolution, see [Changes in
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Implicit Resolution](implicit-resolution.html).
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Other details are available in
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[PR #2065](https://github.com/lampepfl/dotty/pull/2065)
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‎docs/docs/reference/changed/implicit-conversions.md‎

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---
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layout: doc-page
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title: "Implicit Conversions - More Details"
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title: "Implicit Conversions"
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---
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## Implementation
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An _implicit conversion_, also called _view_, is a conversion that
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is applied by the compiler in several situations:
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An implicit conversion, or _view_, from type `S` to type `T` is
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defined by either:
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1. When an expression `e` of type `T` is encountered, but the compiler
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needs an expression of type `S`.
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1. When an expression `e.m` where `e` has type `T` but `T` defines no
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member `m` is encountered.
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- An `implicit def` which has type `S => T` or `(=> S) => T`
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- An implicit value which has type `ImplicitConverter[S, T]`
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In those cases, the compiler looks in the implicit scope for a
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conversion that can convert an expression of type `T` to an expression
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of type `S` (or to a type that defines a member `m` in the second
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case).
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The standard library defines an abstract class `ImplicitConverter`:
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This conversion can be either:
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```scala
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abstract class ImplicitConverter[-T, +U] extends Function1[T, U]
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```
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Function literals are automatically converted to `ImplicitConverter`
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values.
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Views are applied in three situations:
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1. If an expression `e` is of type `T`, and `T` does not conform to
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the expression's expected type `pt`. In this case, an implicit `v`
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which is applicable to `e` and whose result type conforms to `pt`
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is searched. The search proceeds as in the case of implicit
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parameters, where the implicit scope is the one of `T => pt`. If
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such a view is found, the expression `e` is converted to `v(e)`.
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1. In a selection `e.m` with `e` of type `T`, if the selector `m` does
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not denote an accessible member of `T`. In this case, a view `v`
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which is applicable to `e` and whose result contains an accessible
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member named `m` is searched. The search proceeds as in the case of
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implicit parameters, where the implicit scope is the one of `T`. If
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such a view is found, the selection `e.m` is converted to `v(e).m`.
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1. In an application `e.m(args)` with `e` of type `T`, if the selector
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`m` denotes some accessible member(s) of `T`, but none of these
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members is applicable to the arguments `args`. In this case, a view
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`v` which is applicable to `e` and whose result contains a method
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`m` which is applicable to `args` is searched. The search proceeds
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as in the case of implicit parameters, where the implicit scope is
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the one of `T`. If such a view is found, the application
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`e.m(args)` is converted to `v(e).m(args)`.
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1. An `implicit def` of type `T => S` or `(=> T) => S`
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1. An implicit value of type `ImplicitConverter[T, S]`
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# Differences with Scala 2 implicit conversions
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## Examples
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In Scala 2, views whose parameters are passed by-value take precedence
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over views whose parameters are passed by-name. This is no longer the
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case in Scala 3. A type error reporting the ambiguous conversions will
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be emitted in cases where this rule would be applied in Scala 2.
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In Scala 2, implicit values of a function type would be considered as
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potential views. In Scala 3, these implicit value need to have type
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`ImplicitConverter`:
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The first example is taken from `scala.Predef`. Thanks to this
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implicit conversion, it is possible to pass a `scala.Int` to a Java
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method that expects a `java.lang.Integer`
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```scala
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// Scala 2:
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def foo(x: Int)(implicit conv: Int => String): String = x
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// Becomes with Scala 3:
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def foo(x: Int)(implicit conv: ImplicitConverter[Int, String]): String = x
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// Call site is unchanged:
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foo(4)(_.toString)
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// Scala 2:
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implicit val myConverter: Int => String = _.toString
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// Becomes with Scala 3:
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implicit val myConverter: ImplicitConverter[Int, String] = _.toString
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import scala.language.implicitConversions
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implicit def int2Integer(x: Int): java.lang.Integer =
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x.asInstanceOf[java.lang.Integer]
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```
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Note that implicit conversions are also affected by the [changes to
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implicit resolution](implicit-resolution.html) between Scala 2 and
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Scala 3.
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## Motivation for the changes
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The introduction of `ImplicitConverter` in Scala 3 and the decision to
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restrict implicit values of this type to be considered as potential
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views comes from the desire to remove surprising behavior from the
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language:
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The second example shows how to use `ImplicitConverter` to define an
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`Ordering` for an arbitrary type, given existing `Ordering`s for other
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types:
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```scala
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implicit val m: Map[Int, String] = Map(1 -> "abc")
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val x: String = 1 // scalac: assigns "abc" to x
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// Dotty: type error
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implicit def ordT[T, S](
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implicit conv: ImplicitConverter[T, S],
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ordS: Ordering[S]
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): Ordering[T] = {
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// `ordS` compares values of type `S`, but we can convert from `T` to `S`
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(x: T, y: T) => ordS.compare(x, y)
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}
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class A(val x: Int) // The type for which we want an `Ordering`
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// Convert `A` to a type for which an `Ordering` is available:
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implicit val AToInt: ImplicitConverter[A, Int] = _.x
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implicitly[Ordering[Int]] // Ok, exists in the standard library
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implicitly[Ordering[A]] // Ok, will use the implicit conversion from
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// `A` to `Int` and the `Ordering` for `Int`.
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```
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This snippet contains a type error. The right hand side of `val x`
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does not conform to type `String`. In Scala 2, the compiler will use
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`m` as an implicit conversion from `Int` to `String`, whereas Scala 3
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will report a type error, because Map isn't an instance of
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`ImplicitConverter`.
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## Migration path
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Implicit values that are used as views should see their type changed
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to `ImplicitConverter`.
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For the migration of implicit conversions that are affected by the
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changes to implicit resolution, refer to the [Changes in Implicit
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Resolution](implicit-resolution.html) for more information.
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## Reference
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For more information about implicit resolution, see [Changes in
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Implicit Resolution](implicit-resolution.html).
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Other details are available in
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[PR #2065](https://github.com/lampepfl/dotty/pull/2065)
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[More details](implicit-conversions-spec.html)

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