Add copy of reference as TODO

reference taken from 7919cb3

Author: Sporarum

Diff for: docs/_spec/TODOreference/changed-features/changed-features.md

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+---
+layout: index
+title: "Other Changed Features"
+nightlyOf: https://docs.scala-lang.org/scala3/reference/changed-features.html
+---
+
+The following pages document the features that have changed in Scala 3, compared to Scala 2.

Diff for: docs/_spec/TODOreference/changed-features/compiler-plugins.md

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+---
+layout: doc-page
+title: "Changes in Compiler Plugins"
+nightlyOf: https://docs.scala-lang.org/scala3/reference/changed-features/compiler-plugins.html
+---
+
+Compiler plugins are supported by Dotty (and Scala 3) since 0.9. There are two notable changes
+compared to `scalac`:
+
+- No support for analyzer plugins
+- Added support for research plugins
+
+[Analyzer plugins][1] in `scalac` run during type checking and may influence
+normal type checking. This is a very powerful feature but for production usages,
+a predictable and consistent type checker is more important.
+
+For experimentation and research, Scala 3 introduces _research plugin_. Research plugins
+are more powerful than `scalac` analyzer plugins as they let plugin authors customize
+the whole compiler pipeline. One can easily replace the standard typer by a custom one or
+create a parser for a domain-specific language. However, research plugins are only
+enabled for nightly or snaphot releases of Scala 3.
+
+Common plugins that add new phases to the compiler pipeline are called
+_standard plugins_ in Scala 3. In terms of features, they are similar to
+`scalac` plugins, despite minor changes in the API.
+
+## Using Compiler Plugins
+
+Both standard and research plugins can be used with `scalac` by adding the `-Xplugin:` option:
+
+```shell
+scalac -Xplugin:pluginA.jar -Xplugin:pluginB.jar Test.scala
+```
+
+The compiler will examine the jar provided, and look for a property file named
+`plugin.properties` in the root directory of the jar. The property file specifies
+the fully qualified plugin class name. The format of a property file is as follows:
+
+```properties
+pluginClass=dividezero.DivideZero
+```
+
+This is different from `scalac` plugins that required a `scalac-plugin.xml` file.
+
+Starting from 1.1.5, `sbt` also supports Scala 3 compiler plugins. Please refer to the
+[`sbt` documentation][2] for more information.
+
+## Writing a Standard Compiler Plugin
+
+Here is the source code for a simple compiler plugin that reports integer divisions by
+zero as errors.
+
+```scala
+package dividezero
+
+import dotty.tools.dotc.ast.Trees.*
+import dotty.tools.dotc.ast.tpd
+import dotty.tools.dotc.core.Constants.Constant
+import dotty.tools.dotc.core.Contexts.Context
+import dotty.tools.dotc.core.Decorators.*
+import dotty.tools.dotc.core.StdNames.*
+import dotty.tools.dotc.core.Symbols.*
+import dotty.tools.dotc.plugins.{PluginPhase, StandardPlugin}
+import dotty.tools.dotc.transform.{Pickler, Staging}
+
+class DivideZero extends StandardPlugin:
+  val name: String = "divideZero"
+  override val description: String = "divide zero check"
+
+  def init(options: List[String]): List[PluginPhase] =
+    (new DivideZeroPhase) :: Nil
+
+class DivideZeroPhase extends PluginPhase:
+  import tpd.*
+
+  val phaseName = "divideZero"
+
+  override val runsAfter = Set(Pickler.name)
+  override val runsBefore = Set(Staging.name)
+
+  override def transformApply(tree: Apply)(implicit ctx: Context): Tree =
+    tree match
+      case Apply(Select(rcvr, nme.DIV), List(Literal(Constant(0))))
+      if rcvr.tpe <:< defn.IntType =>
+        report.error("dividing by zero", tree.pos)
+      case _ =>
+        ()
+    tree
+end DivideZeroPhase
+```
+
+The plugin main class (`DivideZero`) must extend the trait `StandardPlugin`
+and implement the method `init` that takes the plugin's options as argument
+and returns a list of `PluginPhase`s to be inserted into the compilation pipeline.
+
+Our plugin adds one compiler phase to the pipeline. A compiler phase must extend
+the `PluginPhase` trait. In order to specify when the phase is executed, we also
+need to specify a `runsBefore` and `runsAfter` constraints that are list of phase
+names.
+
+We can now transform trees by overriding methods like `transformXXX`.
+
+## Writing a Research Compiler Plugin
+
+Here is a template for research plugins.
+
+```scala
+import dotty.tools.dotc.core.Contexts.Context
+import dotty.tools.dotc.core.Phases.Phase
+import dotty.tools.dotc.plugins.ResearchPlugin
+
+class DummyResearchPlugin extends ResearchPlugin:
+  val name: String = "dummy"
+  override val description: String = "dummy research plugin"
+
+  def init(options: List[String], phases: List[List[Phase]])(implicit ctx: Context): List[List[Phase]] =
+    phases
+end DummyResearchPlugin
+```
+
+A research plugin must extend the trait `ResearchPlugin`  and implement the
+method `init` that takes the plugin's options as argument as well as the compiler
+pipeline in the form of a list of compiler phases. The method can replace, remove
+or add any phases to the pipeline and return the updated pipeline.
+
+
+[1]: https://github.com/scala/scala/blob/2.13.x/src/compiler/scala/tools/nsc/typechecker/AnalyzerPlugins.scala
+[2]: https://www.scala-sbt.org/1.x/docs/Compiler-Plugins.html

Diff for: docs/_spec/TODOreference/changed-features/eta-expansion-spec.md

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+---
+layout: doc-page
+title: "Automatic Eta Expansion - More Details"
+nightlyOf: https://docs.scala-lang.org/scala3/reference/changed-features/eta-expansion-spec.html
+---
+
+## Motivation
+
+Scala maintains a convenient distinction between _methods_ and _functions_.
+Methods are part of the definition of a class that can be invoked in objects while functions are complete objects themselves, making them first-class entities. For example, they can be assigned to variables.
+These two mechanisms are bridged in Scala by a mechanism called
+[_eta-expansion_](https://www.scala-lang.org/files/archive/spec/2.13/06-expressions.html#eta-expansion-section)
+(also called eta-abstraction), which converts a reference to a method into a function. Intuitively, a method `m` can be passed around by turning it into an object: the function `x => m(x)`.
+
+In this snippet which assigns a method to a `val`, the compiler will perform _automatic eta-expansion_, as shown in the comment:
+
+```scala
+def m(x: Int, y: String) = ???
+val f = m // becomes: val f = (x: Int, y: String) => m(x, y)
+```
+
+In Scala 2, a method reference `m` is converted to a function value only if the expected type is a function type, which means the conversion in the example above would not have been triggered, because `val f` does not have a type ascription. To still get eta-expansion, a shortcut `m _` would force the conversion.
+
+For methods with one or more parameters like in the example above, this restriction has now been dropped. The syntax `m _` is no longer needed and will be deprecated in the future.
+
+## Automatic eta-expansion and partial application
+In the following example `m` can be partially applied to the first two parameters.
+Assigning `m` to `f1` will automatically eta-expand.
+
+```scala
+def m(x: Boolean, y: String)(z: Int): List[Int]
+val f1 = m
+val f2 = m(true, "abc")
+```
+
+This creates two function values:
+
+```scala
+f1: (Boolean, String) => Int => List[Int]
+f2: Int => List[Int]
+```
+
+## Automatic eta-expansion and implicit parameter lists
+
+Methods with implicit parameter lists will always get applied to implicit arguments.
+
+```scala
+def foo(x: Int)(implicit p: Double): Float = ???
+implicit val bla: Double = 1.0
+
+val bar = foo // val bar: Int => Float = ...
+```
+
+## Automatic Eta-Expansion and query types
+
+A method with context parameters can be expanded to a value of a context type by writing the expected context type explicitly.
+
+```scala
+def foo(x: Int)(using p: Double): Float = ???
+val bar: Double ?=> Float = foo(3)
+```
+
+## Rules
+
+- If `m` has an argument list with one or more parameters, we always eta-expand
+- If `m` is has an empty argument list (i.e. has type `()R`):
+    1. If the expected type is of the form `() => T`, we eta expand.
+    2. If m is defined by Java, or overrides a Java defined method, we insert `()`.
+    3. Otherwise we issue an error of the form:
+
+Thus, an unapplied method with an empty argument list is only converted to a function when a function type is expected. It is considered best practice to either explicitly apply the method to `()`, or convert it to a function with `() => m()`.
+
+The method value syntax `m _` is deprecated.
+
+## Reference
+
+For more information, see [PR #2701](https://github.com/lampepfl/dotty/pull/2701).

Diff for: docs/_spec/TODOreference/changed-features/eta-expansion.md

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+---
+layout: doc-page
+title: "Automatic Eta Expansion"
+nightlyOf: https://docs.scala-lang.org/scala3/reference/changed-features/eta-expansion.html
+---
+
+The conversion of _methods_ into _functions_ has been improved and happens automatically for methods with one or more parameters.
+
+```scala
+def m(x: Boolean, y: String)(z: Int): List[Int]
+val f1 = m
+val f2 = m(true, "abc")
+```
+
+This creates two function values:
+```scala
+f1: (Boolean, String) => Int => List[Int]
+f2: Int => List[Int]
+```
+
+The syntax `m _` is no longer needed and will be deprecated in the future.
+
+## Automatic eta-expansion and nullary methods
+
+Automatic eta expansion does not apply to "nullary" methods that take an empty parameter list.
+
+```scala
+def next(): T
+```
+
+Given a simple reference to `next` does not auto-convert to a function.
+One has to write explicitly `() => next()` to achieve that.
+Once again since the `_` is going to be deprecated it's better to write it this way
+rather than `next _`.
+
+The reason for excluding nullary methods from automatic eta expansion
+is that Scala implicitly inserts the `()` argument, which would
+conflict with eta expansion. Automatic `()` insertion is
+[limited](../dropped-features/auto-apply.md) in Scala 3, but the fundamental ambiguity
+remains.
+
+[More details](eta-expansion-spec.md)

Diff for: docs/_spec/TODOreference/changed-features/implicit-conversions-spec.md

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+---
+layout: doc-page
+title: "Implicit Conversions - More Details"
+nightlyOf: https://docs.scala-lang.org/scala3/reference/changed-features/implicit-conversions-spec.html
+---
+
+## Implementation
+
+An implicit conversion, or _view_, from type `S` to type `T` is
+defined by either:
+
+- An `implicit def` which has type `S => T` or `(=> S) => T`
+- An implicit value which has type `Conversion[S, T]`
+
+The standard library defines an abstract class [`Conversion`](https://scala-lang.org/api/3.x/scala/Conversion.html):
+
+```scala
+package scala
+@java.lang.FunctionalInterface
+abstract class Conversion[-T, +U] extends Function1[T, U]:
+  def apply(x: T): U
+```
+
+Function literals are automatically converted to `Conversion` values.
+
+Views are applied in three situations:
+
+1. If an expression `e` is of type `T`, and `T` does not conform to
+   the expression's expected type `pt`. In this case, an implicit `v`
+   which is applicable to `e` and whose result type conforms to `pt`
+   is searched. The search proceeds as in the case of implicit
+   parameters, where the implicit scope is the one of `T => pt`. If
+   such a view is found, the expression `e` is converted to `v(e)`.
+1. In a selection `e.m` with `e` of type `T`, if the selector `m` does
+   not denote an accessible member of `T`. In this case, a view `v`
+   which is applicable to `e` and whose result contains an accessible
+   member named `m` is searched. The search proceeds as in the case of
+   implicit parameters, where the implicit scope is the one of `T`. If
+   such a view is found, the selection `e.m` is converted to `v(e).m`.
+1. In an application `e.m(args)` with `e` of type `T`, if the selector
+   `m` denotes some accessible member(s) of `T`, but none of these
+   members is applicable to the arguments `args`. In this case, a view
+   `v` which is applicable to `e` and whose result contains a method
+   `m` which is applicable to `args` is searched. The search proceeds
+   as in the case of implicit parameters, where the implicit scope is
+   the one of `T`. If such a view is found, the application
+   `e.m(args)` is converted to `v(e).m(args)`.
+
+# Differences with Scala 2 implicit conversions
+
+In Scala 2, views whose parameters are passed by-value take precedence
+over views whose parameters are passed by-name. This is no longer the
+case in Scala 3. A type error reporting the ambiguous conversions will
+be emitted in cases where this rule would be applied in Scala 2:
+
+```scala
+implicit def conv1(x: Int): String = x.toString
+implicit def conv2(x: => Int): String = x.toString
+
+val x: String = 0 // Compiles in Scala2 (uses `conv1`),
+                  // type error in Scala 3 because of ambiguity.
+```
+
+In Scala 2, implicit values of a function type would be considered as
+potential views. In Scala 3, these implicit value need to have type
+`Conversion`:
+
+```scala
+// Scala 2:
+def foo(x: Int)(implicit conv: Int => String): String = x
+
+// Becomes with Scala 3:
+def foo(x: Int)(implicit conv: Conversion[Int, String]): String = x
+
+// Call site is unchanged:
+foo(4)(_.toString)
+
+// Scala 2:
+implicit val myConverter: Int => String = _.toString
+
+// Becomes with Scala 3:
+implicit val myConverter: Conversion[Int, String] = _.toString
+```
+
+Note that implicit conversions are also affected by the [changes to implicit resolution](implicit-resolution.md) between Scala 2 and Scala 3.
+
+## Motivation for the changes
+
+The introduction of [`scala.Conversion`](https://scala-lang.org/api/3.x/scala/Conversion.html)
+in Scala 3 and the decision to restrict implicit values of this type to be
+considered as potential views comes from the desire to remove surprising
+behavior from the language:
+
+```scala
+implicit val m: Map[Int, String] = Map(1 -> "abc")
+
+val x: String = 1  // Scala 2: assigns "abc" to x
+                   // Scala 3: type error
+```
+
+This snippet contains a type error. The right-hand side of `val x`
+does not conform to type `String`. In Scala 2, the compiler will use
+`m` as an implicit conversion from `Int` to `String`, whereas Scala 3
+will report a type error, because `Map` isn't an instance of
+[`Conversion`](https://scala-lang.org/api/3.x/scala/Conversion.html).
+
+## Migration path
+
+Implicit values that are used as views should see their type changed to `Conversion`.
+
+For the migration of implicit conversions that are affected by the
+changes to implicit resolution, refer to the [Changes in Implicit Resolution](implicit-resolution.md) for more information.
+
+## Reference
+
+For more information about implicit resolution, see [Changes in Implicit Resolution](implicit-resolution.md).
+Other details are available in [PR #2065](https://github.com/lampepfl/dotty/pull/2065).