Merge pull request #9577 from dotty-staging/move-Not

Rename scala.implicits.Not to scala.util.Not

Author: odersky

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -743,7 +743,7 @@ class Definitions {
     @tu lazy val TypeBox_CAP: TypeSymbol = TypeBoxClass.requiredType(tpnme.CAP)
 
   @tu lazy val MatchCaseClass: ClassSymbol = requiredClass("scala.internal.MatchCase")
-  @tu lazy val NotClass: ClassSymbol = requiredClass("scala.implicits.Not")
+  @tu lazy val NotClass: ClassSymbol = requiredClass("scala.util.Not")
     @tu lazy val Not_value: Symbol = NotClass.companionModule.requiredMethod(nme.value)
 
   @tu lazy val ValueOfClass: ClassSymbol = requiredClass("scala.ValueOf")

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -1232,7 +1232,7 @@ trait Implicits:
              |According to the new implicit resolution rules this is no longer possible;
              |the search will fail with a global ambiguity error instead.
              |
-             |Consider using the scala.implicits.Not class to implement similar functionality.""",
+             |Consider using the scala.util.Not class to implement similar functionality.""",
              ctx.source.atSpan(span))
 
       /** A relation that influences the order in which implicits are tried.

compiler/test-resources/repl/importFromObj

@@ -11,11 +11,11 @@ scala> buf += xs
   |        Required: Int
 scala> buf ++= xs
 val res0: scala.collection.mutable.ListBuffer[Int] = ListBuffer(1, 2, 3)
-scala> import util.foo
-1 | import util.foo
-  |             ^^^
-  |             value foo is not a member of util
-scala> import util.foo.bar
-1 | import util.foo.bar
-  |        ^^^^^^^^
-  |        value foo is not a member of util
+scala> import util.foobar
+1 | import util.foobar
+  |             ^^^^^^
+  |             value foobar is not a member of util
+scala> import util.foobar.bar
+1 | import util.foobar.bar
+  |        ^^^^^^^^^^^
+  |        value foobar is not a member of util

docs/docs/reference/changed-features/implicit-resolution.md

@@ -107,7 +107,7 @@ which means that the alternative `c` would be chosen as solution!
 Scala 2's somewhat puzzling behavior with respect to ambiguity has been exploited to implement
 the analogue of a "negated" search in implicit resolution, where a query `Q1` fails if some
 other query `Q2` succeeds and `Q1` succeeds if `Q2` fails. With the new cleaned up behavior
-these techniques no longer work. But there is now a new special type `scala.implicits.Not`
+these techniques no longer work. But there is now a new special type `scala.util.Not`
 which implements negation directly. For any query type `Q`: `Not[Q]` succeeds if and only if
 the implicit search for `Q` fails.
 

library/src-bootstrapped/scala/implicits/package.scala

@@ -0,0 +1,6 @@
+package scala
+
+package object implicits {
+  @deprecated("scala.implicits.Not has been renamed scala.util.Not", "0.27.0")
+  type Not[A] = scala.util.Not[A]
+}

library/src-bootstrapped/scala/util/Not.scala

@@ -0,0 +1,45 @@
+package scala.util
+
+/** A special class used to implement negation in implicit search.
+ *
+ *  Consider the problem of using implicit `i1` for a query type `D` if an implicit
+ *  for some other class `C` is available, and using an implicit `i2` if no implicit
+ *  value of type `C` is available. If we do not want to prioritize `i1` and `i2` by
+ *  putting them in different traits we can instead define the following:
+ *
+ *     given i1: D(using ev: C) = ...
+ *     given i2: D(using ev: Not[C]) = ...
+ *
+ *  `Not` is treated specially in implicit search, similar to the way logical negation
+ *  is treated in Prolog: The implicit search for `Not[C]` succeeds if and only if the implicit
+ *  search for `C` fails.
+ *
+ *  In Scala 2 this form of negation can be simulated by setting up a conditional
+ *  ambiguous implicit and an unconditional fallback, the way it is done with the
+ *  `default`, `amb1` and `amb2` methods below. Due to the way these two methods are
+ *  defined, `Not` is also usable from Scala 2.
+ *
+ *  In Dotty, ambiguity is a global error, and therefore cannot be used to implement negation.
+ *  Instead, `Not` is treated natively in implicit search.
+ */
+final class Not[+T] private ()
+
+trait LowPriorityNot {
+
+  /** A fallback method used to emulate negation in Scala 2 */
+  given default[T] as Not[T] = Not.value
+}
+object Not extends LowPriorityNot {
+
+  /** A value of type `Not` to signal a successful search for `Not[C]` (i.e. a failing
+   *  search for `C`). A reference to this value will be explicitly constructed by Dotty's
+   *  implicit search algorithm
+   */
+  def value: Not[Nothing] = new Not[Nothing]()
+
+  /** One of two ambiguous methods used to emulate negation in Scala 2 */
+  given amb1[T](using ev: T) as Not[T] = ???
+
+  /** One of two ambiguous methods used to emulate negation in Scala 2 */
+  given amb2[T](using ev: T) as Not[T] = ???
+}