Disallow _ for wildcard arguments of types and use ?

Author: nicolasstucki

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -26,6 +26,7 @@ object Feature:
   val dependent = experimental("dependent")
   val erasedDefinitions = experimental("erasedDefinitions")
   val symbolLiterals = deprecated("symbolLiterals")
+  val underscoreWildcards = deprecated("underscoreWildcards")
   val fewerBraces = experimental("fewerBraces")
   val saferExceptions = experimental("saferExceptions")
   val clauseInterleaving = experimental("clauseInterleaving")

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1856,9 +1856,12 @@ object Parsers {
           val start = in.skipToken()
           Ident(tpnme.USCOREkw).withSpan(Span(start, in.lastOffset, start))
         else
-          if sourceVersion.isAtLeast(future) then
-            deprecationWarning(em"`_` is deprecated for wildcard arguments of types: use `?` instead")
-            patch(source, Span(in.offset, in.offset + 1), "?")
+          if !in.featureEnabled(Feature.underscoreWildcards) then
+            report.errorOrMigrationWarning(
+                em"`_` is deprecated for wildcard arguments of types: use `?` instead${rewriteNotice(`3.4-migration`)}",
+                in.sourcePos(), from = `3.4`)
+            if sourceVersion == `3.4-migration` then
+              patch(source, Span(in.offset, in.offset + 1), "?")
           val start = in.skipToken()
           typeBounds().withSpan(Span(start, in.lastOffset, start))
       // Allow symbols -_ and +_ through for compatibility with code written using kind-projector in Scala 3 underscore mode.

compiler/test-resources/repl/i13208.scala

@@ -1,8 +1,7 @@
-//> using options -source:future -deprecation
+//> using options -language:`3.4-migration` -deprecation
 scala> type M[X] = X match { case Int => String case _ => Int }
 scala> type N[X] = X match { case List[_] => Int }
-1 warning found
--- Deprecation Warning: --------------------------------------------------------
+-- Error: ----------------------------------------------------------------------
 1 | type N[X] = X match { case List[_] => Int }
   |                                 ^
   |        `_` is deprecated for wildcard arguments of types: use `?` instead

compiler/test-resources/repl/i6643

@@ -3,5 +3,5 @@ scala> import scala.collection._
 scala>:type 1
 Int
 
-scala> object IterableTest { def g[CC[_] <: Iterable[_] with IterableOps[_, _, _]](from: CC[Int]): IterableFactory[CC] = ??? }
+scala> object IterableTest { def g[CC[_] <: Iterable[?] with IterableOps[?, ?, ?]](from: CC[Int]): IterableFactory[CC] = ??? }
 // defined object IterableTest

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -334,7 +334,7 @@ class DottyBytecodeTests extends DottyBytecodeTest {
         |  def test =
         |    try print("foo")
         |    catch {
-        |      case _: scala.runtime.NonLocalReturnControl[_] => ()
+        |      case _: scala.runtime.NonLocalReturnControl[?] => ()
         |    }
         |}
       """.stripMargin

library/src/scala/Tuple.scala

@@ -97,25 +97,25 @@ object Tuple {
 
   /** Type of the head of a tuple */
   type Head[X <: NonEmptyTuple] = X match {
-    case x *: _ => x
+    case x *: xs => x
   }
 
   /** Type of the initial part of the tuple without its last element */
   type Init[X <: Tuple] <: Tuple = X match {
-    case _ *: EmptyTuple => EmptyTuple
+    case x *: EmptyTuple => EmptyTuple
     case x *: xs =>
       x *: Init[xs]
   }
 
   /** Type of the tail of a tuple */
   type Tail[X <: NonEmptyTuple] <: Tuple = X match {
-    case _ *: xs => xs
+    case x *: xs => xs
   }
 
   /** Type of the last element of a tuple */
   type Last[X <: Tuple] = X match {
     case x *: EmptyTuple => x
-    case _ *: xs => Last[xs]
+    case x *: xs => Last[xs]
   }
 
   /** Type of the concatenation of two tuples */
@@ -182,8 +182,8 @@ object Tuple {
    */
   type Zip[T1 <: Tuple, T2 <: Tuple] <: Tuple = (T1, T2) match {
     case (h1 *: t1, h2 *: t2) => (h1, h2) *: Zip[t1, t2]
-    case (EmptyTuple, _) => EmptyTuple
-    case (_, EmptyTuple) => EmptyTuple
+    case (EmptyTuple, ?) => EmptyTuple
+    case (?, EmptyTuple) => EmptyTuple
     case _ => Tuple
   }
 

library/src/scala/runtime/stdLibPatches/language.scala

@@ -114,6 +114,10 @@ object language:
      */
     @compileTimeOnly("`symbolLiterals` can only be used at compile time in import statements")
     object symbolLiterals
+
+    /** TODO */
+    @compileTimeOnly("`underscoreWildcards` can only be used at compile time in import statements")
+    object underscoreWildcards
   end deprecated
 
   /** Where imported, auto-tupling is disabled.

project/Build.scala

@@ -1008,7 +1008,10 @@ object Build {
         Seq("-sourcepath", ((Compile/sourceManaged).value / "scala-library-src").toString)
       },
       Compile / doc / scalacOptions += "-Ydocument-synthetic-types",
-      scalacOptions += "-Ycompile-scala2-library",
+      scalacOptions ++= Seq(
+        "-Ycompile-scala2-library",
+        "-language:deprecated.underscoreWildcards",
+      ),
       scalacOptions -= "-Xfatal-warnings",
       ivyConfigurations += SourceDeps.hide,
       transitiveClassifiers := Seq("sources"),
@@ -1292,6 +1295,7 @@ object Build {
           mtagsSharedSources
         } (Set(mtagsSharedSourceJar)).toSeq
       }.taskValue,
+      scalacOptions += "-language:deprecated.underscoreWildcards",
     )
   }
 
@@ -1361,6 +1365,7 @@ object Build {
     dependsOn(`scala3-library-bootstrappedJS`).
     settings(
       bspEnabled := false,
+      scalacOptions += "-language:deprecated.underscoreWildcards",
       scalacOptions --= Seq("-Xfatal-warnings", "-deprecation"),
 
       // Required to run Scala.js tests.

sbt-test/compilerReporter/i14576/Test.scala

@@ -12,6 +12,3 @@ object Test:
 
   // private[this] and = _ are deprecated under -source:future
   private[this] var x: AnyRef = _
-
-  // under -source:future, `_` is deprecated for wildcard arguments of types: use `?` instead
-  val xs: List[_] = Nil

sbt-test/compilerReporter/i14576/build.sbt

@@ -24,7 +24,7 @@ lazy val root = (project in file("."))
     },
     assertDeprecationSummary := {
       assert {
-        FakePrintWriter.messages.exists(_.contains("there were 3 deprecation warnings; re-run with -deprecation for details"))
+        FakePrintWriter.messages.exists(_.contains("there were 2 deprecation warnings; re-run with -deprecation for details"))
       }
     },
     assertNoDeprecationSummary := {

sbt-test/scala2-compat/erasure/dottyApp/Api.scala

@@ -156,41 +156,41 @@ class Z {
   def int_63(x: AnyVal with Int): Unit = {}
 
   def intARRAY_64(x: Array[Int with Singleton]): Unit = {}
-  def intARRAY_65(x: Array[_ <: Int]): Unit = {}
-  def intARRAY_66(x: Array[_ <: Int with Singleton]): Unit = {}
-  def intARRAY_67(x: Array[_ <: Singleton with Int]): Unit = {}
-  def intARRAY_68(x: Array[_ <: Int with Any]): Unit = {}
-  def intARRAY_69(x: Array[_ <: Any with Int]): Unit = {}
-  def intARRAY_70(x: Array[_ <: Int with AnyVal]): Unit = {}
-  def intARRAY_71(x: Array[_ <: AnyVal with Int]): Unit = {}
-  def intARRAY_71a(x: Array[_ <: Int | Int]): Unit = {}
-  def intARRAY_71b(x: Array[_ <: 1 | 2]): Unit = {}
+  def intARRAY_65(x: Array[? <: Int]): Unit = {}
+  def intARRAY_66(x: Array[? <: Int with Singleton]): Unit = {}
+  def intARRAY_67(x: Array[? <: Singleton with Int]): Unit = {}
+  def intARRAY_68(x: Array[? <: Int with Any]): Unit = {}
+  def intARRAY_69(x: Array[? <: Any with Int]): Unit = {}
+  def intARRAY_70(x: Array[? <: Int with AnyVal]): Unit = {}
+  def intARRAY_71(x: Array[? <: AnyVal with Int]): Unit = {}
+  def intARRAY_71a(x: Array[? <: Int | Int]): Unit = {}
+  def intARRAY_71b(x: Array[? <: 1 | 2]): Unit = {}
 
   def stringARRAY_72(x: Array[String with Singleton]): Unit = {}
-  def stringARRAY_73(x: Array[_ <: String]): Unit = {}
-  def stringARRAY_74(x: Array[_ <: String with Singleton]): Unit = {}
-  def stringARRAY_75(x: Array[_ <: Singleton with String]): Unit = {}
-  def stringARRAY_76(x: Array[_ <: String with Any]): Unit = {}
-  def stringARRAY_77(x: Array[_ <: Any with String]): Unit = {}
-  def stringARRAY_78(x: Array[_ <: String with AnyRef]): Unit = {}
-  def stringARRAY_79(x: Array[_ <: AnyRef with String]): Unit = {}
-  def stringARRAY_79a(x: Array[_ <: String | String]): Unit = {}
-  def stringARRAY_79b(x: Array[_ <: "a" | "b"]): Unit = {}
-
-  def object_80(x: Array[_ <: Singleton]): Unit = {}
-  def object_81(x: Array[_ <: AnyVal]): Unit = {}
-  def objectARRAY_82(x: Array[_ <: AnyRef]): Unit = {}
-  def object_83(x: Array[_ <: Any]): Unit = {}
-  def object_83a(x: Array[_ <: Matchable]): Unit = {}
-  def object_83b(x: Array[_ <: Int | Double]): Unit = {}
-  def object_83c(x: Array[_ <: String | Int]): Unit = {}
-  def object_83d(x: Array[_ <: Int | Matchable]): Unit = {}
-  def object_83e(x: Array[_ <: AnyRef | AnyVal]): Unit = {}
-
-  def serializableARRAY_84(x: Array[_ <: Serializable]): Unit = {}
-  def univARRAY_85(x: Array[_ <: Univ]): Unit = {}
-  def aARRAY_86(x: Array[_ <: A]): Unit = {}
-  def aARRAY_87(x: Array[_ <: A with B]): Unit = {}
+  def stringARRAY_73(x: Array[? <: String]): Unit = {}
+  def stringARRAY_74(x: Array[? <: String with Singleton]): Unit = {}
+  def stringARRAY_75(x: Array[? <: Singleton with String]): Unit = {}
+  def stringARRAY_76(x: Array[? <: String with Any]): Unit = {}
+  def stringARRAY_77(x: Array[? <: Any with String]): Unit = {}
+  def stringARRAY_78(x: Array[? <: String with AnyRef]): Unit = {}
+  def stringARRAY_79(x: Array[? <: AnyRef with String]): Unit = {}
+  def stringARRAY_79a(x: Array[? <: String | String]): Unit = {}
+  def stringARRAY_79b(x: Array[? <: "a" | "b"]): Unit = {}
+
+  def object_80(x: Array[? <: Singleton]): Unit = {}
+  def object_81(x: Array[? <: AnyVal]): Unit = {}
+  def objectARRAY_82(x: Array[? <: AnyRef]): Unit = {}
+  def object_83(x: Array[? <: Any]): Unit = {}
+  def object_83a(x: Array[? <: Matchable]): Unit = {}
+  def object_83b(x: Array[? <: Int | Double]): Unit = {}
+  def object_83c(x: Array[? <: String | Int]): Unit = {}
+  def object_83d(x: Array[? <: Int | Matchable]): Unit = {}
+  def object_83e(x: Array[? <: AnyRef | AnyVal]): Unit = {}
+
+  def serializableARRAY_84(x: Array[? <: Serializable]): Unit = {}
+  def univARRAY_85(x: Array[? <: Univ]): Unit = {}
+  def aARRAY_86(x: Array[? <: A]): Unit = {}
+  def aARRAY_87(x: Array[? <: A with B]): Unit = {}
 
   def objectARRAY_88(x: Array[Any]): Unit = {}
   def objectARRAY_89(x: Array[AnyRef]): Unit = {}

sbt-test/scala2-compat/i11173/app/App.scala

@@ -1,3 +1,3 @@
 import i11173.Bar
 
-def test(x: Bar[_]): Unit = ()
+def test(x: Bar[?]): Unit = ()

scaladoc-testcases/src/tests/exports1.scala

@@ -16,7 +16,7 @@ class A: //unexpected
    = 1
   type HKT[T[_], X] //expected: final type HKT = [T[_], X] =>> a.HKT[T, X]
    = T[X]
-  type SomeRandomType = (List[_] | Seq[_]) & String //expected: final type SomeRandomType = a.SomeRandomType
+  type SomeRandomType = (List[?] | Seq[?]) & String //expected: final type SomeRandomType = a.SomeRandomType
   def x[T[_], X](x: X): HKT[T, X] //expected: def x[T[_], X](x: X): A.this.HKT[T, X]
    = ???
   def fn[T, U]: T => U

scaladoc-testcases/src/tests/hkts.scala

@@ -46,7 +46,7 @@ trait Case14[C[_]]
 class SomeClass extends Case14[List]
 
 
-def method1[E, T](value: List[_ >: E]): Int = 0
+def method1[E, T](value: List[? >: E]): Int = 0
 def method2[F[+X] <: Option[X], A](fa: F[A]): A = fa.get
 
 import scala.collection.immutable.ArraySeq

scaladoc-testcases/src/tests/snippetTestcase2.scala

@@ -2,7 +2,7 @@ package tests
 package snippetTestcase2
 
 trait Quotes2[A] {
-  val r1: r1Module[_] = ???
+  val r1: r1Module[?] = ???
   trait r1Module[A] {
     type X
     object Y {

scaladoc/src/dotty/tools/scaladoc/ScaladocSettings.scala

@@ -133,5 +133,5 @@ class ScaladocSettings extends SettingGroup with AllScalaSettings:
       "List of quick links that is displayed in the header of documentation."
     )
 
-  def scaladocSpecificSettings: Set[Setting[_]] =
+  def scaladocSpecificSettings: Set[Setting[?]] =
     Set(sourceLinks, legacySourceLink, syntax, revision, externalDocumentationMappings, socialLinks, skipById, skipByRegex, deprecatedSkipPackages, docRootContent, snippetCompiler, generateInkuire, defaultTemplate, scastieConfiguration, quickLinks)

scaladoc/src/dotty/tools/scaladoc/site/StaticSiteLoader.scala

@@ -144,7 +144,7 @@ class StaticSiteLoader(val root: File, val args: Scaladoc.Args)(using StaticSite
             (("1900","01","01"), name)
 
       def dateFrom(tf: TemplateFile, default: String = "1900-01-01"): String =
-        val pageSettings = tf.settings.get("page").collect{ case m: Map[String @unchecked, _] => m }
+        val pageSettings = tf.settings.get("page").collect{ case m: Map[String @unchecked, ?] => m }
         pageSettings.flatMap(_.get("date").collect{ case s: String => s}).getOrElse(default) // blogs without date are last
 
       val posts = List(rootPath.resolve("_posts"))

scaladoc/src/dotty/tools/scaladoc/site/common.scala

@@ -94,7 +94,7 @@ def loadTemplateFile(file: File, defaultTitle: Option[TemplateName] = None)(usin
   }.map(_.stripPrefix("\"").stripSuffix("\""))
 
   def listSetting(settings: Map[String, Object], name: String): Option[List[String]] = settings.get(name).map {
-    case elems: List[_] => elems.zipWithIndex.map {
+    case elems: List[?] => elems.zipWithIndex.map {
       case (s: String, _) => s
       case (other, index) =>
         throw new RuntimeException(s"Expected a string at index $index for $name in $file but got $other")

scaladoc/src/dotty/tools/scaladoc/site/templates.scala

@@ -103,10 +103,10 @@ case class TemplateFile(
     )
 
     def asJavaElement(o: Object): Object = o match
-      case m: Map[_, _] => m.transform {
+      case m: Map[?, ?] => m.transform {
         case (k: String, v: Object) => asJavaElement(v)
       }.asJava
-      case l: List[_] => l.map(x => asJavaElement(x.asInstanceOf[Object])).asJava
+      case l: List[?] => l.map(x => asJavaElement(x.asInstanceOf[Object])).asJava
       case other => other
 
     // Library requires mutable maps..

scaladoc/src/dotty/tools/scaladoc/snippets/SnippetCompiler.scala

@@ -21,7 +21,7 @@ import dotty.tools.dotc.util.{ SourcePosition, NoSourcePosition, SourceFile, NoS
 import scala.util.{ Try, Success, Failure }
 
 class SnippetCompiler(
-  val snippetCompilerSettings: Seq[SnippetCompilerSetting[_]],
+  val snippetCompilerSettings: Seq[SnippetCompilerSetting[?]],
   target: AbstractFile = new VirtualDirectory("(memory)")
 ):
   object SnippetDriver extends Driver:

scaladoc/src/dotty/tools/scaladoc/tasty/comments/markdown/DocFlexmarkExtension.scala

@@ -72,7 +72,7 @@ case class DocFlexmarkRenderer(renderLink: (DocLink, String) => String)
         html.raw(renderLink(node.target, node.body))
 
     object Render extends NodeRenderer:
-      override def getNodeRenderingHandlers: JSet[NodeRenderingHandler[_]] =
+      override def getNodeRenderingHandlers: JSet[NodeRenderingHandler[?]] =
         JSet(
           new NodeRenderingHandler(classOf[DocLinkNode], Handler),
         )

scaladoc/src/dotty/tools/scaladoc/tasty/comments/markdown/SectionRenderingExtension.scala

@@ -59,7 +59,7 @@ object SectionRenderingExtension extends HtmlRenderer.HtmlRendererExtension:
 
 
   object Render extends NodeRenderer:
-    override def getNodeRenderingHandlers: JSet[NodeRenderingHandler[_]] =
+    override def getNodeRenderingHandlers: JSet[NodeRenderingHandler[?]] =
       JSet(
         new NodeRenderingHandler(classOf[Section], SectionHandler),
         new NodeRenderingHandler(classOf[AnchorLink], AnchorLinkHandler)

scaladoc/src/dotty/tools/scaladoc/tasty/comments/markdown/SnippetRenderingExtension.scala

@@ -35,7 +35,7 @@ object SnippetRenderingExtension extends HtmlRenderer.HtmlRendererExtension:
       html.raw(SnippetRenderer.renderSnippet(node.getContentChars.toString, node.getInfo.toString.split(" ").headOption))
 
   object Render extends NodeRenderer:
-    override def getNodeRenderingHandlers: JSet[NodeRenderingHandler[_]] =
+    override def getNodeRenderingHandlers: JSet[NodeRenderingHandler[?]] =
       JSet(
         new NodeRenderingHandler(classOf[ExtendedFencedCodeBlock], ExtendedFencedCodeBlockHandler),
         new NodeRenderingHandler(classOf[FencedCodeBlock], FencedCodeBlockHandler)

staging/src/scala/quoted/staging/ExprCompilationUnit.scala

@@ -5,4 +5,4 @@ import dotty.tools.dotc.CompilationUnit
 import dotty.tools.dotc.util.NoSource
 
 /** Compilation unit containing the contents of a quoted expression */
-private class ExprCompilationUnit(val exprBuilder: Quotes => Expr[_]) extends CompilationUnit(NoSource)
+private class ExprCompilationUnit(val exprBuilder: Quotes => Expr[?]) extends CompilationUnit(NoSource)

staging/src/scala/quoted/staging/QuoteCompiler.scala

@@ -114,7 +114,7 @@ private class QuoteCompiler extends Compiler:
     /** Unpickle and optionally compile the expression.
      *  Returns either `Left` with name of the classfile generated or `Right` with the value contained in the expression.
      */
-    def compileExpr(exprBuilder:  Quotes => Expr[_]): Either[String, Any] =
+    def compileExpr(exprBuilder:  Quotes => Expr[?]): Either[String, Any] =
       val units = new ExprCompilationUnit(exprBuilder) :: Nil
       compileUnits(units)
       result

tests/bench/inductive-implicits.scala

@@ -25,7 +25,7 @@ package shapeless {
     def ::[HH](h : HH) : HH :: H :: T = shapeless.::(h, this)
 
     override def toString = head match {
-      case _: ::[_, _] => "("+head.toString+") :: "+tail.toString
+      case _: ::[?, ?] => "("+head.toString+") :: "+tail.toString
       case _ => head.toString+" :: "+tail.toString
     }
   }

tests/explicit-nulls/pos/array.scala

@@ -5,7 +5,7 @@ class Foo {
 
   def test = {
     // accept any array of string
-    def f(xs: Array[_ >: String <: String | Null] | Null): Unit = ???
+    def f(xs: Array[? >: String <: String | Null] | Null): Unit = ???
 
     val a1: Array[String] = ???
     val a2: Array[String] | Null = ???

tests/explicit-nulls/pos/flow-inline.scala

@@ -7,8 +7,8 @@ class TreeOps {
   abstract class Tree[A, B](val key: A, val value: B)
   class RedTree[A, B](override val key: A, override val value: B) extends Tree[A, B](key, value)
 
-  private transparent inline def isRedTree(tree: Tree[_, _] | Null) =
-    (tree != null) && tree.isInstanceOf[RedTree[_, _]]
+  private transparent inline def isRedTree(tree: Tree[?, ?] | Null) =
+    (tree != null) && tree.isInstanceOf[RedTree[?, ?]]
 
   def foo[A, B](tree: Tree[A, B] | Null): Unit = {
     if (isRedTree(tree)) {

tests/generic-java-signatures/arrayBound.scala

@@ -1,16 +1,16 @@
 class Foo[
-  T <: Array[_],
+  T <: Array[?],
   U <: Array[T],
   V <: java.util.List[Array[T]],
-  W <: java.util.List[_ <: java.util.Date],
-  X <: java.util.HashMap[Array[_], java.util.ArrayList[_ <: java.util.Date]],
+  W <: java.util.List[? <: java.util.Date],
+  X <: java.util.HashMap[Array[?], java.util.ArrayList[? <: java.util.Date]],
   T1,
   U1 <: Array[T1],
   V1 <: Array[Array[T1]]
 ]
 object Test {
   def main(args: Array[String]): Unit = {
-    val tParams = classOf[Foo[_, _, _, _, _, _, _, _]].getTypeParameters()
+    val tParams = classOf[Foo[?, ?, ?, ?, ?, ?, ?, ?]].getTypeParameters()
     tParams.foreach { tp =>
       println(tp.getName + " <: " + tp.getBounds.map(_.getTypeName).mkString(", "))
     }