Disallow _ for wildcard arguments of types and use ?

compiler/src/dotty/tools/backend/jvm/ClassfileWriters.scala

@@ -186,7 +186,7 @@ class ClassfileWriters(frontendAccess: PostProcessorFrontendAccess) {
 
   private final class DirEntryWriter(base: Path) extends FileWriter {
     val builtPaths = new ConcurrentHashMap[Path, java.lang.Boolean]()
-    val noAttributes = Array.empty[FileAttribute[_]]
+    val noAttributes = Array.empty[FileAttribute[?]]
     private val isWindows = scala.util.Properties.isWin
 
     private def checkName(component: Path): Unit = if (isWindows) {

compiler/src/dotty/tools/backend/jvm/CoreBTypes.scala

@@ -138,7 +138,7 @@ abstract class CoreBTypesFromSymbols[I <: DottyBackendInterface] extends CoreBTy
   lazy val jlStringBuilderRef       : ClassBType = synchClassBTypeFromSymbol(requiredClass[java.lang.StringBuilder])
   lazy val jlStringBufferRef        : ClassBType = synchClassBTypeFromSymbol(requiredClass[java.lang.StringBuffer])
   lazy val jlCharSequenceRef        : ClassBType = synchClassBTypeFromSymbol(requiredClass[java.lang.CharSequence])
-  lazy val jlClassRef               : ClassBType = synchClassBTypeFromSymbol(requiredClass[java.lang.Class[_]])
+  lazy val jlClassRef               : ClassBType = synchClassBTypeFromSymbol(requiredClass[java.lang.Class[?]])
   lazy val jlThrowableRef           : ClassBType = synchClassBTypeFromSymbol(defn.ThrowableClass)
   lazy val jlCloneableRef           : ClassBType = synchClassBTypeFromSymbol(defn.JavaCloneableClass)
   lazy val jiSerializableRef        : ClassBType = synchClassBTypeFromSymbol(requiredClass[java.io.Serializable])

compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala

@@ -93,7 +93,7 @@ class DottyBackendInterface(val superCallsMap: ReadOnlyMap[Symbol, Set[ClassSymb
 
 object DottyBackendInterface {
 
-  private def erasureString(clazz: Class[_]): String = {
+  private def erasureString(clazz: Class[?]): String = {
     if (clazz.isArray) "Array[" + erasureString(clazz.getComponentType) + "]"
     else clazz.getName
   }

compiler/src/dotty/tools/backend/jvm/GenericSignatureVisitor.scala

@@ -315,7 +315,7 @@ abstract class NestedClassesCollector[T](nestedOnly: Boolean) extends GenericSig
     if (annot.values != null) annot.values.asScala foreach visitConstant
   }
 
-  def visitAnnotations(annots: java.util.List[_ <: AnnotationNode]) = if (annots != null) annots.asScala foreach visitAnnotation
+  def visitAnnotations(annots: java.util.List[? <: AnnotationNode]) = if (annots != null) annots.asScala foreach visitAnnotation
   def visitAnnotationss(annotss: Array[java.util.List[AnnotationNode]]) = if (annotss != null) annotss foreach visitAnnotations
 
   def visitHandle(handle: Handle): Unit = {

compiler/src/dotty/tools/backend/sjs/JSCodeGen.scala

@@ -1089,7 +1089,7 @@ class JSCodeGen()(using genCtx: Context) {
       val exports = List.newBuilder[jsExportsGen.Exported]
       val jsClassCaptures = List.newBuilder[js.ParamDef]
 
-      def add(tree: ConstructorTree[_ <: JSCtor]): Unit = {
+      def add(tree: ConstructorTree[? <: JSCtor]): Unit = {
         val (e, c) = genJSClassCtorDispatch(tree.ctor.sym,
             tree.ctor.paramsAndInfo, tree.overloadNum)
         exports += e
@@ -1270,7 +1270,7 @@ class JSCodeGen()(using genCtx: Context) {
      * here we use the property from building the trees, that a set of
      * descendants always has a range of overload numbers.
      */
-    def ifOverload(tree: ConstructorTree[_], body: js.Tree): js.Tree = body match {
+    def ifOverload(tree: ConstructorTree[?], body: js.Tree): js.Tree = body match {
       case js.Skip() => js.Skip()
 
       case body =>

compiler/src/dotty/tools/backend/sjs/ScopedVar.scala

@@ -28,7 +28,7 @@ object ScopedVar {
 
   implicit def toValue[T](scVar: ScopedVar[T]): T = scVar.get
 
-  def withScopedVars[T](ass: Assignment[_]*)(body: => T): T = {
+  def withScopedVars[T](ass: Assignment[?]*)(body: => T): T = {
     val stack = ass.map(_.push())
     try body
     finally stack.reverse.foreach(_.pop())

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -242,7 +242,7 @@ trait TreeInfo[T <: Untyped] { self: Trees.Instance[T] =>
 
   /** Does this list contain a named argument tree? */
   def hasNamedArg(args: List[Any]): Boolean = args exists isNamedArg
-  val isNamedArg: Any => Boolean = (arg: Any) => arg.isInstanceOf[Trees.NamedArg[_]]
+  val isNamedArg: Any => Boolean = (arg: Any) => arg.isInstanceOf[Trees.NamedArg[?]]
 
   /** Is this pattern node a catch-all (wildcard or variable) pattern? */
   def isDefaultCase(cdef: CaseDef): Boolean = cdef match {
@@ -1060,7 +1060,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
 
   def assertAllPositioned(tree: Tree)(using Context): Unit =
     tree.foreachSubTree {
-      case t: WithoutTypeOrPos[_] =>
+      case t: WithoutTypeOrPos[?] =>
       case t => assert(t.span.exists, i"$t")
     }
 

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -31,7 +31,7 @@ object Trees {
 
   /** Property key for backquoted identifiers and definitions */
   val Backquoted: Property.StickyKey[Unit] = Property.StickyKey()
-  
+
   val SyntheticUnit: Property.StickyKey[Unit] = Property.StickyKey()
 
   /** Trees take a parameter indicating what the type of their `tpe` field
@@ -932,11 +932,11 @@ object Trees {
     def rhs(using Context): Tree[T] = { forceFields(); preRhs.asInstanceOf[Tree[T]] }
 
     def leadingTypeParams(using Context): List[TypeDef[T]] = paramss match
-      case (tparams @ (tparam: TypeDef[_]) :: _) :: _ => tparams.asInstanceOf[List[TypeDef[T]]]
+      case (tparams @ (tparam: TypeDef[?]) :: _) :: _ => tparams.asInstanceOf[List[TypeDef[T]]]
       case _ => Nil
 
     def trailingParamss(using Context): List[ParamClause[T]] = paramss match
-      case ((tparam: TypeDef[_]) :: _) :: paramss1 => paramss1
+      case ((tparam: TypeDef[?]) :: _) :: paramss1 => paramss1
       case _ => paramss
 
     def termParamss(using Context): List[List[ValDef[T]]] =

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/inlines/Inliner.scala

@@ -649,13 +649,13 @@ class Inliner(val call: tpd.Tree)(using Context):
     def treeSize(x: Any): Int =
       var siz = 0
       x match
-        case x: Trees.Inlined[_] =>
+        case x: Trees.Inlined[?] =>
         case x: Positioned =>
           var i = 0
           while i < x.productArity do
             siz += treeSize(x.productElement(i))
             i += 1
-        case x: List[_] =>
+        case x: List[?] =>
           var xs = x
           while xs.nonEmpty do
             siz += treeSize(xs.head)

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/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -160,7 +160,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
       core ~ cs.optionalInfo
 
   private def toTextRetainedElem[T <: Untyped](ref: Tree[T]): Text = ref match
-    case ref: RefTree[_] if ref.typeOpt.exists =>
+    case ref: RefTree[?] if ref.typeOpt.exists =>
       toTextCaptureRef(ref.typeOpt)
     case Apply(fn, Literal(str) :: Nil) if fn.symbol == defn.Caps_capIn =>
       s"cap[${str.stringValue}]"

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2648,7 +2648,7 @@ class ExtensionCanOnlyHaveDefs(mdef: untpd.Tree)(using Context)
         |"""
 }
 
-class UnexpectedPatternForSummonFrom(tree: Tree[_])(using Context)
+class UnexpectedPatternForSummonFrom(tree: Tree[?])(using Context)
   extends SyntaxMsg(UnexpectedPatternForSummonFromID) {
   def msg(using Context) = i"Unexpected pattern for summonFrom. Expected ${hl("`x: T`")} or ${hl("`_`")}"
   def explain(using Context) =

compiler/src/dotty/tools/dotc/util/Stats.scala

@@ -32,7 +32,7 @@ import collection.mutable
       hits(name) += n
     }
 
-  def doRecordSize(fn: String, coll: scala.collection.Iterable[_]): coll.type =
+  def doRecordSize(fn: String, coll: scala.collection.Iterable[?]): coll.type =
     doRecord(fn, coll.size)
     coll
 

compiler/src/scala/quoted/runtime/impl/QuotesImpl.scala

@@ -1779,7 +1779,7 @@ class QuotesImpl private (using val ctx: Context) extends Quotes, QuoteUnpickler
 
         def show(using printer: Printer[TypeRepr]): String = printer.show(self)
 
-        def seal: scala.quoted.Type[_] = self.asType
+        def seal: scala.quoted.Type[?] = self.asType
 
         def asType: scala.quoted.Type[?] =
           new TypeImpl(Inferred(self), SpliceScope.getCurrent)

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

@@ -28,7 +28,7 @@ sealed trait AsmNode[+T] {
 object AsmNode {
   type AsmMethod = AsmNode[MethodNode]
   type AsmField = AsmNode[FieldNode]
-  type AsmMember = AsmNode[_]
+  type AsmMember = AsmNode[?]
 
   implicit class ClassNodeOps(val node: ClassNode) {
     def fieldsAndMethods: List[AsmMember] = {

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

@@ -202,7 +202,7 @@ trait DottyBytecodeTest {
     assert(succ, msg)
   }
 
-  private def sameCharacteristics(clazzA: ClassNode, clazzB: ClassNode)(f: AsmNode[_] => String): (Boolean, String) = {
+  private def sameCharacteristics(clazzA: ClassNode, clazzB: ClassNode)(f: AsmNode[?] => String): (Boolean, String) = {
     val ms1 = clazzA.fieldsAndMethods.toIndexedSeq
     val ms2 = clazzB.fieldsAndMethods.toIndexedSeq
     val name1 = clazzA.name
@@ -254,7 +254,7 @@ trait DottyBytecodeTest {
     }
     .getOrElse(fail("Could not find constructor for object `Test`"))
 
-  private def boxingError(ins: List[_], source: String) =
+  private def boxingError(ins: List[?], source: String) =
     s"""|----------------------------------
         |${ins.mkString("\n")}
         |----------------------------------
@@ -271,7 +271,7 @@ trait DottyBytecodeTest {
     }
     .getOrElse(fail("Could not find constructor for object `Test`"))
 
-  protected def boxingInstructions(method: MethodNode): (List[_], Boolean) = {
+  protected def boxingInstructions(method: MethodNode): (List[?], Boolean) = {
     val ins = instructionsFromMethod(method)
     val boxed = ins.exists {
       case Invoke(op, owner, name, desc, itf) =>