Deprecation warnings for old syntax: _ type wildcards (#18813)

Based on #18887
First part of #18869

* In `3.4` we emit the deprecation warning and enable the patch with
`-rewrite`.
* In `future` we emit we make this syntax an error

```scala
//> using options -source future
def f: F[_] = ??? // error
```
```diff
//> using options -rewrite -source 3.4-migration
- def f: F[_] = ???
+ def f: F[?] = ???
```

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

@@ -1870,9 +1870,15 @@ object Parsers {
           val start = in.skipToken()
           Ident(tpnme.USCOREkw).withSpan(Span(start, in.lastOffset, start))
         else
-          if !inTypeMatchPattern && sourceVersion.isAtLeast(future) then
-            deprecationWarning(em"`_` is deprecated for wildcard arguments of types: use `?` instead")
-            patch(source, Span(in.offset, in.offset + 1), "?")
+          if !inTypeMatchPattern then
+            report.gradualErrorOrMigrationWarning(
+              em"`_` is deprecated for wildcard arguments of types: use `?` instead${rewriteNotice(`3.4-migration`)}",
+              in.sourcePos(),
+              warnFrom = `3.4`,
+              errorFrom = future)
+            if sourceVersion.isMigrating && sourceVersion.isAtLeast(`3.4-migration`) then
+              patch(source, Span(in.offset, in.offset + 1), "?")
+          end if
           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

@@ -1,3 +1,3 @@
-//> using options -source:future -deprecation
+//> using options -source:3.4-migration
 scala> type M[X] = X match { case Int => String case _ => Int }
 scala> type N[X] = X match { case List[_] => Int }

compiler/test-resources/repl/i6643

@@ -1,10 +1,10 @@
 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] = ??? }
 1 warning found
 -- [E003] Syntax Warning: ------------------------------------------------------
-1 | object IterableTest { def g[CC[_] <: Iterable[_] with IterableOps[_, _, _]](from: CC[Int]): IterableFactory[CC] = ??? }
+1 | object IterableTest { def g[CC[_] <: Iterable[?] with IterableOps[?, ?, ?]](from: CC[Int]): IterableFactory[CC] = ??? }
   |                                                  ^^^^
   |                with as a type operator has been deprecated; use & instead
   |

project/Build.scala

@@ -1266,8 +1266,8 @@ object Build {
       libraryDependencies += ("org.scalameta" % "mtags-shared_2.13.12" % mtagsVersion % SourceDeps),
       ivyConfigurations += SourceDeps.hide,
       transitiveClassifiers := Seq("sources"),
+      scalacOptions ++= Seq("-source", "3.3"), // To avoid fatal migration warnings
       Compile / scalacOptions ++= Seq("-Yexplicit-nulls", "-Ysafe-init"),
-      Compile / scalacOptions ++= Seq("-source", "3.3"), // To avoid fatal migration warnings
       Compile / sourceGenerators += Def.task {
         val s = streams.value
         val cacheDir = s.cacheDirectory

sbt-test/compilerReporter/i14576/Test.scala

@@ -10,5 +10,5 @@ object Test:
   def f(x: Text) = println(x.str)
   f("abc")
 
-  // under -source:future, `_` is deprecated for wildcard arguments of types: use `?` instead
-  val xs: List[_] = Nil
+  @deprecated("", "") def deprecatedFun(): Unit = ()
+  deprecatedFun()

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/init/pos/Properties.scala

@@ -15,7 +15,7 @@ import java.util.jar.Attributes.Name as AttributeName
 
 private[scala] trait PropertiesTrait {
   protected def propCategory: String      // specializes the remainder of the values
-  protected def pickJarBasedOn: Class[_]  // props file comes from jar containing this
+  protected def pickJarBasedOn: Class[?]  // props file comes from jar containing this
 
   /** The name of the properties file */
   protected val propFilename = "/" + propCategory + ".properties"

tests/neg-deep-subtype/i4297.scala

@@ -3,11 +3,11 @@
 class Test {
   def test[X <: Option[Int]](x: X) = x.isInstanceOf[Some[Int]]
   def test1[Y <: Int, X <: Option[Y]](x: X) = x.isInstanceOf[Some[Int]]
-  def test2(x: Any) = x.isInstanceOf[Function1[Nothing, _]]
-  def test3a(x: Any) = x.isInstanceOf[Function1[Any, _]] // error
-  def test3b(x: Any) = x.isInstanceOf[Function1[Int, _]] // error
-  def test4[Y <: Int, X <: Function1[Y, Unit]](x: X) = x.isInstanceOf[Function1[Int, _]] // error
+  def test2(x: Any) = x.isInstanceOf[Function1[Nothing, ?]]
+  def test3a(x: Any) = x.isInstanceOf[Function1[Any, ?]] // error
+  def test3b(x: Any) = x.isInstanceOf[Function1[Int, ?]] // error
+  def test4[Y <: Int, X <: Function1[Y, Unit]](x: X) = x.isInstanceOf[Function1[Int, ?]] // error
   def test5[Y <: Int, X <: Function1[Y, Unit]](x: X) = x.isInstanceOf[Function1[Int, Unit]]  // error
   def test6[Y <: Int, X <: Function1[Y, Unit]](x: X) = x.isInstanceOf[Function1[Int, Any]] // error
-  def test7[Y <: Int, X <: Function1[Y, Unit]](x: X) = x.isInstanceOf[Function1[_, Unit]]
+  def test7[Y <: Int, X <: Function1[Y, Unit]](x: X) = x.isInstanceOf[Function1[?, Unit]]
 }

tests/neg-deep-subtype/i5826b.scala

@@ -2,12 +2,12 @@
 
 class Foo {
   def test1[A]: List[Int] | A => Int = {
-    case ls: List[_] => ls.head       // error
+    case ls: List[?] => ls.head       // error
     case _ => 0
   }
 
   def test2[A]: List[Int] | A => Int = {
-    case ls: List[_] => ls.size
+    case ls: List[?] => ls.size
     case _ => 0
   }
 }

tests/neg-deep-subtype/or-type-trees.scala

@@ -14,7 +14,7 @@ object Test1 {
 
   def foo3(myTree: Tree | (Context => Tree)) =
     myTree match
-      case treeFn: (_ => _) =>       // ok
+      case treeFn: (? => ?) =>       // ok
       case _ =>
 }
 
@@ -35,6 +35,6 @@ object Test2 {
 
   def foo3(myTree: Tree[Type] | (Context => Tree[Type])) =
     myTree match
-      case treeFn: (_ => _) =>       // ok
+      case treeFn: (? => ?) =>       // ok
       case _ =>
 }

tests/neg-deep-subtype/t2755.scala

@@ -9,16 +9,16 @@ object Test {
     case x: Array[Float]    => x.sum.toInt
     case x: Array[String]   => x.size
     case x: Array[AnyRef]   => 5
-    case x: Array[_]        => 6
+    case x: Array[?]        => 6
     case _                  => 7
   }
-  def f2(a: Array[_])  = a match {
+  def f2(a: Array[?])  = a match {
     case x: Array[Int]      => x(0)
     case x: Array[Double]   => 2
     case x: Array[Float]    => x.sum.toInt
     case x: Array[String]   => x.size
     case x: Array[AnyRef]   => 5
-    case x: Array[_]        => 6
+    case x: Array[?]        => 6
     case _                  => 7   // error: only null is matched
   }
   def f3[T](a: Array[T]) = a match {
@@ -27,7 +27,7 @@ object Test {
     case x: Array[Float]    => x.sum.toInt
     case x: Array[String]   => x.size
     case x: Array[AnyRef]   => 5
-    case x: Array[_]        => 6
+    case x: Array[?]        => 6
     case _                  => 7   // error: only null is matched
   }
 

tests/neg/IsInstanceOfClassTag.scala

@@ -17,7 +17,7 @@ object IsInstanceOfClassTag {
         xs.head.substring(0)
     }
 
-    safeCast[List[_]](List[Int](1)) match {
+    safeCast[List[?]](List[Int](1)) match {
       case None =>
       case Some(xs) =>
         xs.head.substring(0) // error

tests/neg/IsInstanceOfClassTag2.scala

@@ -16,7 +16,7 @@ object IsInstanceOfClassTag {
       case Some(xs) =>
     }
 
-    safeCast[List[_]](List[Int](1)) match {
+    safeCast[List[?]](List[Int](1)) match {
       case None =>
       case Some(xs) =>
     }

tests/neg/i12284.scala

@@ -1,7 +1,7 @@
 trait I[F[_], A]
 
 def magic[F[_], A](in: I[F, A]): F[A] =
-  val deps: Vector[I[F, _]] = ???
+  val deps: Vector[I[F, ?]] = ???
   val xx = deps.map(i => magic(i))
   val y: Vector[F[Any]] = xx // error
   ???

tests/neg/i15662.scala

@@ -2,12 +2,12 @@
 
 case class Composite[T](v: T)
 
-def m(composite: Composite[_]): Unit =
+def m(composite: Composite[?]): Unit =
   composite match {
     case Composite[Int](v) => println(v)  // error: cannot be checked at runtime
   }
 
-def m2(composite: Composite[_]): Unit =
+def m2(composite: Composite[?]): Unit =
   composite match {
     case Composite(v) => println(v)  // ok
   }

tests/neg/i15893.scala

@@ -27,17 +27,17 @@ transparent inline def transparentInlineMod2(inline n: NatT):  NatT = inline n m
 def dependentlyTypedMod2[N <: NatT](n: N): Mod2[N] = n match
   case Zero(): Zero => Zero() // error
   case Succ(Zero()): Succ[Zero] => Succ(Zero()) // error
-  case Succ(Succ(predPredN)): Succ[Succ[_]] => dependentlyTypedMod2(predPredN) // error
+  case Succ(Succ(predPredN)): Succ[Succ[?]] => dependentlyTypedMod2(predPredN) // error
 
 inline def inlineDependentlyTypedMod2[N <: NatT](inline n: N): Mod2[N] = inline n match
   case Zero(): Zero => Zero() // error
   case Succ(Zero()): Succ[Zero] => Succ(Zero()) // error
-  case Succ(Succ(predPredN)): Succ[Succ[_]] => inlineDependentlyTypedMod2(predPredN) // error
+  case Succ(Succ(predPredN)): Succ[Succ[?]] => inlineDependentlyTypedMod2(predPredN) // error
 
 transparent inline def transparentInlineDependentlyTypedMod2[N <: NatT](inline n: N): Mod2[N] = inline n match
   case Zero(): Zero => Zero() // error
   case Succ(Zero()): Succ[Zero] => Succ(Zero()) // error
-  case Succ(Succ(predPredN)): Succ[Succ[_]] => transparentInlineDependentlyTypedMod2(predPredN) // error
+  case Succ(Succ(predPredN)): Succ[Succ[?]] => transparentInlineDependentlyTypedMod2(predPredN) // error
 
 def foo(n: NatT): NatT = mod2(n) match
   case Succ(Zero()) => Zero()

tests/neg/i18058.check

@@ -1,4 +1,4 @@
 -- Error: tests/neg/i18058.scala:4:21 ----------------------------------------------------------------------------------
-4 |type G = (f: _ <: F) => f.A // error
+4 |type G = (f: ? <: F) => f.A // error
   |                     ^
   |                     invalid new prefix  <: F cannot replace f.type in type f.A

tests/neg/i18058.scala

@@ -1,4 +1,4 @@
 trait F:
   type A
 
-type G = (f: _ <: F) => f.A // error
+type G = (f: ? <: F) => f.A // error

tests/neg/i4382.check

@@ -1,23 +1,23 @@
 -- [E043] Type Error: tests/neg/i4382.scala:3:10 -----------------------------------------------------------------------
-3 |  def v1: Id[_] = ??? // error
+3 |  def v1: Id[?] = ??? // error
   |          ^^^^^
   |          unreducible application of higher-kinded type App.Id to wildcard arguments
   |
   | longer explanation available when compiling with `-explain`
 -- [E043] Type Error: tests/neg/i4382.scala:6:10 -----------------------------------------------------------------------
-6 |  def v2: HkL[_] = ??? // error
+6 |  def v2: HkL[?] = ??? // error
   |          ^^^^^^
   |          unreducible application of higher-kinded type App.HkL to wildcard arguments
   |
   | longer explanation available when compiling with `-explain`
 -- [E043] Type Error: tests/neg/i4382.scala:9:10 -----------------------------------------------------------------------
-9 |  def v3: HkU[_] = ???  // error
+9 |  def v3: HkU[?] = ???  // error
   |          ^^^^^^
   |          unreducible application of higher-kinded type App.HkU to wildcard arguments
   |
   | longer explanation available when compiling with `-explain`
 -- [E043] Type Error: tests/neg/i4382.scala:12:10 ----------------------------------------------------------------------
-12 |  def v4: HkAbs[_] = ??? // error
+12 |  def v4: HkAbs[?] = ??? // error
    |          ^^^^^^^^
    |          unreducible application of higher-kinded type App.HkAbs to wildcard arguments
    |

tests/neg/i4382.scala

@@ -1,13 +1,13 @@
 object App {
   type Id[A] >: A <: A
-  def v1: Id[_] = ??? // error
+  def v1: Id[?] = ??? // error
 
   type HkL[A] >: A
-  def v2: HkL[_] = ??? // error
+  def v2: HkL[?] = ??? // error
 
   type HkU[A] <: A
-  def v3: HkU[_] = ???  // error
+  def v3: HkU[?] = ???  // error
 
   type HkAbs[A]
-  def v4: HkAbs[_] = ??? // error
+  def v4: HkAbs[?] = ??? // error
 }

tests/neg/i4986c.scala

@@ -57,7 +57,7 @@ object Test {
 
   implicitly[U[Int, Option, Map]] // error
 
-  val u = new U[String, List, [A, _] =>> List[Option[_]]] { }
+  val u = new U[String, List, [A, _] =>> List[Option[?]]] { }
   val i = new u.I[Int]
   i.m[Option[Long]] // error
 }

tests/neg/i5077.scala

@@ -7,7 +7,7 @@ case class C[A](is: Is[A], value: A)
 @main
 def Test = {
   val c_string: C[String] = C(IsString, "name")
-  val c_any: C[_] = c_string
+  val c_any: C[?] = c_string
   val any: Any = c_string
 
   // Case 1: error