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[?] = ???
```

Author: nicolasstucki

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"))