Disallow _ for wildcard arguments of types and use ? in future

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

Author: nicolasstucki

Diff for: compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1856,8 +1856,10 @@ 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")
+          report.errorOrMigrationWarning(
+            em"`_` is deprecated for wildcard arguments of types: use `?` instead${rewriteNotice(`future-migration`)}",
+            in.sourcePos(), from = future)
+          if sourceVersion == `future-migration` then
             patch(source, Span(in.offset, in.offset + 1), "?")
           val start = in.skipToken()
           typeBounds().withSpan(Span(start, in.lastOffset, start))

Diff for: 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

Diff for: 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 := {

Diff for: tests/neg/wildcard-type-syntax-future-migration.check

@@ -0,0 +1,5 @@
+-- Error: tests/neg/wildcard-type-syntax-future-migration.scala:7:17 ---------------------------------------------------
+7 |    case _: List[_] => // error
+  |                 ^
+  |                 `_` is deprecated for wildcard arguments of types: use `?` instead
+  |                 This construct can be rewritten automatically under -rewrite -source future-migration.

Diff for: tests/neg/wildcard-type-syntax-future-migration.scala

@@ -0,0 +1,8 @@
+//> using options -Werror
+
+import scala.language.`future-migration`
+
+def test =
+  Seq() match
+    case _: List[_] => // error
+    case _: Seq[?] =>

Diff for: tests/neg/wildcard-type-syntax-future.scala

@@ -0,0 +1,6 @@
+import scala.language.future
+
+def test =
+  Seq() match
+    case _: List[_] => // error
+    case _: Seq[?] =>

Diff for: tests/pos/wildcard-type-syntax-future-migration.scala

@@ -0,0 +1,6 @@
+import scala.language.`future-migration`
+
+def test =
+  Seq() match
+    case _: List[_] => // warn
+    case _: Seq[?] =>

Diff for: tests/pos/wildcard-type-syntax.scala

@@ -0,0 +1,6 @@
+//> using options -Werror
+
+def test =
+  Seq() match
+    case _: List[_] => // error
+    case _: Seq[?] =>