Make private[this] a migration warning

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

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

@@ -3113,15 +3113,20 @@ object Parsers {
       if (in.token == LBRACKET) {
         if (mods.is(Local) || mods.hasPrivateWithin)
           syntaxError(DuplicatePrivateProtectedQualifier())
-        inBrackets {
+        val startOffset = in.offset
+        val mods1 = inBrackets {
           if in.token == THIS then
-            if sourceVersion.isAtLeast(future) then
-              deprecationWarning(
-                em"The [this] qualifier will be deprecated in the future; it should be dropped.")
             in.nextToken()
             mods | Local
           else mods.withPrivateWithin(ident().toTypeName)
         }
+        if mods1.is(Local) then
+          report.errorOrMigrationWarning(
+              em"The [this] qualifier will be deprecated in the future; it should be dropped.${rewriteNotice(`future-migration`)}",
+              in.sourcePos(), from = future)
+          if sourceVersion == `future-migration` then
+              patch(source, Span(startOffset, in.lastOffset), "")
+        mods1
       }
       else mods
 

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -60,6 +60,7 @@ class CompilationTests {
       compileFile("tests/rewrites/rewrites.scala", defaultOptions.and("-source", "3.0-migration").and("-rewrite", "-indent")),
       compileFile("tests/rewrites/rewrites3x.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/rewrites3x-fatal-warnings.scala", defaultOptions.and("-rewrite", "-source", "future-migration", "-Xfatal-warnings")),
+      compileFile("tests/rewrites/private-this.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/filtering-fors.scala", defaultOptions.and("-rewrite", "-source", "3.2-migration")),
       compileFile("tests/rewrites/refutable-pattern-bindings.scala", defaultOptions.and("-rewrite", "-source", "3.2-migration")),
       compileFile("tests/rewrites/i8982.scala", defaultOptions.and("-indent", "-rewrite")),

sbt-test/compilerReporter/i14576/build.sbt

@@ -10,7 +10,7 @@ lazy val resetMessages = taskKey[Unit]("empties the messages list")
 
 lazy val root = (project in file("."))
   .settings(
-    scalacOptions += "-source:future",
+    scalacOptions += "-source:future-migration",
     extraAppenders := { s => Seq(ConsoleAppender(FakePrintWriter)) },
     assertFeatureSummary := {
       assert {
@@ -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 := {

tests/neg/private-this-future-migration.check

@@ -0,0 +1,10 @@
+-- Error: tests/neg/private-this-future-migration.scala:6:16 -----------------------------------------------------------
+6 |  private[this] def foo: Int = ??? // error
+  |                ^
+  |                The [this] qualifier will be deprecated in the future; it should be dropped.
+  |                This construct can be rewritten automatically under -rewrite -source future-migration.
+-- Error: tests/neg/private-this-future-migration.scala:7:18 -----------------------------------------------------------
+7 |  protected[this] def bar: Int = ??? // error
+  |                  ^
+  |                  The [this] qualifier will be deprecated in the future; it should be dropped.
+  |                  This construct can be rewritten automatically under -rewrite -source future-migration.

tests/neg/private-this-future-migration.scala

@@ -0,0 +1,7 @@
+//> using options -Werror
+
+import scala.language.`future-migration`
+
+class Foo:
+  private[this] def foo: Int = ??? // error
+  protected[this] def bar: Int = ??? // error

tests/neg/private-this-future.scala

@@ -0,0 +1,5 @@
+import scala.language.future
+
+class Foo:
+  private[this] def foo: Int = ??? // error
+  protected[this] def bar: Int = ??? // error

tests/pos/private-this-future-migration.scala

@@ -0,0 +1,5 @@
+import scala.language.`future-migration`
+
+class Foo:
+  private[this] def foo: Int = ??? // warn
+  protected[this] def bar: Int = ??? // warn

tests/pos/private-this.scala

@@ -0,0 +1,3 @@
+class Foo:
+  private[this] def foo: Int = ???
+  protected[this] def bar: Int = ???

tests/rewrites/private-this.check

@@ -0,0 +1,12 @@
+class Foo:
+  private def foo1: Int = ???
+  private def foo2: Int = ???
+  private def foo3: Int = ???
+  private def foo4: Int = ???
+  private  def foo5: Int = ???
+
+  protected def bar1: Int = ???
+  protected def bar2: Int = ???
+  protected def bar3: Int = ???
+  protected def bar4: Int = ???
+  protected  def bar5: Int = ???

tests/rewrites/private-this.scala

@@ -0,0 +1,12 @@
+class Foo:
+  private[this] def foo1: Int = ???
+  private[ this] def foo2: Int = ???
+  private[this ] def foo3: Int = ???
+  private[ this ] def foo4: Int = ???
+  private [this] def foo5: Int = ???
+
+  protected[this] def bar1: Int = ???
+  protected[ this] def bar2: Int = ???
+  protected[this ] def bar3: Int = ???
+  protected[ this ] def bar4: Int = ???
+  protected [this] def bar5: Int = ???