Handle 2.13 specially for migration warnings and errors

Move 2.13 back to its original place in front of future, but disable all
migration warnings and errors for it.

Author: odersky

compiler/src/dotty/tools/dotc/config/SourceVersion.scala

@@ -8,7 +8,6 @@ import Feature.isPreviewEnabled
 import util.Property
 
 enum SourceVersion:
-  case `2.13`
 
   case `3.0-migration`, `3.0`
   case `3.1-migration`, `3.1`
@@ -22,6 +21,7 @@ enum SourceVersion:
   case `3.9-migration`, `3.9`
   // Add 3.x-migration and 3.x here
   // !!! Keep in sync with scala.runtime.stdlibPatches.language !!!
+  case `2.13`
   case `future-migration`, `future`
 
   case `never`    // needed for MigrationVersion.errorFrom if we never want to issue an error

compiler/src/dotty/tools/dotc/report.scala

@@ -98,10 +98,12 @@ object report:
     ctx.reporter.report(new Error(fullMsg, NoSourcePosition))
 
   def errorOrMigrationWarning(msg: Message, pos: SrcPos, migrationVersion: MigrationVersion)(using Context): Unit =
-    if sourceVersion.isAtLeast(migrationVersion.errorFrom) then
-      if sourceVersion != migrationVersion.errorFrom.prevMigrating then error(msg, pos)
-      else if ctx.settings.rewrite.value.isEmpty then migrationWarning(msg, pos)
-    else if sourceVersion.isAtLeast(migrationVersion.warnFrom) then warning(msg, pos)
+    if sourceVersion != SourceVersion.`2.13` then
+      // ignore errors or warningsfor Scala 2 stdlib sources
+      if sourceVersion.isAtLeast(migrationVersion.errorFrom) then
+        if sourceVersion != migrationVersion.errorFrom.prevMigrating then error(msg, pos)
+        else if ctx.settings.rewrite.value.isEmpty then migrationWarning(msg, pos)
+      else if sourceVersion.isAtLeast(migrationVersion.warnFrom) then warning(msg, pos)
 
   def restrictionError(msg: Message, pos: SrcPos = NoSourcePosition)(using Context): Unit =
     error(msg.mapMsg("Implementation restriction: " + _), pos)