Tighten requirements for migration mode

We now issue a migration warning only if the source version is migrating and
exactly one before the current source. Previously, a migration warning intended
for 3.0-migration would also have been issued for future-migration.

Also, fix error message for viewbounds.

Author: odersky

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

@@ -1824,7 +1824,7 @@ object Parsers {
         } :: contextBounds(pname)
       case VIEWBOUND =>
         report.errorOrMigrationWarning(
-          "view bounds `<%' are deprecated, use a context bound `:' instead",
+          "view bounds `<%' are no longer supported, use a context bound `:' instead",
           in.sourcePos())
         atSpan(in.skipToken()) {
           Function(Ident(pname) :: Nil, toplevelTyp())

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

@@ -78,7 +78,7 @@ object report:
   def errorOrMigrationWarning(msg: Message, pos: SrcPos = NoSourcePosition,
       from: SourceVersion = SourceVersion.defaultSourceVersion)(using Context): Unit =
     if sourceVersion.isAtLeast(from) then
-      if sourceVersion.isMigrating then migrationWarning(msg, pos)
+      if sourceVersion.ordinal < from.ordinal then migrationWarning(msg, pos)
       else error(msg, pos)
 
   def restrictionError(msg: Message, pos: SrcPos = NoSourcePosition)(using Context): Unit =

tests/neg/i11567.scala

@@ -0,0 +1,5 @@
+import language.`future-migration`
+class Test
+object Test {
+  def foo[A <% Test](x: A) = x  // error
+}