Add a 3.6-migration warning for MT lubbing (#21336)

Fixes #21258, all the other parts having been addressed (reverting a
change in binary erasure, which shipped in patch releases)

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -40,7 +40,7 @@ import annotation.tailrec
 import Implicits.*
 import util.Stats.record
 import config.Printers.{gadts, typr}
-import config.Feature, Feature.{sourceVersion, migrateTo3, modularity}
+import config.Feature, Feature.{migrateTo3, modularity, sourceVersion, warnOnMigration}
 import config.SourceVersion.*
 import rewrites.Rewrites, Rewrites.patch
 import staging.StagingLevel
@@ -2615,7 +2615,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         if !acc.exists then NoType
         else if case1.body.tpe.isProvisional then NoType
         else acc | case1.body.tpe
-      if lub.exists then TypeTree(lub, inferred = true)
+      if lub.exists then
+        if !lub.isAny then
+          val msg = em"Match type upper bound inferred as $lub, where previously it was defaulted to Any"
+          warnOnMigration(msg, tree, `3.6`)
+        TypeTree(lub, inferred = true)
       else bound1
     else bound1
     assignType(cpy.MatchTypeTree(tree)(bound2, sel1, cases1), bound2, sel1, cases1)

tests/warn/i21258.check

@@ -0,0 +1,6 @@
+-- Migration Warning: tests/warn/i21258.scala:4:17 ---------------------------------------------------------------------
+4 |  type MT[X] = X match { // warn
+  |               ^
+  |               Match type upper bound inferred as String, where previously it was defaulted to Any
+5 |    case Int => String
+6 |  }

tests/warn/i21258.scala

@@ -0,0 +1,14 @@
+import scala.language.`3.6-migration`
+
+object Test {
+  type MT[X] = X match { // warn
+    case Int => String
+  }
+
+  def unboundUnreducibleSig[X](x: X): MT[X] = ???
+
+  type MT2[X] = X match { // no warning
+    case Int => String
+    case String => Any
+  }
+}