Par back to only widening skolems and inline proxies

Author: dwijnand

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -1007,7 +1007,10 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       case tp1: MatchType =>
         def compareMatch = tp2 match {
           case tp2: MatchType =>
-            isSubType(tp1.scrutinee, tp2.scrutinee) &&
+            val scrutinee1 = tp1.scrutinee match
+              case tp: TermRef if tp.symbol.is(InlineProxy) => tp.info
+              case tp                                       => tp.widenSkolem
+            isSameType(scrutinee1, tp2.scrutinee) &&
             tp1.cases.corresponds(tp2.cases)(isSubType)
           case _ => false
         }

tests/neg/mt-subtyping-transitivity.scala

@@ -0,0 +1,14 @@
+final class A
+final class B
+
+type MT[X] = X match
+  case A => String
+  case B => Int
+
+def test: MT[A | B] = ??? : MT[A] // error
+// testing that
+//    MT[A]  !<: MT[A | B]
+// otherwise
+//    String <: MT[A] <: MT[A | B]
+// but
+//    String !<: MT[A | B]