In compareMatch, check scrutinees are subtypes

From the Match Type paper, typing rule S-Match5 requires that the LHS
scrutinee "Ss" is a subtype of the RHS scrutinee "Ts": Ss <: Ts.  It's
not required that they are the same type.  This covers widening a skolem
or inline proxy tp1 scrutinee, without erroneously widening the RHS tp2
scrutinee.

Author: dwijnand

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

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

tests/neg/mt-scrutinee-widen2.scala

@@ -0,0 +1,13 @@
+// A test case showing how we shouldn't widen
+// both IsX scrutinees and make "def test" typecheck
+import scala.util.Random
+val x = 42
+
+type IsX[T] =
+    T match
+        case x.type => true
+        case _ => false
+
+def bothXOrNot(a: Int, b: Int)(using IsX[a.type] =:= IsX[b.type]) = ???
+
+def test = bothXOrNot(Random.nextInt(), Random.nextInt()) // error

tests/pos/mt-scrutinee-widen3.scala

@@ -0,0 +1,12 @@
+// Like widen2, but using a.type only, meaning it should typecheck
+import scala.util.Random
+val x = 42
+
+type IsX[T] =
+    T match
+        case x.type => true
+        case _ => false
+
+def bothXOrNot(a: Int, b: Int)(using IsX[a.type] =:= IsX[a.type]) = ???
+
+def test = bothXOrNot(Random.nextInt(), Random.nextInt())