Fix a case of MT reduction using abstract types

The original motivation wasn't with a test case.  And reproducing it
is not convincing.  So I'd rather revert and fix this issue.

Author: dwijnand

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

@@ -1030,7 +1030,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             tp1.cases.corresponds(tp2.cases)(isSubType)
           case _ => false
         }
-        (!caseLambda.exists || canWidenAbstract) && recur(tp1.underlying, tp2) || compareMatch
+        recur(tp1.underlying, tp2) || compareMatch
       case tp1: AnnotatedType if tp1.isRefining =>
         isNewSubType(tp1.parent)
       case JavaArrayType(elem1) =>

compiler/test/dotc/pos-test-pickling.blacklist

@@ -53,6 +53,7 @@ i15827.scala
 i17149.scala
 tuple-fold.scala
 mt-redux-norm.perspective.scala
+i18202.scala
 
 # Opaque type
 i5720.scala

tests/pos/i18202.scala

@@ -0,0 +1,7 @@
+import compiletime.ops.int.*
+
+type TupleIndex[T <: Tuple, A, I <: Int] <: (Any, Int) = T match
+  case A *: _ => (A, I)
+  case _ *: t => TupleIndex[t, A, S[I]]
+
+def tupleIndex[T <: Tuple, A](using i: ValueOf[Tuple.Elem[TupleIndex[T, A, 0], 1]]): Int = i.value