Fix redundancy (unreachability) warning (#16179)

Author: KacperFKorban

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -534,16 +534,15 @@ class SpaceEngine(using Context) extends SpaceLogic {
     val mt: MethodType = unapp.widen match {
       case mt: MethodType => mt
       case pt: PolyType   =>
-        inContext(ctx.fresh.setExploreTyperState()) {
           val tvars = pt.paramInfos.map(newTypeVar(_))
           val mt = pt.instantiate(tvars).asInstanceOf[MethodType]
           scrutineeTp <:< mt.paramInfos(0)
           // force type inference to infer a narrower type: could be singleton
           // see tests/patmat/i4227.scala
           mt.paramInfos(0) <:< scrutineeTp
+          instantiateSelected(mt, tvars)
           isFullyDefined(mt, ForceDegree.all)
           mt
-        }
     }
 
     // Case unapply:

tests/pos/i16123.scala

@@ -0,0 +1,12 @@
+// scalac: -Werror
+sealed trait Nat
+case class Zero() extends Nat
+case class Succ[N <: Nat](n: N) extends Nat
+
+class Test:
+  def foo(n: Nat): Int = n match
+    case Zero() => 0
+    case Succ(Zero()) => 1
+    case _ => 2 // was: warning for this line
+
+  def test = foo(Succ(Succ(Zero()))) // evaluates to 2