Fix PreserveSubPatBinders not storing subparts that are used only for type tests.

Author: DarkDimius

src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -428,6 +428,7 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
 
       private lazy val (stored, substed) = (subPatBinders, subPatRefs).zipped.partition{ case (sym, _) => storedBinders(sym) }
 
+      // dd: this didn't yet trigger error. But I believe it would. if this causes double denition of symbol error this can be replaced with NoRebindings
       protected lazy val introducedRebindings:  Rebindings = if (!emitVars) Rebindings(subPatBinders, subPatRefs)
       else {
         val (subPatBindersSubstituted, subPatRefsSubstituted) = substed.unzip
@@ -948,7 +949,7 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
         case Bind(nme.WILDCARD, _)                => true // don't skip when binding an interesting symbol!
         case Ident(nme.WILDCARD)                  => true
         case Alternative(ps)                      => ps forall unapply
-        case Typed(PatternBoundToUnderscore(), _) => true
+        case Typed(PatternBoundToUnderscore(), _) => false // true // Dmitry: change in dotty. Type test will be performed and the field must be stored
         case _                                    => false
       }
     }