Fix ClassCastException in patmat when using Product1.

Author: DarkDimius

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

@@ -1443,7 +1443,7 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
       // require (nbSubPats > 0 && (!lastIsStar || isSeq))
       protected def subPatRefs(binder: Symbol): List[Tree] = {
         val refs = if (totalArity > 0 && isSeq) subPatRefsSeq(binder)
-        else if (totalArity > 1 && !isSeq) productElemsToN(binder, totalArity)
+        else if (defn.isProductSubType(binder.info) && !isSeq) productElemsToN(binder, totalArity)
         else ref(binder):: Nil
         refs
       }

tests/run/value-class-extractor.scala

@@ -6,7 +6,7 @@ object NonNullChar {
   @inline final val None = new NonNullChar(0.toChar)
 }
 
-final class SomeProduct /*extends Product3[String, Int, List[String]]*/ {
+final class SomeProduct extends Product3[String, Int, List[String]] {
   def canEqual(x: Any) = x.isInstanceOf[SomeProduct]
   def _1 = "abc"
   def _2 = 5