SI-6624 set info of case pattern binder to help find case field acces…

…sors

sometimes the type checker infers a weird type for a sub-pattern of a case class/extractor pattern
this confuses the pattern matcher and it can't find the case field accessors for the sub-pattern

use the expected argument type of the extractor corresponding to the case class that we're matching
as the info for the sub-pattern binder -- this type more readily admits querying its caseFieldAccessors

src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala

@@ -417,7 +417,17 @@ trait PatternMatching extends Transform with TypingTransformers with ast.TreeDSL
             // (the prefix of the argument passed to the unapply must equal the prefix of the type of the binder)
             val treeMaker = TypeTestTreeMaker(patBinder, patBinder, extractor.paramType, extractor.paramType)(pos, extractorArgTypeTest = true)
             (List(treeMaker), treeMaker.nextBinder)
-          } else (Nil, patBinder)
+          } else {
+            // no type test needed, but the tree maker relies on `patBinderOrCasted` having type `extractor.paramType` (and not just some type compatible with it)
+            // SI-6624 shows this is necessary because apparently patBinder may have an unfortunate type (.decls don't have the case field accessors)
+            // TODO: get to the bottom of this -- I assume it happens when type checking infers a weird type for an unapply call
+            // by going back to the parameterType for the extractor call we get a saner type, so let's just do that for now
+            /* TODO: uncomment when `settings.developer` and `devWarning` become available
+              if (settings.developer.value && !(patBinder.info =:= extractor.paramType))
+                devWarning(s"resetting info of $patBinder: ${patBinder.info} to ${extractor.paramType}")
+            */
+            (Nil, patBinder setInfo extractor.paramType)
+          }
 
         withSubPats(typeTestTreeMaker :+ extractor.treeMaker(patBinderOrCasted, pos), extractor.subBindersAndPatterns: _*)
       }

test/files/pos/t6624.scala

@@ -0,0 +1,28 @@
+sealed trait KList[+M[_]]
+
+case class KCons[M[_], +T <: KList[M]](
+  tail: T
+) extends KList[M]
+
+case class KNil[M[_]]() extends KList[M]
+
+object Test {
+  val klist: KCons[Option, KCons[Option, KCons[Option, KNil[Nothing]]]] = ???
+
+  // crashes with
+  // "Exception in thread "main" scala.reflect.internal.Types$TypeError: value _1 is not a member 
+  // of KCons[Option,KCons[Option,KNil[Nothing]]]"
+  klist match {
+   case KCons(KCons(KCons(_))) =>
+  }
+
+  // fails with a similar message as an error, rather than a crash.
+  klist match {
+    case KCons(KCons(_)) =>
+  }
+
+  // succeeds
+  klist match {
+    case KCons(_) =>
+  }
+}