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

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

1 files changed, 28 insertions, 0 deletions

diff --git a/test/files/pos/t6624.scala b/test/files/pos/t6624.scala
new file mode 100644
index 0000000000..1a92b92d53
--- /dev/null
+++ b/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(_) =>
+  }
+}
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