From befe9a74443254ee6c4e91e037154b77f4fe9d3d Mon Sep 17 00:00:00 2001
From: Liu Fengyun <liu@fengy.me>
Date: Fri, 4 Sep 2020 15:40:29 +0200
Subject: [PATCH 1/3] Add further minimized test

---
 tests/patmat/i9631.scala | 12 ++++++++++++
 1 file changed, 12 insertions(+)
 create mode 100644 tests/patmat/i9631.scala

diff --git a/tests/patmat/i9631.scala b/tests/patmat/i9631.scala
new file mode 100644
index 000000000000..17600db8517a
--- /dev/null
+++ b/tests/patmat/i9631.scala
@@ -0,0 +1,12 @@
+trait Txn[T <: Txn[T]]
+
+sealed trait SkipList[T <: Txn[T]]
+
+trait Set[T <: Txn[T]] extends SkipList[T]
+
+object HASkipList {
+  def debug[T <: Txn[T]](in: SkipList[T]): Unit = in match {
+    case impl: Set[T] =>
+    case _ =>
+  }
+}

From a5b60252e097943df9e18c3c54c1b39a0afc96ee Mon Sep 17 00:00:00 2001
From: Liu Fengyun <liu@fengy.me>
Date: Fri, 4 Sep 2020 17:37:37 +0200
Subject: [PATCH 2/3] Fix #9631: handle F-bounds in type expansion

---
 compiler/src/dotty/tools/dotc/core/TypeOps.scala | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/core/TypeOps.scala b/compiler/src/dotty/tools/dotc/core/TypeOps.scala
index 8cfb7759b6d0..05f8b8b850f3 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeOps.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeOps.scala
@@ -673,8 +673,8 @@ object TypeOps:
           tp // break cycles
 
         case tp: TypeRef if isBounds(tp.underlying) =>
-          val lo = this(tp.info.loBound)
-          val hi = this(tp.info.hiBound)
+          val lo = this(tp.info.loBound.subst(tp.symbol :: Nil, WildcardType :: Nil))
+          val hi = this(tp.info.hiBound.subst(tp.symbol :: Nil, WildcardType :: Nil))
           // See tests/patmat/gadt.scala  tests/patmat/exhausting.scala  tests/patmat/t9657.scala
           val exposed = expose(lo, hi)
           typr.println(s"$tp exposed to =====> $exposed")

From 4ad9d8a45923953eec1f924cd9a2148f2c18cee2 Mon Sep 17 00:00:00 2001
From: Liu Fengyun <liu@fengy.me>
Date: Fri, 4 Sep 2020 17:41:46 +0200
Subject: [PATCH 3/3] Add original test

---
 tests/pos/i9631.scala | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)
 create mode 100644 tests/pos/i9631.scala

diff --git a/tests/pos/i9631.scala b/tests/pos/i9631.scala
new file mode 100644
index 000000000000..5806bda33b4f
--- /dev/null
+++ b/tests/pos/i9631.scala
@@ -0,0 +1,19 @@
+trait Txn[T <: Txn[T]]
+
+object SkipList {
+  trait Set[T <: Txn[T], A] extends SkipList[T, A, A]
+}
+sealed trait SkipList[T <: Txn[T], A, E]
+
+object HASkipList {
+  def debug[T <: Txn[T], A](in: SkipList[T, A, _], key: A)(implicit tx: T): Int = in match {
+    case impl: Impl[T, A, _] => impl.foo(key)
+    case _ => -1
+  }
+
+  private trait Impl[T <: Txn[T], A, E] {
+    self: SkipList[T, A, E] =>
+
+    def foo(key: A)(implicit tx: T): Int
+  }
+}