Fix #9631: handle F-bounds in type expansion

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")

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 _ =>
+  }
+}

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
+  }
+}