diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index 9152d6d38a51..f50d1a9a9896 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -171,6 +171,8 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
             derefCount += 1
             if (derefCount >= DerefLimit) NoType
             else try mapOver(t.ref) finally derefCount -= 1
+          case tp: TypeVar =>
+            tp
           case _ =>
             mapOver(t)
         }
diff --git a/tests/pos/i5666.scala b/tests/pos/i5666.scala
new file mode 100644
index 000000000000..fb4be4e02da3
--- /dev/null
+++ b/tests/pos/i5666.scala
@@ -0,0 +1,20 @@
+// This file should still compile if Config.LogPrendingSubtypesthreshold is set to 9.
+sealed trait HList
+trait HNil extends HList
+trait HCons[+H, +T] extends HList
+
+trait Concat[L1, L2] { type Out }
+object Concat {
+  implicit def i0[L]:
+    Concat[HNil, L] { type Out = L } = null
+
+  implicit def i1[H, T, L, O]
+    (implicit c: Concat[T, L] { type Out = O }):
+      Concat[HCons[H, T], L] { type Out = HCons[H, O] } = null
+}
+
+object Test {
+  type L1 = HCons[Unit, HNil]
+
+  implicitly[Concat[L1, L1]]
+}
\ No newline at end of file