Make MonoidK[Endo] stack safe on combineK

core/src/main/scala/cats/instances/function.scala

@@ -3,6 +3,7 @@ package instances
 
 import cats.Contravariant
 import cats.arrow.{ArrowChoice, Category, CommutativeArrow}
+import cats.data.AndThen
 
 import annotation.tailrec
 
@@ -151,8 +152,15 @@ sealed private[instances] trait Function1Instances extends Function1Instances0 {
       def compose[A, B, C](f: B => C, g: A => B): A => C = f.compose(g)
     }
 
-  implicit val catsStdMonoidKForFunction1: MonoidK[Endo] =
-    Category[Function1].algebraK
+  implicit val catsStdMonoidKForFunction1: MonoidK[Endo] = new MonoidK[Endo] {
+
+    val category: Category[Function] = Category[Function1]
+
+    override def empty[A]: Endo[A] = category.id
+
+    override def combineK[A](x: Endo[A], y: Endo[A]): Endo[A] =
+      AndThen(category.compose(x, y))
+  }
 
 }
 

tests/src/test/scala/cats/tests/FunctionSuite.scala

@@ -138,4 +138,12 @@ class FunctionSuite extends CatsSuite {
   checkAll("CommutativeGroup[String => CGrp]", SerializableTests.serializable(CommutativeGroup[String => CGrp]))
   checkAll("ContravariantMonoidal[Function1[?, Monoid]]",
            SerializableTests.serializable(ContravariantMonoidal[? => Long]))
+
+  test("MonoidK[Endo] is stack safe on combineK") {
+    def incrementAll(as: Int): Int = as + 1
+    val bigList: List[Int => Int] = List.fill(50000)(incrementAll)
+
+    val sumAll = bigList.combineAll(MonoidK[Endo].algebra)
+    List(1, 1, 1).map(sumAll)
+  }
 }