Fix scala#6088: add test

Author: liufengyun

tests/patmat/i6088.scala

@@ -0,0 +1,31 @@
+/** Natural transformation. */
+trait ~>[F[_], G[_]] {
+  def apply[A](fa: F[A]): G[A]
+}
+
+/** Higher-kinded pattern functor typeclass. */
+trait HFunctor[H[f[_], i]] {
+  def hmap[A[_], B[_]](nt: A ~> B): ([x] =>> H[A,x]) ~> ([x] =>> H[B,x])
+}
+
+/** Some HK pattern functor. */
+enum ExprF[R[_],I] {
+  case Const[R[_]](i: Int) extends ExprF[R,Int]
+  case Neg[R[_]](l: R[Int]) extends ExprF[R,Int]
+  case Eq[R[_]](l: R[Int], r: R[Int]) extends ExprF[R,Boolean]
+}
+
+/** Companion. */
+object ExprF {
+  given hfunctor as HFunctor[ExprF] {
+    def hmap[A[_], B[_]](nt: A ~> B): ([x] =>> ExprF[A,x]) ~> ([x] =>> ExprF[B,x]) = {
+      new ~>[[x] =>> ExprF[A,x], [x] =>> ExprF[B,x]] {
+        def apply[I](fa: ExprF[A,I]): ExprF[B,I] = fa match {
+          case Const(i) => Const(i)
+          case Neg(l) => Neg(nt(l))
+          case Eq(l, r) => Eq(nt(l), nt(r))
+        }
+      }
+    }
+  }
+}