Re-Implementation of Hypergeometric PFQ gradient function

stan/math/fwd/fun/hypergeometric_pFq.hpp

@@ -22,32 +22,42 @@ namespace math {
  * @return Generalized hypergeometric function
  */
 template <typename Ta, typename Tb, typename Tz,
-          require_all_matrix_t<Ta, Tb>* = nullptr,
-          require_return_type_t<is_fvar, Ta, Tb, Tz>* = nullptr>
-inline return_type_t<Ta, Tb, Tz> hypergeometric_pFq(const Ta& a, const Tb& b,
-                                                    const Tz& z) {
-  using fvar_t = return_type_t<Ta, Tb, Tz>;
-  ref_type_t<Ta> a_ref = a;
-  ref_type_t<Tb> b_ref = b;
-  auto grad_tuple = grad_pFq(a_ref, b_ref, z);
-
-  typename fvar_t::Scalar grad = 0;
-
-  if (!is_constant<Ta>::value) {
-    grad += dot_product(forward_as<promote_scalar_t<fvar_t, Ta>>(a_ref).d(),
-                        std::get<0>(grad_tuple));
+          typename FvarT = return_type_t<Ta, Tb, Tz>,
+          bool grad_a = !is_constant<Ta>::value,
+          bool grad_b = !is_constant<Tb>::value,
+          bool grad_z = !is_constant<Tz>::value,
+          require_all_vector_t<Ta, Tb>* = nullptr,
+          require_fvar_t<FvarT>* = nullptr>
+inline FvarT hypergeometric_pFq(const Ta& a, const Tb& b, const Tz& z) {
+  using PartialsT = partials_type_t<FvarT>;
+  using ARefT = ref_type_t<Ta>;
+  using BRefT = ref_type_t<Tb>;
+
+  ARefT a_ref = a;
+  BRefT b_ref = b;
+  auto&& a_val = value_of(a_ref);
+  auto&& b_val = value_of(b_ref);
+  auto&& z_val = value_of(z);
+  PartialsT pfq_val = hypergeometric_pFq(a_val, b_val, z_val);
+  auto grad_tuple
+      = grad_pFq<grad_a, grad_b, grad_z>(pfq_val, a_val, b_val, z_val);
+
+  FvarT rtn = FvarT(pfq_val, 0.0);
+
+  if (grad_a) {
+    rtn.d_ += dot_product(forward_as<promote_scalar_t<FvarT, ARefT>>(a_ref).d(),
+                          std::get<0>(grad_tuple));
   }
-  if (!is_constant<Tb>::value) {
-    grad += dot_product(forward_as<promote_scalar_t<fvar_t, Tb>>(b_ref).d(),
-                        std::get<1>(grad_tuple));
+  if (grad_b) {
+    rtn.d_ += dot_product(forward_as<promote_scalar_t<FvarT, BRefT>>(b_ref).d(),
+                          std::get<1>(grad_tuple));
   }
-  if (!is_constant<Tz>::value) {
-    grad += forward_as<promote_scalar_t<fvar_t, Tz>>(z).d_
-            * std::get<2>(grad_tuple);
+  if (grad_z) {
+    rtn.d_ += forward_as<promote_scalar_t<FvarT, Tz>>(z).d_
+              * std::get<2>(grad_tuple);
   }
 
-  return fvar_t(
-      hypergeometric_pFq(value_of(a_ref), value_of(b_ref), value_of(z)), grad);
+  return rtn;
 }
 
 }  // namespace math