Changes per @SteveBronder

Author: WardBrian

stan/math/prim/constraint/sum_to_zero_constrain.hpp

@@ -47,9 +47,9 @@ inline plain_type_t<Vec> sum_to_zero_constrain(const Vec& y) {
 
   auto&& y_ref = to_ref(y);
 
-  typename plain_type_t<Vec>::Scalar sum_w(0);
+  value_type_t<Vec> sum_w(0);
   for (int i = N; i > 0; --i) {
-    double n = i;
+    double n = static_cast<double>(i);
     auto w = y_ref(i - 1) * inv_sqrt(n * (n + 1));
     sum_w += w;
 

stan/math/prim/constraint/sum_to_zero_free.hpp

@@ -49,10 +49,10 @@ inline plain_type_t<Vec> sum_to_zero_free(const Vec& z) {
 
   y.coeffRef(N - 1) = -z_ref(N) * sqrt(N * (N + 1)) / N;
 
-  typename plain_type_t<Vec>::Scalar sum_w(0);
+  value_type_t<Vec> sum_w(0);
 
   for (int i = N - 2; i >= 0; --i) {
-    double n = i + 1;
+    double n = static_cast<double>(i + 1);
     auto w = y(i + 1) / sqrt((n + 1) * (n + 2));
     sum_w += w;
     y.coeffRef(i) = (sum_w - z_ref(i + 1)) * sqrt(n * (n + 1)) / n;

stan/math/rev/constraint/sum_to_zero_constrain.hpp

@@ -39,20 +39,20 @@ namespace math {
  * @return Zero-sum vector of dimensionality K.
  */
 template <typename T, require_rev_col_vector_t<T>* = nullptr>
-inline auto sum_to_zero_constrain(const T& y) {
+inline auto sum_to_zero_constrain(T&& y) {
   using ret_type = plain_type_t<T>;
   if (unlikely(y.size() == 0)) {
     return arena_t<ret_type>(Eigen::VectorXd{{0}});
   }
-  auto arena_y = to_arena(y);
+  auto arena_y = to_arena(std::forward<T>(y));
   arena_t<ret_type> arena_z = sum_to_zero_constrain(arena_y.val());
 
   reverse_pass_callback([arena_y, arena_z]() mutable {
     const auto N = arena_y.size();
 
     double sum_u_adj = 0;
     for (int i = 0; i < N; ++i) {
-      double n = i + 1;
+      double n = static_cast<double>(i + 1);
 
       // adjoint of the reverse cumulative sum computed in the forward mode
       sum_u_adj += arena_z.adj()(i);
@@ -95,8 +95,8 @@ inline auto sum_to_zero_constrain(const T& y) {
  * @return Zero-sum vector of dimensionality K.
  */
 template <typename T, require_rev_col_vector_t<T>* = nullptr>
-inline auto sum_to_zero_constrain(const T& y, scalar_type_t<T>& lp) {
-  return sum_to_zero_constrain(y);
+inline auto sum_to_zero_constrain(T&& y, scalar_type_t<T>& lp) {
+  return sum_to_zero_constrain(std::forward<T>(y));
 }
 
 }  // namespace math