cleanups for promote_elements

stan/math/prim/arr/fun/promote_elements.hpp

@@ -19,16 +19,15 @@ namespace math {
  * @tparam S type of input elements, must be assignable to T
  */
 template <typename T, typename S>
-struct promote_elements<std::vector<T>, std::vector<S> > {
+struct promote_elements<std::vector<T>, std::vector<S>> {
   /**
    * Return input vector of type S as vector of type T.
    *
    * @param u vector of type S, assignable to type T
    * @returns vector of type T
    */
   inline static std::vector<T> promote(const std::vector<S>& u) {
-    std::vector<T> t({u.data(), u.data() + u.size()});
-    return t;
+    return {u.begin(), u.end()};
   }
 };
 
@@ -40,7 +39,7 @@ struct promote_elements<std::vector<T>, std::vector<S> > {
  * @tparam T type of elements
  */
 template <typename T>
-struct promote_elements<std::vector<T>, std::vector<T> > {
+struct promote_elements<std::vector<T>, std::vector<T>> {
   /**
    * Return input vector.
    *

stan/math/prim/mat/fun/promote_elements.hpp

@@ -26,11 +26,7 @@ struct promote_elements<Eigen::Matrix<T, R, C>, Eigen::Matrix<S, R, C> > {
    */
   inline static Eigen::Matrix<T, R, C> promote(
       const Eigen::Matrix<S, R, C>& u) {
-    Eigen::Matrix<T, Eigen::Dynamic, Eigen::Dynamic> t(u.rows(), u.cols());
-    for (int i = 0; i < u.size(); ++i) {
-      t(i) = promote_elements<T, S>::promote(u(i));
-    }
-    return t;
+    return u.template cast<T>();
   }
 };
 

test/unit/math/rev/mat/fun/dot_product2_test.cpp

@@ -996,14 +996,16 @@ TEST(AgradRevMatrix, promoteElts) {
   EXPECT_FLOAT_EQ(5.0, c.val());
 
   vector<double> x(5);
-  for (int i = 0; i < 5; ++i)
-    x[i] = i * i;
-  vector<double> y = promote_common<vector<double>, vector<double> >(x);
+  for (int i = 0; i < 5; ++i) {
+    x[i] = i * i;    
+  }
+  vector<double> y = promote_common<vector<double>, vector<double>>(x);
   EXPECT_EQ(5U, y.size());
-  for (int i = 0; i < 5; ++i)
+  for (int i = 0; i < 5; ++i) {
     EXPECT_FLOAT_EQ(x[i], y[i]);
+  }
   std::vector<var> z
-      = promote_common<std::vector<double>, std::vector<var> >(x);
+      = promote_common<std::vector<double>, std::vector<var>>(x);
   EXPECT_EQ(5U, z.size());
   for (int i = 0; i < 5; ++i)
     EXPECT_FLOAT_EQ(x[i], z[i].val());