addressed review comments

Author: t4c1

stan/math/fwd/fun/quad_form.hpp

@@ -31,7 +31,7 @@ inline promote_scalar_t<return_type_t<EigMat1, EigMat2>, EigMat2> quad_form(
     const EigMat1& A, const EigMat2& B) {
   check_square("quad_form", "A", A);
   check_multiplicable("quad_form", "A", A, "B", B);
-  return multiply(transpose(B), multiply(A, B));
+  return multiply(B.transpose(), multiply(A, B));
 }
 
 /**

stan/math/prim/fun/quad_form.hpp

@@ -18,7 +18,7 @@ namespace math {
  *
  * @param A square matrix
  * @param B second matrix
- * @return The quadratic form, which is a symmetric matrix of size CB.
+ * @return The quadratic form, which is a symmetric matrix.
  * @throws std::invalid_argument if A is not square, or if A cannot be
  * multiplied by B
  */
@@ -27,7 +27,9 @@ template <typename EigMat1, typename EigMat2,
           require_not_eigen_col_vector_t<EigMat2>* = nullptr,
           require_vt_same<EigMat1, EigMat2>* = nullptr,
           require_all_vt_arithmetic<EigMat1, EigMat2>* = nullptr>
-inline plain_type_t<EigMat2> quad_form(const EigMat1& A, const EigMat2& B) {
+inline Eigen::Matrix<value_type_t<EigMat2>, EigMat2::ColsAtCompileTime,
+                     EigMat2::ColsAtCompileTime>
+quad_form(const EigMat1& A, const EigMat2& B) {
   check_square("quad_form", "A", A);
   check_multiplicable("quad_form", "A", A, "B", B);
   return B.transpose() * A * B;

stan/math/prim/fun/quad_form_sym.hpp

@@ -17,7 +17,7 @@ namespace math {
  *
  * @param A symmetric matrix
  * @param B second matrix
- * @return The quadratic form, which is a symmetric matrix of size CB.
+ * @return The quadratic form, which is a symmetric matrix.
  * @throws std::invalid_argument if A is not symmetric, or if A cannot be
  * multiplied by B
  */

stan/math/prim/fun/rep_matrix.hpp

@@ -21,20 +21,14 @@ template <typename ColVec, require_eigen_col_vector_t<ColVec>* = nullptr>
 inline Eigen::Matrix<value_type_t<ColVec>, Eigen::Dynamic, Eigen::Dynamic>
 rep_matrix(const ColVec& v, int n) {
   check_nonnegative("rep_matrix", "rows", n);
-  Eigen::Matrix<value_type_t<ColVec>, Eigen::Dynamic, Eigen::Dynamic> result(
-      v.size(), n);
-  result.colwise() = v;
-  return result;
+  return v.replicate(1, n);
 }
 
 template <typename RowVec, require_eigen_row_vector_t<RowVec>* = nullptr>
 inline Eigen::Matrix<value_type_t<RowVec>, Eigen::Dynamic, Eigen::Dynamic>
 rep_matrix(const RowVec& rv, int m) {
   check_nonnegative("rep_matrix", "cols", m);
-  Eigen::Matrix<value_type_t<RowVec>, Eigen::Dynamic, Eigen::Dynamic> result(
-      m, rv.size());
-  result.rowwise() = rv;
-  return result;
+  return rv.replicate(m, 1);
 }
 }  // namespace math
 }  // namespace stan

stan/math/rev/fun/quad_form.hpp

@@ -106,7 +106,7 @@ class quad_form_vari : public vari {
  *
  * @param A square matrix
  * @param B second matrix
- * @return The quadratic form, which is a symmetric matrix of size Cb.
+ * @return The quadratic form, which is a symmetric matrix.
  * @throws std::invalid_argument if A is not square, or if A cannot be
  * multiplied by B
  */