addressed remaining review comments

Author: t4c1

stan/math/fwd/fun/multiply.hpp

@@ -35,8 +35,7 @@ inline auto multiply(const Mat1& m1, const Mat2& m2) {
     Eigen::Matrix<value_type_t<Mat1>, 1, Mat1::ColsAtCompileTime> crow
         = m1.row(i);
     for (size_type j = 0; j < m2.cols(); j++) {
-      auto ccol = m2.col(j);
-      result(i, j) = dot_product(crow, ccol);
+      result(i, j) = dot_product(crow, m2.col(j));
     }
   }
   return result;

stan/math/prim/fun/trace_gen_quad_form.hpp

@@ -32,7 +32,7 @@ template <typename TD, typename TA, typename TB,
           typename = require_all_eigen_t<TD, TA, TB>,
           typename = require_all_not_var_t<value_type_t<TD>, value_type_t<TA>,
                                            value_type_t<TB>>,
-          typename = require_any_not_same_vt<double, TD, TA, TB>>
+          typename = require_any_not_eigen_vt<std::is_arithmetic, TD, TA, TB>>
 inline auto trace_gen_quad_form(const TD &D, const TA &A, const TB &B) {
   check_square("trace_gen_quad_form", "A", A);
   check_square("trace_gen_quad_form", "D", D);
@@ -44,7 +44,7 @@ inline auto trace_gen_quad_form(const TD &D, const TA &A, const TB &B) {
 /**
  * Return the trace of D times the quadratic form of B and A.
  * That is, `trace_gen_quad_form(D, A, B) = trace(D * B' * A * B).`
- * This is the double-only overload to allow Eigen's expression
+ * This is the overload for arithmetic types to allow Eigen's expression
  * templates to be used for efficiency.
  *
  * @tparam TD type of the first matrix or expression
@@ -60,8 +60,7 @@ inline auto trace_gen_quad_form(const TD &D, const TA &A, const TB &B) {
  * be multiplied by D.
  */
 template <typename TD, typename TA, typename TB,
-          typename = require_all_eigen_t<TD, TA, TB>,
-          typename = require_all_same_vt<double, TD, TA, TB>>
+          typename = require_all_eigen_vt<std::is_arithmetic, TD, TA, TB>>
 inline double trace_gen_quad_form(const TD &D, const TA &A, const TB &B) {
   check_square("trace_gen_quad_form", "A", A);
   check_square("trace_gen_quad_form", "D", D);