update const auto& in binary_op

Author: SteveBronder

stan/math/prim/functor/apply_scalar_binary.hpp

@@ -203,7 +203,7 @@ inline auto apply_scalar_binary(F&& f, T1&& x, T2&& y) {
   return make_holder(
       [](auto&& f_inner, auto&& x_inner, auto&& y_inner) {
         return x_inner.unaryExpr(
-            [f_inner, y_inner](const auto&& v) { return f_inner(v, y_inner); });
+            [f_inner, y_inner](const auto& v) { return f_inner(v, y_inner); });
       },
       std::forward<F>(f), std::forward<T1>(x), std::forward<T2>(y));
 }
@@ -229,7 +229,7 @@ inline auto apply_scalar_binary(F&& f, T1&& x, T2&& y) {
   return make_holder(
       [](auto&& f_inner, auto&& x_inner, auto&& y_inner) {
         return y_inner.unaryExpr(
-            [f_inner, x_inner](const auto&& v) { return f_inner(x_inner, v); });
+            [f_inner, x_inner](const auto& v) { return f_inner(x_inner, v); });
       },
       std::forward<F>(f), std::forward<T1>(x), std::forward<T2>(y));
 }
@@ -290,7 +290,7 @@ inline auto apply_scalar_binary(const F& f, const T1& x, const T2& y) {
   using T_return = std::decay_t<decltype(f(x[0], y))>;
   std::vector<T_return> result(x.size());
   Eigen::Map<Eigen::Matrix<T_return, -1, 1>>(result.data(), result.size())
-      = x_vec.unaryExpr([f, y](const auto&& v) { return f(v, y); });
+      = x_vec.unaryExpr([f, y](const auto& v) { return f(v, y); });
   return result;
 }
 
@@ -320,7 +320,7 @@ inline auto apply_scalar_binary(const F& f, const T1& x, const T2& y) {
   using T_return = std::decay_t<decltype(f(x, y[0]))>;
   std::vector<T_return> result(y.size());
   Eigen::Map<Eigen::Matrix<T_return, -1, 1>>(result.data(), result.size())
-      = y_vec.unaryExpr([f, x](const auto&& v) { return f(x, v); });
+      = y_vec.unaryExpr([f, x](const auto& v) { return f(x, v); });
   return result;
 }