corrected the error

Author: Franzi2114

stan/math/prim/prob/wiener4_lccdf.hpp

@@ -56,20 +56,20 @@ inline auto wiener_prob_derivative_term(const T_a& a, const T_v& v,
                                         const T_w& w) noexcept {
   using ret_t = return_type_t<T_a, T_w, T_v>;
   const auto exponent_m1 = log1m(1.1 * 1.0e-8);
-  ret_t ans;
   const auto neg_v = -v;
   const auto one_m_w = 1 - w;
   int sign_v = neg_v < 0 ? 1 : -1;
   const auto two_a_neg_v = 2.0 * a * neg_v;
   const auto exponent_with_1mw = sign_v * two_a_neg_v * w;
-  const auto exponent = (sign_v * two_a_neg_v);
+  const auto exponent = sign_v * two_a_neg_v;
   const auto exponent_with_w = two_a_neg_v * one_m_w;
   // truncating longer calculations, for numerical stability
   if (unlikely((exponent_with_1mw >= exponent_m1)
                || ((exponent_with_w >= exponent_m1) && (sign_v == 1))
                || (exponent >= exponent_m1) || neg_v == 0)) {
     return ret_t(-one_m_w);
   }
+  ret_t ans;
   ret_t diff_term;
   const auto log_w = log(one_m_w);
   if (neg_v < 0) {
@@ -133,7 +133,7 @@ inline auto wiener4_ccdf_grad_a(const T_y& y, const T_a& a, const T_v& v,
   // derivative of the wiener probability w.r.t. 'a' (on log-scale)
   auto prob_grad_a = -wiener_prob_derivative_term(a, v, w) * v;
   if (!is_scal_finite(prob_grad_a)) {
-    prob_grad_a = ret_t(NEGATIVE_INFTY);
+    return ret_t(NEGATIVE_INFTY);
   }
   const auto log_prob_hit_upper = log_wiener_prob_hit_upper(a, v, w);
   const auto cdf_grad_a = wiener4_cdf_grad_a(y, a, v, w, cdf, log_err);

stan/math/prim/prob/wiener4_lcdf.hpp

@@ -9,6 +9,9 @@ namespace internal {
 
 /**
  * Make the expression finite
+ *
+ * @param x Expression to test
+ * @return Expression or limited to maximum numeric_limit
  */
 template <typename T_x>
 inline auto make_finite(const T_x& x) {
@@ -18,6 +21,7 @@ inline auto make_finite(const T_x& x) {
     return std::numeric_limits<T_x>::max();
   }
 }
+
 /**
  * Calculate the probability term 'P' on log scale for distribution
  *
@@ -106,9 +110,9 @@ inline auto log_probability_GradAV(const T_a& a, const T_v& v, const T_w& w) {
       log_quotient = NEGATIVE_INFTY;
     }
     if (log(w) > log_quotient) {
-      return -exp(log_prob) * (w - exp(log_quotient));
-    } else {
-      return exp(log_prob) * (exp(log_quotient) - w);
+      return -exp(log_prob + log_diff_exp(log(w), log_quotient));
+    } else { 
+      return exp(log_prob + log_diff_exp(log_quotient, log(w)));
     }
   }
 }
@@ -728,4 +732,4 @@ inline auto wiener_lcdf(const T_y& y, const T_a& a, const T_t0& t0,
 }
 }  // namespace math
 }  // namespace stan
-#endif
+#endif