Additional template args missed

Author: andrjohns

stan/math/prim/fun/hypergeometric_2F1.hpp

@@ -43,7 +43,7 @@ namespace internal {
  * @return Gauss hypergeometric function
  */
 template <typename Ta1, typename Ta2, typename Tb, typename Tz,
-          typename RtnT = boost::optional<return_type_t<Ta1, Ta1, Tb, Tz>>,
+          typename RtnT = boost::optional<return_type_t<Ta1, Ta2, Tb, Tz>>,
           require_all_arithmetic_t<Ta1, Ta2, Tb, Tz>* = nullptr>
 inline RtnT hyper_2F1_special_cases(const Ta1& a1, const Ta2& a2, const Tb& b,
                                     const Tz& z) {
@@ -148,7 +148,7 @@ inline RtnT hyper_2F1_special_cases(const Ta1& a1, const Ta2& a2, const Tb& b,
  * @return Gauss hypergeometric function
  */
 template <typename Ta1, typename Ta2, typename Tb, typename Tz,
-          typename ScalarT = return_type_t<Ta1, Ta1, Tb, Tz>,
+          typename ScalarT = return_type_t<Ta1, Ta2, Tb, Tz>,
           typename OptT = boost::optional<ScalarT>,
           require_all_arithmetic_t<Ta1, Ta2, Tb, Tz>* = nullptr>
 inline return_type_t<Ta1, Ta2, Tb, Tz> hypergeometric_2F1(const Ta1& a1,