Allow further promotion to be disabled

Author: mborland

include/boost/math/special_functions/logistic_sigmoid.hpp

@@ -14,12 +14,12 @@
 namespace boost {
 namespace math {
 
-template <typename RealType, typename Policy>
+template <bool further_promotion = true, typename RealType, typename Policy>
 RealType logistic_sigmoid(RealType x, const Policy&)
 {
     BOOST_MATH_STD_USING
 
-    using promoted_real_type = typename policies::evaluation<RealType, Policy>::type;
+    using promoted_real_type = std::conditional_t<further_promotion, typename policies::evaluation<RealType, Policy>::type, RealType>;
 
     if(-x >= tools::log_max_value<RealType>())
     {
@@ -34,10 +34,10 @@ RealType logistic_sigmoid(RealType x, const Policy&)
     return res;
 }
 
-template <typename RealType>
+template <bool further_promotion = true, typename RealType>
 RealType logistic_sigmoid(RealType x)
 {
-    return logistic_sigmoid(x, policies::policy<>());
+    return logistic_sigmoid<further_promotion>(x, policies::policy<>());
 }
 
 } // namespace math