Implement suggestions by jzmaddock

Author: dedekindring

include/boost/math/special_functions/gamma.hpp

@@ -1481,14 +1481,14 @@ BOOST_MATH_GPU_ENABLED T gamma_incomplete_imp_final(T a, T x, bool normalised, b
                #ifdef BOOST_MATH_HAS_NVRTC
                if (boost::math::is_same_v<T, float>)
                {
-                  init_value = (normalised ? 1 : ::tgammaf(a));
+                  init_value = (normalised ? T(1) : ::tgammaf(a));
                }
                else
                {
-                  init_value = (normalised ? 1 : ::tgamma(a));
+                  init_value = (normalised ? T(1) : ::tgamma(a));
                }
                #else
-               init_value = (normalised ? 1 : boost::math::tgamma(a, pol));
+               init_value = (normalised ? T(1) : boost::math::tgamma(a, pol));
                #endif
 
                if(normalised || (result >= 1) || (tools::max_value<T>() * result > init_value))
@@ -1620,29 +1620,29 @@ BOOST_MATH_GPU_ENABLED T gamma_incomplete_imp_final(T a, T x, bool normalised, b
       T gam;
       if (boost::math::is_same_v<T, float>)
       {
-         gam = normalised ? 1 : ::tgammaf(a);
+         gam = normalised ? T(1) : ::tgammaf(a);
       }
       else
       {
-         gam = normalised ? 1 : ::tgamma(a);
+         gam = normalised ? T(1) : ::tgamma(a);
       }
       #else
-      T gam = normalised ? 1 : boost::math::tgamma(a, pol);
+      T gam = normalised ? T(1) : boost::math::tgamma(a, pol);
       #endif
       result = gam - result;
    }
-   if(p_derivative && x > 0)
+   if(p_derivative)
    {
       //
       // Need to convert prefix term to derivative:
       //
-      if((x < 1) && (tools::max_value<T>() * x < *p_derivative))
+      if(x == 0 || ((x < 1) && (tools::max_value<T>() * x < *p_derivative)))
       {
          // overflow, just return an arbitrarily large value:
          *p_derivative = tools::max_value<T>() / 2;
       }
-
-      *p_derivative /= x;
+      else
+         *p_derivative /= x;
    }
 
    return result;
@@ -2110,8 +2110,8 @@ BOOST_MATH_GPU_ENABLED T gamma_p_derivative_imp(T a, T x, const Policy& pol)
    //
    if(x == 0)
    {
-      return (a > 1) ? 0 :
-         (a == 1) ? 1 : policies::raise_overflow_error<T>("boost::math::gamma_p_derivative<%1%>(%1%, %1%)", nullptr, pol);
+      return (a > 1) ? T(0) :
+         (a == 1) ? T(1) : policies::raise_overflow_error<T>("boost::math::gamma_p_derivative<%1%>(%1%, %1%)", nullptr, pol);
    }
    //
    // Normal case:

test/git_issue_1249.cpp

@@ -127,11 +127,11 @@ void test_impl()
 
 void test_derivative()
 {
+   using namespace boost::math::detail;
    double derivative = 0;
-   double result = boost::math::detail::gamma_incomplete_imp(1.0, 0.0, 
-     true, false, c_policy(), &derivative);
+   double result = gamma_incomplete_imp(1.0, 0.0, true, false, c_policy(), &derivative);
    BOOST_CHECK(errno == 0);
-   BOOST_CHECK_EQUAL(derivative, 0);
+   BOOST_CHECK_EQUAL(derivative, tools::max_value<double>() / 2);
    BOOST_CHECK_EQUAL(result, 0);
 }