From 6b580bd6f18d485b9656d6918cfbca1e929c123e Mon Sep 17 00:00:00 2001
From: Nicholas McKibben <nicholas.bgp@gmail.com>
Date: Sat, 4 Feb 2023 22:43:51 -0700
Subject: [PATCH] FIX: avoid overflow on overflow check in
 gamma_p_derivative_imp

---
 include/boost/math/special_functions/gamma.hpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/boost/math/special_functions/gamma.hpp b/include/boost/math/special_functions/gamma.hpp
index 68afe66e30..efd9eef713 100644
--- a/include/boost/math/special_functions/gamma.hpp
+++ b/include/boost/math/special_functions/gamma.hpp
@@ -1795,7 +1795,8 @@ T gamma_p_derivative_imp(T a, T x, const Policy& pol)
    //
    typedef typename lanczos::lanczos<T, Policy>::type lanczos_type;
    T f1 = detail::regularised_gamma_prefix(a, x, pol, lanczos_type());
-   if((x < 1) && (tools::max_value<T>() * x < f1))
+   static const T inv_max_value = 1.0 / tools::max_value<T>();
+   if((x < 1) && (x < inv_max_value * f1))
    {
       // overflow:
       return policies::raise_overflow_error<T>("boost::math::gamma_p_derivative<%1%>(%1%, %1%)", nullptr, pol);