From ceea26d144f5d87adc38b818e04a7bc31db21059 Mon Sep 17 00:00:00 2001
From: Matt Borland <matt.borland1@gmail.com>
Date: Sun, 5 Jul 2020 12:33:37 -0500
Subject: [PATCH] boost::math::pow is not constexpr #354 (#388)

* Made pow constexpr

* Added static_assert to test

* Changed constexpr to BOOST_CXX14_CONSTEXPR
---
 .../boost/math/special_functions/math_fwd.hpp   |  4 ++--
 include/boost/math/special_functions/pow.hpp    | 17 ++++++++---------
 test/pow_test.cpp                               |  4 ++++
 3 files changed, 14 insertions(+), 11 deletions(-)

diff --git a/include/boost/math/special_functions/math_fwd.hpp b/include/boost/math/special_functions/math_fwd.hpp
index f143c08bc..09cf24c3c 100644
--- a/include/boost/math/special_functions/math_fwd.hpp
+++ b/include/boost/math/special_functions/math_fwd.hpp
@@ -1019,10 +1019,10 @@ namespace boost
 
    // pow:
    template <int N, typename T, class Policy>
-   typename tools::promote_args<T>::type pow(T base, const Policy& policy);
+   BOOST_CXX14_CONSTEXPR typename tools::promote_args<T>::type pow(T base, const Policy& policy);
 
    template <int N, typename T>
-   typename tools::promote_args<T>::type pow(T base);
+   BOOST_CXX14_CONSTEXPR typename tools::promote_args<T>::type pow(T base);
 
    // next:
    template <class T, class U, class Policy>
diff --git a/include/boost/math/special_functions/pow.hpp b/include/boost/math/special_functions/pow.hpp
index 9c92116ac..2706fc1b5 100644
--- a/include/boost/math/special_functions/pow.hpp
+++ b/include/boost/math/special_functions/pow.hpp
@@ -35,7 +35,7 @@ template <int N, int M = N%2>
 struct positive_power
 {
     template <typename T>
-    static T result(T base)
+    static BOOST_CXX14_CONSTEXPR T result(T base)
     {
         T power = positive_power<N/2>::result(base);
         return power * power;
@@ -46,7 +46,7 @@ template <int N>
 struct positive_power<N, 1>
 {
     template <typename T>
-    static T result(T base)
+    static BOOST_CXX14_CONSTEXPR T result(T base)
     {
         T power = positive_power<N/2>::result(base);
         return base * power * power;
@@ -57,7 +57,7 @@ template <>
 struct positive_power<1, 1>
 {
     template <typename T>
-    static T result(T base){ return base; }
+    static BOOST_CXX14_CONSTEXPR T result(T base){ return base; }
 };
 
 
@@ -65,7 +65,7 @@ template <int N, bool>
 struct power_if_positive
 {
     template <typename T, class Policy>
-    static T result(T base, const Policy&)
+    static BOOST_CXX14_CONSTEXPR T result(T base, const Policy&)
     { return positive_power<N>::result(base); }
 };
 
@@ -73,7 +73,7 @@ template <int N>
 struct power_if_positive<N, false>
 {
     template <typename T, class Policy>
-    static T result(T base, const Policy& policy)
+    static BOOST_CXX14_CONSTEXPR T result(T base, const Policy& policy)
     {
         if (base == 0)
         {
@@ -92,7 +92,7 @@ template <>
 struct power_if_positive<0, true>
 {
     template <typename T, class Policy>
-    static T result(T base, const Policy& policy)
+    static BOOST_CXX14_CONSTEXPR T result(T base, const Policy& policy)
     {
         if (base == 0)
         {
@@ -126,15 +126,14 @@ struct select_power_if_positive
 
 
 template <int N, typename T, class Policy>
-inline typename tools::promote_args<T>::type pow(T base, const Policy& policy)
+BOOST_CXX14_CONSTEXPR inline typename tools::promote_args<T>::type pow(T base, const Policy& policy)
 { 
    typedef typename tools::promote_args<T>::type result_type;
    return detail::select_power_if_positive<N>::type::result(static_cast<result_type>(base), policy); 
 }
 
-
 template <int N, typename T>
-inline typename tools::promote_args<T>::type pow(T base)
+BOOST_CXX14_CONSTEXPR inline typename tools::promote_args<T>::type pow(T base)
 { return pow<N>(base, policies::policy<>()); }
 
 #ifdef BOOST_MSVC
diff --git a/test/pow_test.cpp b/test/pow_test.cpp
index d004f0d9b..eef8e3e6d 100644
--- a/test/pow_test.cpp
+++ b/test/pow_test.cpp
@@ -194,6 +194,10 @@ BOOST_AUTO_TEST_CASE( test_main )
     test_with_big_exponents<boost::math::concepts::real_concept, -1>();
 #endif
 
+    #ifndef BOOST_NO_CXX14_CONSTEXPR
+    static_assert(boost::math::pow<8>(2)==256, "Pow is not constexpr");
+    #endif
+
     test_return_types();
 
     test_error_policy();