diff --git a/include/boost/random/detail/integer_log2.hpp b/include/boost/random/detail/integer_log2.hpp
index f1d7ae0..e794509 100644
--- a/include/boost/random/detail/integer_log2.hpp
+++ b/include/boost/random/detail/integer_log2.hpp
@@ -22,9 +22,7 @@ namespace boost {
 namespace random {
 namespace detail {
 
-// Daniel James: Disabled use of constexpr because integer_log2_impl is not a
-// valid constexpr.
-#if 0 && !defined(BOOST_NO_CONSTEXPR)
+#if !defined(BOOST_NO_CONSTEXPR)
 #define BOOST_RANDOM_DETAIL_CONSTEXPR constexpr
 #elif defined(BOOST_MSVC)
 #define BOOST_RANDOM_DETAIL_CONSTEXPR __forceinline
@@ -38,10 +36,11 @@ template<int Shift>
 struct integer_log2_impl
 {
     template<class T>
-    BOOST_RANDOM_DETAIL_CONSTEXPR static int apply(T t, int accum)
+    BOOST_RANDOM_DETAIL_CONSTEXPR static int apply(T t, int accum,
+            int update = 0)
     {
-        int update = ((t >> Shift) != 0) * Shift;
-        return integer_log2_impl<Shift / 2>::apply(t >> update, accum + update);
+        return update = ((t >> Shift) != 0) * Shift,
+            integer_log2_impl<Shift / 2>::apply(t >> update, accum + update);
     }
 };