Improve error handling

Author: mborland

include/boost/math/special_functions/next.hpp

@@ -18,11 +18,11 @@
 #include <boost/math/tools/traits.hpp>
 #include <boost/math/tools/config.hpp>
 #include <type_traits>
+#include <stdexcept>
 #include <cfloat>
 #include <cstdint>
 #include <cstring>
 
-
 #if !defined(_CRAYC) && !defined(__CUDACC__) && (!defined(__GNUC__) || (__GNUC__ > 3) || ((__GNUC__ == 3) && (__GNUC_MINOR__ > 3)))
 #if (defined(_M_IX86_FP) && (_M_IX86_FP >= 2)) || defined(__SSE2__)
 #include "xmmintrin.h"
@@ -725,6 +725,7 @@ typename tools::promote_args<T, U>::type float_distance(const T& a, const U& b)
 inline std::int32_t float_distance(float a, float b)
 {
    using std::abs;
+   using std::isfinite;
    constexpr auto tol = 2 * (std::numeric_limits<float>::min)();
 
    // 0, very small, and large magnitude distances all need special handling
@@ -736,6 +737,10 @@ inline std::int32_t float_distance(float a, float b)
    {
       return static_cast<std::int32_t>(float_distance(a, b, policies::policy<>()));
    }
+   else if (!(isfinite)(a) || !(isfinite)(b))
+   {
+      throw std::domain_error("a and b must both be finite");
+   }
 
    static_assert(sizeof(float) == sizeof(std::int32_t), "float is incorrect size.");
 
@@ -757,6 +762,7 @@ inline std::int32_t float_distance(float a, float b)
 inline std::int64_t float_distance(double a, double b)
 {
    using std::abs;
+   using std::isfinite;
    constexpr auto tol = 2 * (std::numeric_limits<double>::min)();
 
    // 0, very small, and large magnitude distances all need special handling
@@ -768,6 +774,11 @@ inline std::int64_t float_distance(double a, double b)
    {
       return static_cast<std::int64_t>(float_distance(a, b, policies::policy<>()));
    }
+   else if (!(isfinite)(a) || !(isfinite)(b))
+   {
+      throw std::domain_error("a and b must both be finite");
+   }
+
 
    static_assert(sizeof(double) == sizeof(std::int64_t), "double is incorrect size.");