Missing special function hypot(3 args)

[gpeterhoff]

I am missing hypot(3 args). For an exact implementation there is just the discussion https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77776. Maybe you have some ideas.
hypot(2 args) must then of course be adapted.
math_fwd.hpp:

   // Hypotenuse function sqrt(x ^ 2 + y ^ 2).

   template <class T1, class T2>
   tools::promote_args_t<T1, T2>
         hypot(T1 x, T2 y);

   template <class T1, class T2, class Policy>
   std::enable_if_t<policies::is_policy<Policy>::value, tools::promote_args_t<T1, T2>>
         hypot(T1 x, T2 y, const Policy&);

   // Hypotenuse function sqrt(x ^ 2 + y ^ 2 + z ^ 2).

   template <class T1, class T2, class T3>
   std::enable_if_t<!policies::is_policy<T3>::value, tools::promote_args_t<T1, T2, T3>>
         hypot(T1 x, T2 y, T3 z);

   template <class T1, class T2, class T3, class Policy>
   std::enable_if_t<!policies::is_policy<T3>::value, tools::promote_args_t<T1, T2, T3>>
         hypot(T1 x, T2 y, T3 z, const Policy&);

add to BOOST_MATH_DECLARE_SPECIAL_FUNCTIONS(Policy)

   template <class T1, class T2, class T3>\
   inline std::enable_if_t<!policies::is_policy<T3>::value, tools::promote_args_t<T1, T2, T3>> \
   hypot(T1 x, T2 y, T3 z){ return boost::math::hypot(x, y, z, Policy()); } \

and of course the implementations in hypot.hpp

Questions

• Why are you making this so complicated? Yes, the functions were implemented before C++11. But then no more updates have taken place, although boost.math requires C++14. I think the whole #define's and forwarddeclarations (math_fwd.hpp) can be saved if you declare the functions cleanly via std::enable_if_t (because of the policies), whereby the question then arises whether the policies are still up to date at all (we have C++23, C++26 is just around the corner).
• Your special-functions throw exceptions, but std::special-functions do not (normally). So your functions are incompatible - which is of course due to your very old implementations.

regards
Gero