Autodiff Order 0 Fvar Error in C++11 and C++14

[shawn-mcadam]

When compiled with -std=c++11 or c++14 every math function in this code

#include <iostream>
#include <boost/math/differentiation/autodiff.hpp>

int main(){
  using boost::math::differentiation::make_fvar;
  std::cout << log(make_fvar<double,0>(1)) << std::endl;
  std::cout << asin(make_fvar<double,0>(0)) << std::endl;
  std::cout << asinh(make_fvar<double,0>(0)) << std::endl;
  std::cout << tan(make_fvar<double,0>(0)) << std::endl;
  std::cout << atan(make_fvar<double,0>(0)) << std::endl;
  std::cout << atanh(make_fvar<double,0>(0)) << std::endl;
  std::cout << acos(make_fvar<double,0>(0)) << std::endl;
  std::cout << acosh(make_fvar<double,0>(1)) << std::endl;
  std::cout << erf(make_fvar<double,0>(0)) << std::endl;
}

produces error in g++ 10.2.0 (the specific command being g++-10 -std=c++11 -I/usr/local/include fvar_order_0.cpp) along the lines of

fvar_order_0.cpp:5:43:   required from here
/usr/local/include/boost/math/differentiation/autodiff.hpp:681:81: error: too many initializers for 'std::__array_traits<double, 0>::_Type'
  681 | fvar<RealType, Order>::fvar(RealType2 const& ca) : v{{static_cast<RealType>(ca)}} {}

and similar errors occur with the same standards in clang 11.0.3

/usr/local/include/boost/math/differentiation/autodiff.hpp:676:55: error: type 'double' cannot be narrowed to 'std::__1::array<double, 0>::_CharType' (aka 'char') in
      initializer list [-Wc++11-narrowing]
fvar<RealType, Order>::fvar(root_type const& ca) : v{{static_cast<RealType>(ca)}} {}

The code compiles without a problem when using -std=c++17 or c++20. I think the problem is that each of those functions has a line like

if BOOST_AUTODIFF_IF_CONSTEXPR (order == 0)
    return fvar<RealType, Order>(d0);
  else {
    auto x = make_fvar<root_type, order - 1>(static_cast<root_type>(cr));

and since BOOST_AUTODIFF_IF_CONSTEXPR is null in c++11 and c++14 the compiler will try calling make_fvar<double,-1>(). The main problem is just that the error messages are really confusing, so I'd be happy with something along the lines of a static_assert(ORDER != 0,""). Thank you!

[pulver]

Thank you @ssm259 both for the report and diagnosis. This will be fixed shortly.

[pulver]

@NAThompson I just realized I probably should have submitted this via a separate branch and then requested a PR to develop. My apologies for any inconvenience - feel free to make any changes or delete this commit.

[NAThompson]

@pulver : You're right that would've been better, but my feeling aren't hurt.

@ssm259 : Is the issue fixed for you?

[pulver]

@ssm259 : Is the issue fixed for you?

In case you are testing against a slightly older version of boost, you may have to manually define BOOST_IF_CONSTEXPR like so prior to the #include, which was only recently added:

#include <boost/config.hpp>
//
// C++17 if constexpr
//
#if !defined(BOOST_NO_CXX17_IF_CONSTEXPR)
#  define BOOST_IF_CONSTEXPR if constexpr
#else
#  define BOOST_IF_CONSTEXPR if
#endif
#include <boost/math/differentiation/autodiff.hpp>

[shawn-mcadam]

@pulver @NAThompson commit d739449 seems to have fixed things. Thanks, guys!