constexpr signbit (#793)

Implements constexpr signbit with annotated caveats during compile time.

doc/sf/ccmath.qbk

@@ -195,6 +195,9 @@ All of the following functions require C++17 or greater.
         template <typename T>
         constexpr Promoted nexttoward(T from, long double to)
 
+        template <typename T>
+        constexpr bool signbit(T arg)
+
     } // Namespaces
 
 [endsect] [/section:ccmath Constexpr CMath]

include/boost/math/ccmath/ccmath.hpp

@@ -39,5 +39,7 @@
 #include <boost/math/ccmath/islessequal.hpp>
 #include <boost/math/ccmath/isunordered.hpp>
 #include <boost/math/ccmath/fma.hpp>
+#include <boost/math/ccmath/next.hpp>
+#include <boost/math/ccmath/signbit.hpp>
 
 #endif // BOOST_MATH_CCMATH_HPP

include/boost/math/ccmath/signbit.hpp

@@ -0,0 +1,60 @@
+//  (C) Copyright Matt Borland 2022.
+//  Use, modification and distribution are subject to the
+//  Boost Software License, Version 1.0. (See accompanying file
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_MATH_CCMATH_SIGNBIT_HPP
+#define BOOST_MATH_CCMATH_SIGNBIT_HPP
+
+#include <cmath>
+#include <limits>
+#include <type_traits>
+#include <boost/math/tools/is_constant_evaluated.hpp>
+#include <boost/math/ccmath/isnan.hpp>
+
+namespace boost::math::ccmath {
+
+namespace detail {
+
+// Typical implementations of signbit involve type punning via union and manipulating
+// overflow (see libc++ or musl). Neither of these are allowed in constexpr contexts
+// (technically type punning via union in general is UB in c++ but well defined in C) 
+// therefore NANs and 0s are treated as positive
+
+template <typename T>
+constexpr bool signbit_impl(T arg)
+{
+    if (boost::math::ccmath::isnan(arg))
+    {
+        return false;
+    }
+    
+    return arg < static_cast<T>(0);
+}
+
+}
+
+// Return value: true if arg is negative, false if arg is 0, NAN, or positive
+template <typename Real, std::enable_if_t<!std::is_integral_v<Real>, bool> = true>
+constexpr bool signbit(Real arg)
+{
+    if (BOOST_MATH_IS_CONSTANT_EVALUATED(arg))
+    {
+        return boost::math::ccmath::detail::signbit_impl(arg);
+    }
+    else
+    {
+        using std::signbit;
+        return signbit(arg);
+    }
+}
+
+template <typename Z, std::enable_if_t<std::is_integral_v<Z>, bool> = true>
+constexpr bool signbit(Z arg)
+{
+    return boost::math::ccmath::signbit(static_cast<double>(arg));
+}
+
+} // Namespaces
+
+#endif // BOOST_MATH_CCMATH_SIGNBIT_HPP

test/Jamfile.v2

@@ -161,6 +161,7 @@ test-suite special_fun :
    [ run ccmath_isunordered_test.cpp ../../test/build//boost_unit_test_framework : : : [ requires cxx17_if_constexpr ] ]
    [ run ccmath_next_test.cpp ../../test/build//boost_unit_test_framework : : : [ requires cxx17_if_constexpr ] ]
    [ run ccmath_fma_test.cpp ../../test/build//boost_unit_test_framework : : : [ requires cxx17_if_constexpr ] ]
+   [ run ccmath_signbit_test.cpp ../../test/build//boost_unit_test_framework : : : [ requires cxx17_if_constexpr ] ]
    [ run log1p_expm1_test.cpp test_instances//test_instances pch_light ../../test/build//boost_unit_test_framework  ]
    [ run powm1_sqrtp1m1_test.cpp test_instances//test_instances pch_light ../../test/build//boost_unit_test_framework  ]
    [ run git_issue_705.cpp ../../test/build//boost_unit_test_framework  ]

test/ccmath_signbit_test.cpp

@@ -0,0 +1,43 @@
+//  (C) Copyright Matt Borland 2022.
+//  Use, modification and distribution are subject to the
+//  Boost Software License, Version 1.0. (See accompanying file
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <type_traits>
+#include <boost/math/ccmath/signbit.hpp>
+
+#if !defined(BOOST_MATH_NO_CONSTEXPR_DETECTION) && !defined(BOOST_MATH_USING_BUILTIN_CONSTANT_P)
+template <typename T>
+void test()
+{
+    // Edge cases
+    static_assert(boost::math::ccmath::signbit(T(0)) == false);
+    static_assert(boost::math::ccmath::signbit(std::numeric_limits<T>::quiet_NaN()) == false);
+    static_assert(boost::math::ccmath::signbit(std::numeric_limits<T>::signaling_NaN()) == false);
+
+    // Positive numbers
+    static_assert(boost::math::ccmath::signbit(std::numeric_limits<T>::infinity()) == false);
+    static_assert(boost::math::ccmath::signbit(T(1)) == false);
+
+    // Negative numbers
+    static_assert(boost::math::ccmath::signbit(-std::numeric_limits<T>::infinity()) == true);
+    static_assert(boost::math::ccmath::signbit(T(-1)) == true);
+}
+
+int main(void)
+{
+    test<float>();
+    test<double>();
+
+    #ifndef BOOST_MATH_NO_LONG_DOUBLE_MATH_FUNCTIONS
+    test<long double>();
+    #endif
+
+    return 0;
+}
+#else
+int main(void)
+{
+    return 0;
+}
+#endif 

test/compile_test/ccmath_signbit_incl_test.cpp

@@ -0,0 +1,16 @@
+//  (C) Copyright Matt Borland 2022.
+//  Use, modification and distribution are subject to the
+//  Boost Software License, Version 1.0. (See accompanying file
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/math/ccmath/signbit.hpp>
+#include "test_compile_result.hpp"
+
+void compile_and_link_test()
+{
+   check_result<bool>(boost::math::ccmath::signbit(1.0F));
+   check_result<bool>(boost::math::ccmath::signbit(1.0));
+#ifndef BOOST_MATH_NO_LONG_DOUBLE_MATH_FUNCTIONS
+   check_result<bool>(boost::math::ccmath::signbit(1.0L));
+#endif
+}