Fix conflict between cstdfloat.hpp and multiprecision. (#474)

* Tentatively fix conflict between cstdfloat.hpp and multiprecision.

* Add missing struct to new code path.

* Reconfigure again for cstdfloat.hpp interop.

* Add missing #includes to float128.hpp.
Make constexpr __float128 arithmetic tests behave the same way, irrespective of whether numberic_limits is specialized for that type.

* Make tests conditional on BOOST_HAS_FLOAT128

include/boost/multiprecision/detail/float128_functions.hpp

@@ -11,7 +11,14 @@
 
 #include <boost/multiprecision/detail/standalone_config.hpp>
 
-#ifdef BOOST_HAS_FLOAT128
+#ifndef BOOST_MP_STANDALONE
+#include <boost/cstdfloat.hpp>
+#if defined(BOOST_MATH_USE_FLOAT128) && !defined(BOOST_CSTDFLOAT_NO_LIBQUADMATH_SUPPORT)
+#  define BOOST_MP_HAVE_CSTDFLOAT
+#endif
+#endif
+
+#if defined(BOOST_HAS_FLOAT128)
 
 namespace boost 
 {
@@ -27,10 +34,15 @@ extern "C" int        isinfq(__float128) throw();
 extern "C" int        isnanq(__float128) throw();
 extern "C" __float128 strtoflt128(const char*, char**) throw();
 
+#ifdef BOOST_MP_HAVE_CSTDFLOAT
+using std::ldexp;
+using std::frexp;
+using std::floor;
+#else
 inline __float128 ldexp(__float128 f, int i) throw() { return ldexpq(f, i); }
 inline __float128 frexp(__float128 f, int* p) throw() { return frexpq(f, p); }
 inline __float128 floor(__float128 f) throw() { return floorq(f); }
-
+#endif
 }
 
 namespace detail {

include/boost/multiprecision/float128.hpp

@@ -16,7 +16,10 @@
 #endif
 
 #include <memory>
+#include <climits>
+#include <cfloat>
 #include <tuple>
+#include <cstring>
 #include <boost/multiprecision/detail/standalone_config.hpp>
 #include <boost/multiprecision/number.hpp>
 #include <boost/multiprecision/detail/hash.hpp>

test/constexpr_arithmetric_test.hpp

@@ -9,11 +9,17 @@
 
 // clang-format off
 
+#ifdef BOOST_HAS_FLOAT128
+#define OR_IS_FLOAT128 || std::is_same<__float128, U>::value
+#else
+#define OR_IS_FLOAT128
+#endif
+
 template <class T, class U>
 BOOST_CXX14_CONSTEXPR T do_test_constexpr_add_subtract(T a, U b)
 {
    a = +b;
-   if constexpr(std::numeric_limits<U>::is_signed && std::numeric_limits<T>::is_signed)
+   if constexpr((std::numeric_limits<U>::is_signed OR_IS_FLOAT128) && std::numeric_limits<T>::is_signed)
       b = -b;
    a += b;
    a += a;
@@ -26,13 +32,13 @@ BOOST_CXX14_CONSTEXPR T do_test_constexpr_add_subtract(T a, U b)
    a += bb--;
    a = a + b;
    a += a - b;
-   if constexpr(std::numeric_limits<U>::is_signed && std::numeric_limits<T>::is_signed)
+   if constexpr((std::numeric_limits<U>::is_signed OR_IS_FLOAT128) && std::numeric_limits<T>::is_signed)
       a -= b - -a;
    a += b + a;
    if constexpr(std::numeric_limits<T>::is_signed)
    {
       a = -a;
-      if constexpr(std::numeric_limits<U>::is_signed)
+      if constexpr(std::numeric_limits<U>::is_signed OR_IS_FLOAT128)
          a -= b;
    }
    return a;

test/constexpr_test_float128.cpp

@@ -16,13 +16,13 @@ int main()
       constexpr float128 b = test_constexpr_add_subtract(a);
 
       constexpr __float128 f128 = (__float128)b;
-      static_assert(f128 == -134.0f);
+      static_assert(f128 == -108.0f);
 
       constexpr int i = (int)b;
-      static_assert(i == -134);
+      static_assert(i == -108);
 
       constexpr short s = (short)b;
-      static_assert(s == -134);
+      static_assert(s == -108);
    }
    {
       constexpr float128 a(22);

test/test.hpp

@@ -167,7 +167,7 @@ std::ostream& operator<<(std::ostream& os, unsigned __int128 val)
 }
 
 #endif
-#ifdef BOOST_HAS_FLOAT128
+#if defined(BOOST_HAS_FLOAT128) && !defined(BOOST_MP_HAVE_CSTDFLOAT)
 std::ostream& operator<<(std::ostream& os, __float128 f)
 {
    return os << static_cast<long double>(f);