Use signcopy workaround

If constexpr signbit is not available, we make
use of signcopy(), which has been constexpr for
quite a while in Clang.

Author: robincaloudis

libcxx/include/__math/traits.h

@@ -30,19 +30,39 @@ namespace __math {
 
 template <class _A1, __enable_if_t<is_floating_point<_A1>::value, int> = 0>
 _LIBCPP_NODISCARD inline _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI bool signbit(_A1 __x) _NOEXCEPT {
+// TODO(LLVM 22): Remove `__builtin_copysign`-workaround once support for Clang 19 is dropped.
+#if !__has_constexpr_builtin(__builtin_signbit) && _LIBCPP_STD_VER >= 23
+  return __builtin_copysign(1.0, __x) == -1.0;
+#else
   return __builtin_signbit(__x);
+#endif
 }
 
 _LIBCPP_NODISCARD inline _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI bool signbit(float __x) _NOEXCEPT {
+// TODO(LLVM 22): Remove `__builtin_copysign`-workaround once support for Clang 19 is dropped.
+#if !__has_constexpr_builtin(__builtin_signbit) && _LIBCPP_STD_VER >= 23
+  return __builtin_copysign(1.0, __x) == -1.0;
+#else
   return __builtin_signbit(__x);
+#endif
 }
 
 _LIBCPP_NODISCARD inline _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI bool signbit(double __x) _NOEXCEPT {
+// TODO(LLVM 22): Remove `__builtin_copysign`-workaround once support for Clang 19 is dropped.
+#if !__has_constexpr_builtin(__builtin_signbit) && _LIBCPP_STD_VER >= 23
+  return __builtin_copysign(1.0, __x) == -1.0;
+#else
   return __builtin_signbit(__x);
+#endif
 }
 
 _LIBCPP_NODISCARD inline _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI bool signbit(long double __x) _NOEXCEPT {
+// TODO(LLVM 22): Remove `__builtin_copysign`-workaround once support for Clang 19 is dropped.
+#if !__has_constexpr_builtin(__builtin_signbit) && _LIBCPP_STD_VER >= 23
+  return __builtin_copysign(1.0, __x) == -1.0;
+#else
   return __builtin_signbit(__x);
+#endif
 }
 
 template <class _A1, __enable_if_t<is_integral<_A1>::value && is_signed<_A1>::value, int> = 0>