Apply constexpr only when builtin is

Author: robincaloudis

libcxx/include/__math/traits.h

@@ -28,13 +28,14 @@ namespace __math {
 // signbit
 
 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);
+_LIBCPP_NODISCARD inline
+// TODO(LLVM 22): Remove conditional once support for Clang 19 is dropped.
+#if !defined(__clang__) || __has_constexpr_builtin(__builtin_signbit)
+    _LIBCPP_CONSTEXPR_SINCE_CXX23
 #endif
+    _LIBCPP_HIDE_FROM_ABI bool
+    signbit(_A1 __x) _NOEXCEPT {
+  return __builtin_signbit(__x);
 }
 
 _LIBCPP_NODISCARD inline _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI

libcxx/test/libcxx/numerics/c.math/constexpr-cxx23-clang.pass.cpp

@@ -220,9 +220,16 @@ int main(int, char**) {
   ASSERT_CONSTEXPR_CXX23(std::isnormal(-1.0) == 1);
   ASSERT_CONSTEXPR_CXX23(std::isnormal(-1.0L) == 1);
 
+// TODO(LLVM 22): Remove `__has_constexpr_builtin` conditional once support for Clang 19 is dropped.
+#if !__has_constexpr_builtin(__builtin_signbit)
+  ASSERT_NOT_CONSTEXPR_CXX23(std::signbit(-1.0f) == 1);
+  ASSERT_NOT_CONSTEXPR_CXX23(std::signbit(-1.0) == 1);
+  ASSERT_NOT_CONSTEXPR_CXX23(std::signbit(-1.0L) == 1);
+#else
   ASSERT_CONSTEXPR_CXX23(std::signbit(-1.0f) == 1);
   ASSERT_CONSTEXPR_CXX23(std::signbit(-1.0) == 1);
   ASSERT_CONSTEXPR_CXX23(std::signbit(-1.0L) == 1);
+#endif
 
   ASSERT_NOT_CONSTEXPR_CXX23(std::isgreater(-1.0f, 0.0f) == 0);
   ASSERT_NOT_CONSTEXPR_CXX23(std::isgreater(-1.0, 0.0) == 0);

libcxx/test/std/numerics/c.math/signbit.pass.cpp

@@ -11,6 +11,9 @@
 // We don't control the implementation on windows
 // UNSUPPORTED: windows
 
+// These compilers don't support constexpr `__builtin_signbit` yet.
+// UNSUPPORTED: clang-17, clang-18, clang-19, apple-clang-16, apple-clang-17
+
 #include <cassert>
 #include <cmath>
 #include <limits>