Revert "[libc++] Remove dead implementation of is_nothrow_convertible and merge the remaining code into is_convertible.h (llvm#137717)"

This reverts commit e43e8ec.

Author: AnthonyLatsis

libcxx/include/CMakeLists.txt

@@ -847,6 +847,7 @@ set(files
   __type_traits/is_member_pointer.h
   __type_traits/is_nothrow_assignable.h
   __type_traits/is_nothrow_constructible.h
+  __type_traits/is_nothrow_convertible.h
   __type_traits/is_nothrow_destructible.h
   __type_traits/is_null_pointer.h
   __type_traits/is_object.h

libcxx/include/__type_traits/is_convertible.h

@@ -26,16 +26,6 @@ template <class _From, class _To>
 _LIBCPP_NO_SPECIALIZATIONS inline constexpr bool is_convertible_v = __is_convertible(_From, _To);
 #endif
 
-#if _LIBCPP_STD_VER >= 20
-
-template <class _Tp, class _Up>
-struct _LIBCPP_NO_SPECIALIZATIONS is_nothrow_convertible : bool_constant<__is_nothrow_convertible(_Tp, _Up)> {};
-
-template <class _Tp, class _Up>
-_LIBCPP_NO_SPECIALIZATIONS inline constexpr bool is_nothrow_convertible_v = __is_nothrow_convertible(_Tp, _Up);
-
-#endif // _LIBCPP_STD_VER >= 20
-
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___TYPE_TRAITS_IS_CONVERTIBLE_H

libcxx/include/__type_traits/is_nothrow_convertible.h

@@ -0,0 +1,62 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___TYPE_TRAITS_IS_NOTHROW_CONVERTIBLE_H
+#define _LIBCPP___TYPE_TRAITS_IS_NOTHROW_CONVERTIBLE_H
+
+#include <__config>
+#include <__type_traits/conjunction.h>
+#include <__type_traits/disjunction.h>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/is_convertible.h>
+#include <__type_traits/is_void.h>
+#include <__type_traits/lazy.h>
+#include <__utility/declval.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 20
+
+#  if __has_builtin(__is_nothrow_convertible)
+
+template <class _Tp, class _Up>
+struct _LIBCPP_NO_SPECIALIZATIONS is_nothrow_convertible : bool_constant<__is_nothrow_convertible(_Tp, _Up)> {};
+
+template <class _Tp, class _Up>
+_LIBCPP_NO_SPECIALIZATIONS inline constexpr bool is_nothrow_convertible_v = __is_nothrow_convertible(_Tp, _Up);
+
+#  else // __has_builtin(__is_nothrow_convertible)
+
+template <typename _Tp>
+void __test_noexcept(_Tp) noexcept;
+
+template <typename _Fm, typename _To>
+bool_constant<noexcept(std::__test_noexcept<_To>(std::declval<_Fm>()))> __is_nothrow_convertible_test();
+
+template <typename _Fm, typename _To>
+struct __is_nothrow_convertible_helper : decltype(std::__is_nothrow_convertible_test<_Fm, _To>()) {};
+
+template <typename _Fm, typename _To>
+struct is_nothrow_convertible
+    : _Or<_And<is_void<_To>, is_void<_Fm>>,
+          _Lazy<_And, is_convertible<_Fm, _To>, __is_nothrow_convertible_helper<_Fm, _To> > >::type {};
+
+template <typename _Fm, typename _To>
+inline constexpr bool is_nothrow_convertible_v = is_nothrow_convertible<_Fm, _To>::value;
+
+#  endif // __has_builtin(__is_nothrow_convertible)
+
+#endif // _LIBCPP_STD_VER >= 20
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___TYPE_TRAITS_IS_NOTHROW_CONVERTIBLE_H

libcxx/include/module.modulemap.in

@@ -232,6 +232,10 @@ module std_core [system] {
       header "__type_traits/is_nothrow_constructible.h"
       export std_core.type_traits.integral_constant
     }
+    module is_nothrow_convertible {
+      header "__type_traits/is_nothrow_convertible.h"
+      export std_core.type_traits.integral_constant
+    }
     module is_nothrow_destructible {
       header "__type_traits/is_nothrow_destructible.h"
       export std_core.type_traits.integral_constant

libcxx/include/type_traits

@@ -548,6 +548,7 @@ namespace std
 #    include <__type_traits/common_reference.h>
 #    include <__type_traits/is_bounded_array.h>
 #    include <__type_traits/is_constant_evaluated.h>
+#    include <__type_traits/is_nothrow_convertible.h>
 #    include <__type_traits/is_unbounded_array.h>
 #    include <__type_traits/type_identity.h>
 #    include <__type_traits/unwrap_ref.h>