diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -99,6 +99,7 @@
   __algorithm/unique_copy.h
   __algorithm/unwrap_iter.h
   __algorithm/upper_bound.h
+  __assert
   __availability
   __bit/bit_cast.h
   __bit/byteswap.h
diff --git a/libcxx/include/__algorithm/clamp.h b/libcxx/include/__algorithm/clamp.h
--- a/libcxx/include/__algorithm/clamp.h
+++ b/libcxx/include/__algorithm/clamp.h
@@ -10,8 +10,8 @@
 #define _LIBCPP___ALGORITHM_CLAMP_H
 
 #include <__algorithm/comp.h>
+#include <__assert>
 #include <__config>
-#include <__debug>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
diff --git a/libcxx/include/__algorithm/comp_ref_type.h b/libcxx/include/__algorithm/comp_ref_type.h
--- a/libcxx/include/__algorithm/comp_ref_type.h
+++ b/libcxx/include/__algorithm/comp_ref_type.h
@@ -9,6 +9,7 @@
 #ifndef _LIBCPP___ALGORITHM_COMP_REF_TYPE_H
 #define _LIBCPP___ALGORITHM_COMP_REF_TYPE_H
 
+#include <__assert>
 #include <__config>
 #include <__debug>
 #include <__utility/declval.h>
diff --git a/libcxx/include/__algorithm/sample.h b/libcxx/include/__algorithm/sample.h
--- a/libcxx/include/__algorithm/sample.h
+++ b/libcxx/include/__algorithm/sample.h
@@ -10,8 +10,8 @@
 #define _LIBCPP___ALGORITHM_SAMPLE_H
 
 #include <__algorithm/min.h>
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__random/uniform_int_distribution.h>
 #include <iterator>
 
diff --git a/libcxx/include/__assert b/libcxx/include/__assert
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__assert
@@ -0,0 +1,69 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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___ASSERT
+#define _LIBCPP___ASSERT
+
+#include <__config>
+#include <iosfwd> // for std::string
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_DEBUG_LEVEL >= 1
+#   define _LIBCPP_ASSERT_IMPL(x, m) ((x) ? (void)0 : ::std::__libcpp_debug_function(::std::__libcpp_debug_info(__FILE__, __LINE__, #x, m)))
+#else
+#   define _LIBCPP_ASSERT_IMPL(x, m) ((void)0)
+#endif
+
+// We do this dance because some of our tests re-define _LIBCPP_ASSERT to something else.
+// In the future, we should find other ways to test our assertions and disallow re-defining
+// _LIBCPP_ASSERT.
+#if !defined(_LIBCPP_ASSERT)
+#   define _LIBCPP_ASSERT(x, m) _LIBCPP_ASSERT_IMPL(x, m)
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+struct _LIBCPP_TEMPLATE_VIS __libcpp_debug_info {
+  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+  __libcpp_debug_info()
+      : __file_(nullptr), __line_(-1), __pred_(nullptr), __msg_(nullptr) {}
+  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+  __libcpp_debug_info(const char* __f, int __l, const char* __p, const char* __m)
+    : __file_(__f), __line_(__l), __pred_(__p), __msg_(__m) {}
+
+  _LIBCPP_FUNC_VIS string what() const;
+
+  const char* __file_;
+  int __line_;
+  const char* __pred_;
+  const char* __msg_;
+};
+
+/// __libcpp_debug_function_type - The type of the assertion failure handler.
+typedef void(*__libcpp_debug_function_type)(__libcpp_debug_info const&);
+
+/// __libcpp_debug_function - The handler function called when a _LIBCPP_ASSERT
+///    fails.
+extern _LIBCPP_EXPORTED_FROM_ABI __libcpp_debug_function_type __libcpp_debug_function;
+
+/// __libcpp_abort_debug_function - A debug handler that aborts when called.
+_LIBCPP_NORETURN _LIBCPP_FUNC_VIS
+void __libcpp_abort_debug_function(__libcpp_debug_info const&);
+
+/// __libcpp_set_debug_function - Set the debug handler to the specified
+///    function.
+_LIBCPP_FUNC_VIS
+bool __libcpp_set_debug_function(__libcpp_debug_function_type __func);
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___ASSERT
diff --git a/libcxx/include/__coroutine/coroutine_handle.h b/libcxx/include/__coroutine/coroutine_handle.h
--- a/libcxx/include/__coroutine/coroutine_handle.h
+++ b/libcxx/include/__coroutine/coroutine_handle.h
@@ -9,8 +9,8 @@
 #ifndef _LIBCPP___COROUTINE_COROUTINE_HANDLE_H
 #define _LIBCPP___COROUTINE_COROUTINE_HANDLE_H
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__functional/hash.h>
 #include <__memory/addressof.h>
 #include <compare>
diff --git a/libcxx/include/__debug b/libcxx/include/__debug
--- a/libcxx/include/__debug
+++ b/libcxx/include/__debug
@@ -24,57 +24,16 @@
 #   include <cstdlib>
 #endif
 
-#if _LIBCPP_DEBUG_LEVEL == 0
+#if _LIBCPP_DEBUG_LEVEL < 2
 #   define _LIBCPP_DEBUG_ASSERT(x, m) ((void)0)
-#   define _LIBCPP_ASSERT_IMPL(x, m) ((void)0)
-#elif _LIBCPP_DEBUG_LEVEL == 1
-#   define _LIBCPP_DEBUG_ASSERT(x, m) ((void)0)
-#   define _LIBCPP_ASSERT_IMPL(x, m) ((x) ? (void)0 : ::std::__libcpp_debug_function(::std::__libcpp_debug_info(__FILE__, __LINE__, #x, m)))
 #elif _LIBCPP_DEBUG_LEVEL == 2
 #   define _LIBCPP_DEBUG_ASSERT(x, m) _LIBCPP_ASSERT(::std::__libcpp_is_constant_evaluated() || (x), m)
-#   define _LIBCPP_ASSERT_IMPL(x, m) ((x) ? (void)0 : ::std::__libcpp_debug_function(::std::__libcpp_debug_info(__FILE__, __LINE__, #x, m)))
 #else
 #   error _LIBCPP_DEBUG_LEVEL must be one of 0, 1, 2
 #endif
 
-#if !defined(_LIBCPP_ASSERT)
-#   define _LIBCPP_ASSERT(x, m) _LIBCPP_ASSERT_IMPL(x, m)
-#endif
-
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-struct _LIBCPP_TEMPLATE_VIS __libcpp_debug_info {
-  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
-  __libcpp_debug_info()
-      : __file_(nullptr), __line_(-1), __pred_(nullptr), __msg_(nullptr) {}
-  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
-  __libcpp_debug_info(const char* __f, int __l, const char* __p, const char* __m)
-    : __file_(__f), __line_(__l), __pred_(__p), __msg_(__m) {}
-
-  _LIBCPP_FUNC_VIS string what() const;
-
-  const char* __file_;
-  int __line_;
-  const char* __pred_;
-  const char* __msg_;
-};
-
-/// __libcpp_debug_function_type - The type of the assertion failure handler.
-typedef void(*__libcpp_debug_function_type)(__libcpp_debug_info const&);
-
-/// __libcpp_debug_function - The handler function called when a _LIBCPP_ASSERT
-///    fails.
-extern _LIBCPP_EXPORTED_FROM_ABI __libcpp_debug_function_type __libcpp_debug_function;
-
-/// __libcpp_abort_debug_function - A debug handler that aborts when called.
-_LIBCPP_NORETURN _LIBCPP_FUNC_VIS
-void __libcpp_abort_debug_function(__libcpp_debug_info const&);
-
-/// __libcpp_set_debug_function - Set the debug handler to the specified
-///    function.
-_LIBCPP_FUNC_VIS
-bool __libcpp_set_debug_function(__libcpp_debug_function_type __func);
-
 #if _LIBCPP_DEBUG_LEVEL == 2 || defined(_LIBCPP_BUILDING_LIBRARY)
 
 struct _LIBCPP_TYPE_VIS __c_node;
diff --git a/libcxx/include/__filesystem/directory_iterator.h b/libcxx/include/__filesystem/directory_iterator.h
--- a/libcxx/include/__filesystem/directory_iterator.h
+++ b/libcxx/include/__filesystem/directory_iterator.h
@@ -10,9 +10,9 @@
 #ifndef _LIBCPP___FILESYSTEM_DIRECTORY_ITERATOR_H
 #define _LIBCPP___FILESYSTEM_DIRECTORY_ITERATOR_H
 
+#include <__assert>
 #include <__availability>
 #include <__config>
-#include <__debug>
 #include <__filesystem/directory_entry.h>
 #include <__filesystem/directory_options.h>
 #include <__filesystem/path.h>
diff --git a/libcxx/include/__filesystem/path_iterator.h b/libcxx/include/__filesystem/path_iterator.h
--- a/libcxx/include/__filesystem/path_iterator.h
+++ b/libcxx/include/__filesystem/path_iterator.h
@@ -10,9 +10,9 @@
 #ifndef _LIBCPP___FILESYSTEM_PATH_ITERATOR_H
 #define _LIBCPP___FILESYSTEM_PATH_ITERATOR_H
 
+#include <__assert>
 #include <__availability>
 #include <__config>
-#include <__debug>
 #include <__filesystem/path.h>
 #include <__iterator/iterator_traits.h>
 #include <cstddef>
diff --git a/libcxx/include/__format/format_arg.h b/libcxx/include/__format/format_arg.h
--- a/libcxx/include/__format/format_arg.h
+++ b/libcxx/include/__format/format_arg.h
@@ -10,6 +10,7 @@
 #ifndef _LIBCPP___FORMAT_FORMAT_ARG_H
 #define _LIBCPP___FORMAT_FORMAT_ARG_H
 
+#include <__assert>
 #include <__concepts/arithmetic.h>
 #include <__config>
 #include <__format/format_error.h>
diff --git a/libcxx/include/__format/format_string.h b/libcxx/include/__format/format_string.h
--- a/libcxx/include/__format/format_string.h
+++ b/libcxx/include/__format/format_string.h
@@ -10,8 +10,8 @@
 #ifndef _LIBCPP___FORMAT_FORMAT_STRING_H
 #define _LIBCPP___FORMAT_FORMAT_STRING_H
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__format/format_error.h>
 #include <cstddef>
 #include <cstdint>
diff --git a/libcxx/include/__format/formatter.h b/libcxx/include/__format/formatter.h
--- a/libcxx/include/__format/formatter.h
+++ b/libcxx/include/__format/formatter.h
@@ -12,6 +12,7 @@
 
 #include <__algorithm/copy.h>
 #include <__algorithm/fill_n.h>
+#include <__assert>
 #include <__availability>
 #include <__config>
 #include <__format/format_error.h>
diff --git a/libcxx/include/__format/formatter_floating_point.h b/libcxx/include/__format/formatter_floating_point.h
--- a/libcxx/include/__format/formatter_floating_point.h
+++ b/libcxx/include/__format/formatter_floating_point.h
@@ -17,9 +17,9 @@
 #include <__algorithm/min.h>
 #include <__algorithm/rotate.h>
 #include <__algorithm/transform.h>
+#include <__assert>
 #include <__concepts/arithmetic.h>
 #include <__config>
-#include <__debug>
 #include <__format/format_error.h>
 #include <__format/format_fwd.h>
 #include <__format/format_string.h>
diff --git a/libcxx/include/__format/formatter_integral.h b/libcxx/include/__format/formatter_integral.h
--- a/libcxx/include/__format/formatter_integral.h
+++ b/libcxx/include/__format/formatter_integral.h
@@ -14,6 +14,7 @@
 #include <__algorithm/copy_n.h>
 #include <__algorithm/fill_n.h>
 #include <__algorithm/transform.h>
+#include <__assert>
 #include <__config>
 #include <__format/format_error.h>
 #include <__format/format_fwd.h>
diff --git a/libcxx/include/__format/formatter_pointer.h b/libcxx/include/__format/formatter_pointer.h
--- a/libcxx/include/__format/formatter_pointer.h
+++ b/libcxx/include/__format/formatter_pointer.h
@@ -11,9 +11,9 @@
 #define _LIBCPP___FORMAT_FORMATTER_POINTER_H
 
 #include <__algorithm/copy.h>
+#include <__assert>
 #include <__availability>
 #include <__config>
-#include <__debug>
 #include <__format/format_error.h>
 #include <__format/format_fwd.h>
 #include <__format/formatter.h>
diff --git a/libcxx/include/__format/formatter_string.h b/libcxx/include/__format/formatter_string.h
--- a/libcxx/include/__format/formatter_string.h
+++ b/libcxx/include/__format/formatter_string.h
@@ -10,6 +10,7 @@
 #ifndef _LIBCPP___FORMAT_FORMATTER_STRING_H
 #define _LIBCPP___FORMAT_FORMATTER_STRING_H
 
+#include <__assert>
 #include <__config>
 #include <__format/format_error.h>
 #include <__format/format_fwd.h>
diff --git a/libcxx/include/__format/parser_std_format_spec.h b/libcxx/include/__format/parser_std_format_spec.h
--- a/libcxx/include/__format/parser_std_format_spec.h
+++ b/libcxx/include/__format/parser_std_format_spec.h
@@ -12,8 +12,8 @@
 
 #include <__algorithm/find_if.h>
 #include <__algorithm/min.h>
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__format/format_arg.h>
 #include <__format/format_error.h>
 #include <__format/format_string.h>
diff --git a/libcxx/include/__functional/function.h b/libcxx/include/__functional/function.h
--- a/libcxx/include/__functional/function.h
+++ b/libcxx/include/__functional/function.h
@@ -10,8 +10,8 @@
 #ifndef _LIBCPP___FUNCTIONAL_FUNCTION_H
 #define _LIBCPP___FUNCTIONAL_FUNCTION_H
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__functional/binary_function.h>
 #include <__functional/invoke.h>
 #include <__functional/unary_function.h>
diff --git a/libcxx/include/__hash_table b/libcxx/include/__hash_table
--- a/libcxx/include/__hash_table
+++ b/libcxx/include/__hash_table
@@ -10,6 +10,7 @@
 #ifndef _LIBCPP__HASH_TABLE
 #define _LIBCPP__HASH_TABLE
 
+#include <__assert>
 #include <__bits> // __libcpp_clz
 #include <__config>
 #include <__debug>
diff --git a/libcxx/include/__iterator/advance.h b/libcxx/include/__iterator/advance.h
--- a/libcxx/include/__iterator/advance.h
+++ b/libcxx/include/__iterator/advance.h
@@ -10,8 +10,8 @@
 #ifndef _LIBCPP___ITERATOR_ADVANCE_H
 #define _LIBCPP___ITERATOR_ADVANCE_H
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__iterator/concepts.h>
 #include <__iterator/incrementable_traits.h>
 #include <__iterator/iterator_traits.h>
diff --git a/libcxx/include/__iterator/common_iterator.h b/libcxx/include/__iterator/common_iterator.h
--- a/libcxx/include/__iterator/common_iterator.h
+++ b/libcxx/include/__iterator/common_iterator.h
@@ -10,8 +10,8 @@
 #ifndef _LIBCPP___ITERATOR_COMMON_ITERATOR_H
 #define _LIBCPP___ITERATOR_COMMON_ITERATOR_H
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__iterator/concepts.h>
 #include <__iterator/incrementable_traits.h>
 #include <__iterator/iter_move.h>
diff --git a/libcxx/include/__iterator/counted_iterator.h b/libcxx/include/__iterator/counted_iterator.h
--- a/libcxx/include/__iterator/counted_iterator.h
+++ b/libcxx/include/__iterator/counted_iterator.h
@@ -9,8 +9,8 @@
 #ifndef _LIBCPP___ITERATOR_COUNTED_ITERATOR_H
 #define _LIBCPP___ITERATOR_COUNTED_ITERATOR_H
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__iterator/concepts.h>
 #include <__iterator/default_sentinel.h>
 #include <__iterator/incrementable_traits.h>
diff --git a/libcxx/include/__iterator/next.h b/libcxx/include/__iterator/next.h
--- a/libcxx/include/__iterator/next.h
+++ b/libcxx/include/__iterator/next.h
@@ -10,8 +10,8 @@
 #ifndef _LIBCPP___ITERATOR_NEXT_H
 #define _LIBCPP___ITERATOR_NEXT_H
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__iterator/advance.h>
 #include <__iterator/concepts.h>
 #include <__iterator/incrementable_traits.h>
diff --git a/libcxx/include/__iterator/prev.h b/libcxx/include/__iterator/prev.h
--- a/libcxx/include/__iterator/prev.h
+++ b/libcxx/include/__iterator/prev.h
@@ -10,8 +10,8 @@
 #ifndef _LIBCPP___ITERATOR_PREV_H
 #define _LIBCPP___ITERATOR_PREV_H
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__iterator/advance.h>
 #include <__iterator/concepts.h>
 #include <__iterator/incrementable_traits.h>
diff --git a/libcxx/include/__memory/construct_at.h b/libcxx/include/__memory/construct_at.h
--- a/libcxx/include/__memory/construct_at.h
+++ b/libcxx/include/__memory/construct_at.h
@@ -10,8 +10,8 @@
 #ifndef _LIBCPP___MEMORY_CONSTRUCT_AT_H
 #define _LIBCPP___MEMORY_CONSTRUCT_AT_H
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__iterator/access.h>
 #include <__memory/addressof.h>
 #include <__memory/voidify.h>
diff --git a/libcxx/include/__node_handle b/libcxx/include/__node_handle
--- a/libcxx/include/__node_handle
+++ b/libcxx/include/__node_handle
@@ -58,8 +58,8 @@
 
 */
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <memory>
 #include <optional>
 
diff --git a/libcxx/include/__numeric/gcd_lcm.h b/libcxx/include/__numeric/gcd_lcm.h
--- a/libcxx/include/__numeric/gcd_lcm.h
+++ b/libcxx/include/__numeric/gcd_lcm.h
@@ -10,8 +10,8 @@
 #ifndef _LIBCPP___NUMERIC_GCD_LCM_H
 #define _LIBCPP___NUMERIC_GCD_LCM_H
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <limits>
 #include <type_traits>
 
diff --git a/libcxx/include/__ranges/drop_view.h b/libcxx/include/__ranges/drop_view.h
--- a/libcxx/include/__ranges/drop_view.h
+++ b/libcxx/include/__ranges/drop_view.h
@@ -9,8 +9,8 @@
 #ifndef _LIBCPP___RANGES_DROP_VIEW_H
 #define _LIBCPP___RANGES_DROP_VIEW_H
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__iterator/concepts.h>
 #include <__iterator/iterator_traits.h>
 #include <__iterator/next.h>
diff --git a/libcxx/include/__ranges/iota_view.h b/libcxx/include/__ranges/iota_view.h
--- a/libcxx/include/__ranges/iota_view.h
+++ b/libcxx/include/__ranges/iota_view.h
@@ -9,6 +9,7 @@
 #ifndef _LIBCPP___RANGES_IOTA_VIEW_H
 #define _LIBCPP___RANGES_IOTA_VIEW_H
 
+#include <__assert>
 #include <__compare/three_way_comparable.h>
 #include <__concepts/arithmetic.h>
 #include <__concepts/constructible.h>
@@ -20,7 +21,6 @@
 #include <__concepts/semiregular.h>
 #include <__concepts/totally_ordered.h>
 #include <__config>
-#include <__debug>
 #include <__functional/ranges_operations.h>
 #include <__iterator/concepts.h>
 #include <__iterator/incrementable_traits.h>
diff --git a/libcxx/include/__ranges/subrange.h b/libcxx/include/__ranges/subrange.h
--- a/libcxx/include/__ranges/subrange.h
+++ b/libcxx/include/__ranges/subrange.h
@@ -9,13 +9,13 @@
 #ifndef _LIBCPP___RANGES_SUBRANGE_H
 #define _LIBCPP___RANGES_SUBRANGE_H
 
+#include <__assert>
 #include <__concepts/constructible.h>
 #include <__concepts/convertible_to.h>
 #include <__concepts/copyable.h>
 #include <__concepts/derived_from.h>
 #include <__concepts/different_from.h>
 #include <__config>
-#include <__debug>
 #include <__iterator/advance.h>
 #include <__iterator/concepts.h>
 #include <__iterator/incrementable_traits.h>
diff --git a/libcxx/include/__ranges/view_interface.h b/libcxx/include/__ranges/view_interface.h
--- a/libcxx/include/__ranges/view_interface.h
+++ b/libcxx/include/__ranges/view_interface.h
@@ -9,8 +9,8 @@
 #ifndef _LIBCPP___RANGES_VIEW_INTERFACE_H
 #define _LIBCPP___RANGES_VIEW_INTERFACE_H
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__iterator/concepts.h>
 #include <__iterator/iterator_traits.h>
 #include <__iterator/prev.h>
diff --git a/libcxx/include/__string b/libcxx/include/__string
--- a/libcxx/include/__string
+++ b/libcxx/include/__string
@@ -17,7 +17,9 @@
 #include <__algorithm/find_end.h>
 #include <__algorithm/find_first_of.h>
 #include <__algorithm/min.h>
+#include <__assert>
 #include <__config>
+#include <__debug>
 #include <__functional/hash.h>     // for __murmur2_or_cityhash
 #include <__iterator/iterator_traits.h>
 #include <cstdint>     // for uint_least16_t
@@ -30,8 +32,6 @@
 #   include <cwchar> // for wmemcpy
 #endif
 
-#include <__debug>
-
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/array b/libcxx/include/array
--- a/libcxx/include/array
+++ b/libcxx/include/array
@@ -108,8 +108,8 @@
 
 */
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__tuple>
 #include <__utility/unreachable.h>
 #include <algorithm>
diff --git a/libcxx/include/bit b/libcxx/include/bit
--- a/libcxx/include/bit
+++ b/libcxx/include/bit
@@ -61,11 +61,11 @@
 
 */
 
+#include <__assert>
 #include <__bit/bit_cast.h>
 #include <__bit/byteswap.h>
 #include <__bits> // __libcpp_clz
 #include <__config>
-#include <__debug>
 #include <limits>
 #include <type_traits>
 #include <version>
diff --git a/libcxx/include/charconv b/libcxx/include/charconv
--- a/libcxx/include/charconv
+++ b/libcxx/include/charconv
@@ -77,6 +77,7 @@
 
 */
 
+#include <__assert>
 #include <__availability>
 #include <__bits>
 #include <__charconv/chars_format.h>
diff --git a/libcxx/include/deque b/libcxx/include/deque
--- a/libcxx/include/deque
+++ b/libcxx/include/deque
@@ -160,8 +160,8 @@
 
 */
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__iterator/iterator_traits.h>
 #include <__split_buffer>
 #include <__utility/forward.h>
diff --git a/libcxx/include/experimental/coroutine b/libcxx/include/experimental/coroutine
--- a/libcxx/include/experimental/coroutine
+++ b/libcxx/include/experimental/coroutine
@@ -45,7 +45,7 @@
 
  */
 
-#include <__debug>
+#include <__assert>
 #include <cstddef>
 #include <experimental/__config>
 #include <functional>
diff --git a/libcxx/include/experimental/memory_resource b/libcxx/include/experimental/memory_resource
--- a/libcxx/include/experimental/memory_resource
+++ b/libcxx/include/experimental/memory_resource
@@ -64,7 +64,7 @@
 
  */
 
-#include <__debug>
+#include <__assert>
 #include <__tuple>
 #include <cstddef>
 #include <cstdlib>
diff --git a/libcxx/include/fstream b/libcxx/include/fstream
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -179,9 +179,9 @@
 
 */
 
+#include <__assert>
 #include <__availability>
 #include <__config>
-#include <__debug>
 #include <__locale>
 #include <__utility/unreachable.h>
 #include <cstdio>
diff --git a/libcxx/include/future b/libcxx/include/future
--- a/libcxx/include/future
+++ b/libcxx/include/future
@@ -361,9 +361,9 @@
 
 */
 
+#include <__assert>
 #include <__availability>
 #include <__config>
-#include <__debug>
 #include <__memory/allocator_arg_t.h>
 #include <__memory/uses_allocator.h>
 #include <__utility/auto_cast.h>
diff --git a/libcxx/include/list b/libcxx/include/list
--- a/libcxx/include/list
+++ b/libcxx/include/list
@@ -180,6 +180,7 @@
 
 */
 
+#include <__assert>
 #include <__config>
 #include <__debug>
 #include <__utility/forward.h>
diff --git a/libcxx/include/locale b/libcxx/include/locale
--- a/libcxx/include/locale
+++ b/libcxx/include/locale
@@ -187,6 +187,7 @@
 
 */
 
+#include <__assert>
 #include <__config>
 #include <__debug>
 #include <__locale>
diff --git a/libcxx/include/map b/libcxx/include/map
--- a/libcxx/include/map
+++ b/libcxx/include/map
@@ -528,8 +528,8 @@
 
 */
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__functional/is_transparent.h>
 #include <__iterator/iterator_traits.h>
 #include <__node_handle>
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -989,6 +989,7 @@
 
   // __config not modularised due to a bug in Clang
   // FIXME: These should be private.
+  module __assert            { private header "__assert"            export * }
   module __availability      { private header "__availability"      export * }
   module __bit_reference     { private header "__bit_reference"     export * }
   module __bits              { private header "__bits"              export * }
diff --git a/libcxx/include/optional b/libcxx/include/optional
--- a/libcxx/include/optional
+++ b/libcxx/include/optional
@@ -158,10 +158,10 @@
 
 */
 
+#include <__assert>
 #include <__availability>
 #include <__concepts/invocable.h>
 #include <__config>
-#include <__debug>
 #include <compare>
 #include <functional>
 #include <initializer_list>
diff --git a/libcxx/include/regex b/libcxx/include/regex
--- a/libcxx/include/regex
+++ b/libcxx/include/regex
@@ -762,8 +762,8 @@
 } // std
 */
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__iterator/wrap_iter.h>
 #include <__locale>
 #include <compare>
diff --git a/libcxx/include/set b/libcxx/include/set
--- a/libcxx/include/set
+++ b/libcxx/include/set
@@ -471,8 +471,8 @@
 
 */
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__functional/is_transparent.h>
 #include <__iterator/iterator_traits.h>
 #include <__node_handle>
diff --git a/libcxx/include/span b/libcxx/include/span
--- a/libcxx/include/span
+++ b/libcxx/include/span
@@ -127,6 +127,7 @@
 
 */
 
+#include <__assert>
 #include <__config>
 #include <__debug>
 #include <__iterator/concepts.h>
diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -518,6 +518,7 @@
 
 */
 
+#include <__assert>
 #include <__config>
 #include <__debug>
 #include <__ios/fpos.h>
diff --git a/libcxx/include/string_view b/libcxx/include/string_view
--- a/libcxx/include/string_view
+++ b/libcxx/include/string_view
@@ -195,8 +195,8 @@
 
 */
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__ranges/concepts.h>
 #include <__ranges/data.h>
 #include <__ranges/enable_borrowed_range.h>
diff --git a/libcxx/include/thread b/libcxx/include/thread
--- a/libcxx/include/thread
+++ b/libcxx/include/thread
@@ -82,8 +82,8 @@
 
 */
 
+#include <__assert>
 #include <__config>
-#include <__debug>
 #include <__mutex_base>
 #include <__thread/poll_with_backoff.h>
 #include <__thread/timed_backoff_policy.h>
diff --git a/libcxx/include/unordered_map b/libcxx/include/unordered_map
--- a/libcxx/include/unordered_map
+++ b/libcxx/include/unordered_map
@@ -514,6 +514,7 @@
 
 */
 
+#include <__assert>
 #include <__config>
 #include <__debug>
 #include <__functional/is_transparent.h>
diff --git a/libcxx/include/unordered_set b/libcxx/include/unordered_set
--- a/libcxx/include/unordered_set
+++ b/libcxx/include/unordered_set
@@ -459,6 +459,7 @@
 
 */
 
+#include <__assert>
 #include <__config>
 #include <__debug>
 #include <__functional/is_transparent.h>
diff --git a/libcxx/include/vector b/libcxx/include/vector
--- a/libcxx/include/vector
+++ b/libcxx/include/vector
@@ -271,6 +271,7 @@
 
 */
 
+#include <__assert>
 #include <__bit_reference>
 #include <__config>
 #include <__debug>
diff --git a/libcxx/src/CMakeLists.txt b/libcxx/src/CMakeLists.txt
--- a/libcxx/src/CMakeLists.txt
+++ b/libcxx/src/CMakeLists.txt
@@ -65,6 +65,7 @@
 
 if (LIBCXX_ENABLE_DEBUG_MODE_SUPPORT)
   list(APPEND LIBCXX_SOURCES
+    assert.cpp
     debug.cpp
     )
 endif()
diff --git a/libcxx/src/assert.cpp b/libcxx/src/assert.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/src/assert.cpp
@@ -0,0 +1,39 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include <__assert>
+#include <__config>
+#include <cstdio>
+#include <cstdlib>
+#include <string>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+std::string __libcpp_debug_info::what() const {
+  string msg = __file_;
+  msg += ":" + to_string(__line_) + ": _LIBCPP_ASSERT '";
+  msg += __pred_;
+  msg += "' failed. ";
+  msg += __msg_;
+  return msg;
+}
+
+_LIBCPP_NORETURN void __libcpp_abort_debug_function(__libcpp_debug_info const& info) {
+    std::fprintf(stderr, "%s\n", info.what().c_str());
+    std::abort();
+}
+
+_LIBCPP_SAFE_STATIC __libcpp_debug_function_type
+    __libcpp_debug_function = __libcpp_abort_debug_function;
+
+bool __libcpp_set_debug_function(__libcpp_debug_function_type __func) {
+  __libcpp_debug_function = __func;
+  return true;
+}
+
+_LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/src/debug.cpp b/libcxx/src/debug.cpp
--- a/libcxx/src/debug.cpp
+++ b/libcxx/src/debug.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "__assert"
 #include "__config"
 #include "__debug"
 #include "functional"
@@ -22,27 +23,6 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-std::string __libcpp_debug_info::what() const {
-  string msg = __file_;
-  msg += ":" + to_string(__line_) + ": _LIBCPP_ASSERT '";
-  msg += __pred_;
-  msg += "' failed. ";
-  msg += __msg_;
-  return msg;
-}
-_LIBCPP_NORETURN void __libcpp_abort_debug_function(__libcpp_debug_info const& info) {
-    std::fprintf(stderr, "%s\n", info.what().c_str());
-    std::abort();
-}
-
-_LIBCPP_SAFE_STATIC __libcpp_debug_function_type
-    __libcpp_debug_function = __libcpp_abort_debug_function;
-
-bool __libcpp_set_debug_function(__libcpp_debug_function_type __func) {
-  __libcpp_debug_function = __func;
-  return true;
-}
-
 _LIBCPP_FUNC_VIS
 __libcpp_db*
 __get_db()
diff --git a/libcxx/src/filesystem/directory_iterator.cpp b/libcxx/src/filesystem/directory_iterator.cpp
--- a/libcxx/src/filesystem/directory_iterator.cpp
+++ b/libcxx/src/filesystem/directory_iterator.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <__assert>
 #include "__config"
 #include "filesystem"
 #include "stack"
diff --git a/libcxx/src/filesystem/filesystem_common.h b/libcxx/src/filesystem/filesystem_common.h
--- a/libcxx/src/filesystem/filesystem_common.h
+++ b/libcxx/src/filesystem/filesystem_common.h
@@ -9,6 +9,7 @@
 #ifndef FILESYSTEM_COMMON_H
 #define FILESYSTEM_COMMON_H
 
+#include <__assert>
 #include "__config"
 #include "array"
 #include "chrono"
diff --git a/libcxx/src/filesystem/operations.cpp b/libcxx/src/filesystem/operations.cpp
--- a/libcxx/src/filesystem/operations.cpp
+++ b/libcxx/src/filesystem/operations.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <__assert>
 #include <__utility/unreachable.h>
 #include "filesystem"
 #include "array"
diff --git a/libcxx/src/filesystem/posix_compat.h b/libcxx/src/filesystem/posix_compat.h
--- a/libcxx/src/filesystem/posix_compat.h
+++ b/libcxx/src/filesystem/posix_compat.h
@@ -23,6 +23,7 @@
 #ifndef POSIX_COMPAT_H
 #define POSIX_COMPAT_H
 
+#include <__assert>
 #include "filesystem"
 
 #include "filesystem_common.h"
diff --git a/libcxx/src/include/ryu/common.h b/libcxx/src/include/ryu/common.h
--- a/libcxx/src/include/ryu/common.h
+++ b/libcxx/src/include/ryu/common.h
@@ -42,6 +42,7 @@
 // Avoid formatting to keep the changes with the original code minimal.
 // clang-format off
 
+#include <__assert>
 #include "__config"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
diff --git a/libcxx/src/include/ryu/d2s_intrinsics.h b/libcxx/src/include/ryu/d2s_intrinsics.h
--- a/libcxx/src/include/ryu/d2s_intrinsics.h
+++ b/libcxx/src/include/ryu/d2s_intrinsics.h
@@ -42,6 +42,7 @@
 // Avoid formatting to keep the changes with the original code minimal.
 // clang-format off
 
+#include <__assert>
 #include "__config"
 
 #include "include/ryu/ryu.h"
diff --git a/libcxx/src/include/to_chars_floating_point.h b/libcxx/src/include/to_chars_floating_point.h
--- a/libcxx/src/include/to_chars_floating_point.h
+++ b/libcxx/src/include/to_chars_floating_point.h
@@ -21,6 +21,7 @@
 #include "__algorithm/find_if.h"
 #include "__algorithm/lower_bound.h"
 #include "__algorithm/min.h"
+#include <__assert>
 #include "__config"
 #include "__iterator/access.h"
 #include "__iterator/size.h"
diff --git a/libcxx/src/mutex.cpp b/libcxx/src/mutex.cpp
--- a/libcxx/src/mutex.cpp
+++ b/libcxx/src/mutex.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <__assert>
 #include "mutex"
 #include "limits"
 #include "system_error"
diff --git a/libcxx/src/ryu/d2fixed.cpp b/libcxx/src/ryu/d2fixed.cpp
--- a/libcxx/src/ryu/d2fixed.cpp
+++ b/libcxx/src/ryu/d2fixed.cpp
@@ -39,6 +39,7 @@
 // Avoid formatting to keep the changes with the original code minimal.
 // clang-format off
 
+#include <__assert>
 #include "__config"
 #include "charconv"
 #include "cstring"
diff --git a/libcxx/src/ryu/d2s.cpp b/libcxx/src/ryu/d2s.cpp
--- a/libcxx/src/ryu/d2s.cpp
+++ b/libcxx/src/ryu/d2s.cpp
@@ -39,6 +39,7 @@
 // Avoid formatting to keep the changes with the original code minimal.
 // clang-format off
 
+#include <__assert>
 #include "__config"
 #include "charconv"
 
diff --git a/libcxx/src/ryu/f2s.cpp b/libcxx/src/ryu/f2s.cpp
--- a/libcxx/src/ryu/f2s.cpp
+++ b/libcxx/src/ryu/f2s.cpp
@@ -39,6 +39,7 @@
 // Avoid formatting to keep the changes with the original code minimal.
 // clang-format off
 
+#include <__assert>
 #include "__config"
 #include "charconv"
 
diff --git a/libcxx/src/string.cpp b/libcxx/src/string.cpp
--- a/libcxx/src/string.cpp
+++ b/libcxx/src/string.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <__assert>
 #include "string"
 #include "charconv"
 #include "cstdlib"
@@ -13,7 +14,6 @@
 #include "limits"
 #include "stdexcept"
 #include <stdio.h>
-#include "__debug"
 
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 #   include "cwchar"
diff --git a/libcxx/src/strstream.cpp b/libcxx/src/strstream.cpp
--- a/libcxx/src/strstream.cpp
+++ b/libcxx/src/strstream.cpp
@@ -6,13 +6,13 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <__assert>
 #include <__utility/unreachable.h>
 #include "strstream"
 #include "algorithm"
 #include "climits"
 #include "cstring"
 #include "cstdlib"
-#include "__debug"
 #include "__undef_macros"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
diff --git a/libcxx/src/support/ibm/xlocale_zos.cpp b/libcxx/src/support/ibm/xlocale_zos.cpp
--- a/libcxx/src/support/ibm/xlocale_zos.cpp
+++ b/libcxx/src/support/ibm/xlocale_zos.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <__assert>
 #include <__support/ibm/xlocale.h>
 #include <sstream>
 #include <vector>
diff --git a/libcxx/src/system_error.cpp b/libcxx/src/system_error.cpp
--- a/libcxx/src/system_error.cpp
+++ b/libcxx/src/system_error.cpp
@@ -10,6 +10,7 @@
 
 #include "system_error"
 
+#include <__assert>
 #include "include/config_elast.h"
 #include "cerrno"
 #include "cstring"
@@ -17,7 +18,6 @@
 #include "cstdlib"
 #include "string"
 #include "string.h"
-#include "__debug"
 
 #if defined(__ANDROID__)
 #include <android/api-level.h>
diff --git a/libcxx/test/libcxx/diagnostics/detail.headers/assert.module.verify.cpp b/libcxx/test/libcxx/diagnostics/detail.headers/assert.module.verify.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/diagnostics/detail.headers/assert.module.verify.cpp
@@ -0,0 +1,15 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: modules-build
+
+// WARNING: This test was generated by 'generate_private_header_tests.py'
+// and should not be edited manually.
+
+// expected-error@*:* {{use of private header from outside its module: '__assert'}}
+#include <__assert>