diff --git a/libcxx/docs/UsingLibcxx.rst b/libcxx/docs/UsingLibcxx.rst
--- a/libcxx/docs/UsingLibcxx.rst
+++ b/libcxx/docs/UsingLibcxx.rst
@@ -196,6 +196,32 @@
   warning saying that `std::auto_ptr` is deprecated. If the macro is defined,
   no warning will be emitted. By default, this macro is not defined.
 
+**_LIBCPP_ENABLE_INCOMPLETE_FEATURES**:
+  Some library features in libc++ are marked as incomplete. Including
+  incomplete headers will result in a compilation failure, unless the
+  appropriate macro is defined. This is used for features that:
+
+    * Are large and partial implementations aren't easily detected during
+      compilation. For example:
+
+      * Calling a missing ranges algorithm leads to a compilation failure and
+        is easily detected.
+      * Calling an incomplete ``std::format`` function may compile, but at
+        run-time its behavior may not match the required behavior.
+
+    * Aren't guaranteed ABI stable yet, due to:
+
+      * The feature is still being developed.
+      * The C++ committee considers backporting ABI breaking changes to an
+        older version of the Standard. During the June 2021 plenary several ABI
+        breaking changes were retroactively applied to the C++20 Standard.
+
+  This macro is only used for features that are in a ratified version of the
+  C++ Standard. It's possible to determine the implementation status by testing
+  the appropriate feature test macro. Once a feature is complete it will be
+  available without defining a macro. By default, this macro is not defined.
+
+
 C++17 Specific Configuration Macros
 -----------------------------------
 **_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES**:
@@ -227,6 +253,20 @@
   ``[[nodiscard]]`` in dialects after C++17.  See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>`
   for more information.
 
+**_LIBCPP_ENABLE_CXX20_INCOMPLETE_FORMAT**:
+  This macros is used to enable the incomplete C++20 ``<format>`` header. This
+  macro automatically defined when defining
+  ``_LIBCPP_ENABLE_INCOMPLETE_FEATURES``. The implementation status of the
+  ``<format>`` header can be tested with the ``__cpp_lib_format`` feature
+  test macro.
+
+**_LIBCPP_ENABLE_CXX20_INCOMPLETE_RANGES**:
+  This macros is used to enable the incomplete C++20 ``<ranges>`` header. This
+  macro automatically defined when defining
+  ``_LIBCPP_ENABLE_INCOMPLETE_FEATURES``. The implementation status of the
+  ``<ranges>`` header can be tested with the ``__cpp_lib_ranges`` feature
+  test macro.
+
 **_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES**:
   This macro is used to re-enable all the features removed in C++20. The effect
   is equivalent to manually defining each macro listed below.
diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -1362,6 +1362,11 @@
 #define _LIBCPP_ENABLE_CXX20_REMOVED_TYPE_TRAITS
 #endif // _LIBCPP_ENABLE_CXX20_REMOVED_FEATURES
 
+#if defined(_LIBCPP_ENABLE_INCOMPLETE_FEATURES)
+#define _LIBCPP_ENABLE_CXX20_INCOMPLETE_FORMAT
+#define _LIBCPP_ENABLE_CXX20_INCOMPLETE_RANGES
+#endif // _LIBCPP_ENABLE_INCOMPLETE_FEATURES
+
 #if !defined(__cpp_deduction_guides) || __cpp_deduction_guides < 201611
 #define _LIBCPP_HAS_NO_DEDUCTION_GUIDES
 #endif
diff --git a/libcxx/include/format b/libcxx/include/format
--- a/libcxx/include/format
+++ b/libcxx/include/format
@@ -60,6 +60,10 @@
 #include <__format/format_parse_context.h>
 #include <version>
 
+#ifndef _LIBCPP_ENABLE_CXX20_INCOMPLETE_FORMAT
+#error The implementation of the format library is incomplete define _LIBCPP_ENABLE_CXX20_INCOMPLETE_FORMAT to use it.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/ranges b/libcxx/include/ranges
--- a/libcxx/include/ranges
+++ b/libcxx/include/ranges
@@ -180,6 +180,10 @@
 #include <type_traits>
 #include <version>
 
+#ifndef _LIBCPP_ENABLE_CXX20_INCOMPLETE_RANGES
+#error The implementation of the ranges library is incomplete define _LIBCPP_ENABLE_CXX20_INCOMPLETE_RANGES to use it.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
diff --git a/libcxx/test/std/utilities/format/format.error/format.error.pass.cpp b/libcxx/test/std/utilities/format/format.error/format.error.pass.cpp
--- a/libcxx/test/std/utilities/format/format.error/format.error.pass.cpp
+++ b/libcxx/test/std/utilities/format/format.error/format.error.pass.cpp
@@ -15,6 +15,9 @@
 
 // class format_error;
 
+// This define should be automatically defined by the test runner.
+// And there should be additional tests to see that the compilation fails when the proper defines aren't set.
+#define _LIBCPP_ENABLE_INCOMPLETE_FEATURES
 #include <format>
 #include <type_traits>
 #include <cstring>