diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index e098bd574eec7..190a97db9462f 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -784,8 +784,8 @@ config_define_if_not(LIBCXX_ENABLE_WIDE_CHARACTERS _LIBCPP_HAS_NO_WIDE_CHARACTER
 config_define_if_not(LIBCXX_ENABLE_TIME_ZONE_DATABASE _LIBCPP_HAS_NO_TIME_ZONE_DATABASE)
 config_define_if_not(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
 
-# TODO(LLVM 19): Produce a deprecation warning.
 if (LIBCXX_ENABLE_ASSERTIONS)
+  message(DEPRECATION "LIBCXX_ENABLE_ASSERTIONS is deprecated and will be removed in LLVM 20. Please use LIBCXX_HARDENING_MODE instead.")
   set(LIBCXX_HARDENING_MODE "extensive")
 endif()
 if (LIBCXX_HARDENING_MODE STREQUAL "none")
diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index b9c6bc84892c5..a4cf411d2a6c1 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -89,7 +89,7 @@ Deprecations and Removals
   This decision is based on LEWGs discussion on `P3122 <https://wg21.link/P3122>` and `P3162 <https://wg21.link/P3162>`
   to not use ``[[nodiscard]]`` in the standard.
 
-- TODO: The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable that was used to enable the safe mode has been deprecated and setting
+- The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable that was used to enable the safe mode has been deprecated and setting
   it triggers an error; use the ``LIBCXX_HARDENING_MODE`` CMake variable with the value ``extensive`` instead. Similarly,
   the ``_LIBCPP_ENABLE_ASSERTIONS`` macro has been deprecated (setting it to ``1`` still enables the extensive mode in
   the LLVM 19 release while also issuing a deprecation warning). See :ref:`the hardening documentation
diff --git a/libcxx/include/__config b/libcxx/include/__config
index dfb14fd6a380c..91bc6fc21df8e 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -38,10 +38,10 @@
 
 // HARDENING {
 
-// TODO(hardening): deprecate this in LLVM 19.
 // This is for backward compatibility -- make enabling `_LIBCPP_ENABLE_ASSERTIONS` (which predates hardening modes)
-// equivalent to setting the extensive mode.
+// equivalent to setting the extensive mode. This is deprecated and will be removed in LLVM 20.
 #  ifdef _LIBCPP_ENABLE_ASSERTIONS
+#    warning "_LIBCPP_ENABLE_ASSERTIONS is deprecated, please use _LIBCPP_HARDENING_MODE instead"
 #    if _LIBCPP_ENABLE_ASSERTIONS != 0 && _LIBCPP_ENABLE_ASSERTIONS != 1
 #      error "_LIBCPP_ENABLE_ASSERTIONS must be set to 0 or 1"
 #    endif
diff --git a/libcxx/test/libcxx/assertions/modes/enabling_assertions_enables_extensive_mode.pass.cpp b/libcxx/test/libcxx/assertions/modes/enabling_assertions_enables_extensive_mode.pass.cpp
index d4dad8b71fe92..c496fc32dc939 100644
--- a/libcxx/test/libcxx/assertions/modes/enabling_assertions_enables_extensive_mode.pass.cpp
+++ b/libcxx/test/libcxx/assertions/modes/enabling_assertions_enables_extensive_mode.pass.cpp
@@ -19,7 +19,7 @@
 // HWASAN replaces TRAP with abort or error exit code.
 // XFAIL: hwasan
 // Note that GCC doesn't support `-Wno-macro-redefined`.
-// ADDITIONAL_COMPILE_FLAGS: -U_LIBCPP_HARDENING_MODE -D_LIBCPP_ENABLE_ASSERTIONS=1
+// ADDITIONAL_COMPILE_FLAGS: -U_LIBCPP_HARDENING_MODE -D_LIBCPP_ENABLE_ASSERTIONS=1 -Wno-#warnings -Wno-cpp
 
 #include <cassert>
 #include "check_assertion.h"