[libc++][hardening] Allow setting the assertion semantic via CMake. (llvm#167636)

var-const · web-flow · commit c29b29bb6a7f · 2025-11-14T10:39:06.000-08:00
Add a new CMake variable, `LIBCXX_ASSERTION_SEMANTIC`, that largely
mirrors `LIBCXX_HARDENING_MODE`, except that it also supports a
special value `hardening_dependent` that indicates the semantic will be
selected based on the hardening mode in effect:
- `fast` and `extensive` map to `quick_enforce`;
- `debug` maps to `enforce`.
diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
@@ -66,6 +66,19 @@ if (NOT "${LIBCXX_HARDENING_MODE}" IN_LIST LIBCXX_SUPPORTED_HARDENING_MODES)
   message(FATAL_ERROR
     "Unsupported hardening mode: '${LIBCXX_HARDENING_MODE}'. Supported values are ${LIBCXX_SUPPORTED_HARDENING_MODES}.")
 endif()
+set(LIBCXX_SUPPORTED_ASSERTION_SEMANTICS hardening_dependent ignore observe quick_enforce enforce)
+set(LIBCXX_ASSERTION_SEMANTIC "hardening_dependent" CACHE STRING
+  "Specify the default assertion semantic to use. This semantic will be used
+  inside the compiled library and will be the default when compiling user code.
+  Note that users can override this setting in their own code. This does not
+  affect the ABI. Supported values are ${LIBCXX_SUPPORTED_ASSERTION_SEMANTICS}.
+  `hardening_dependent` is a special value that instructs the library to select
+  the assertion semantic based on the hardening mode in effect.")
+
+if (NOT "${LIBCXX_ASSERTION_SEMANTIC}" IN_LIST LIBCXX_SUPPORTED_ASSERTION_SEMANTICS)
+  message(FATAL_ERROR
+    "Unsupported assertion semantic: '${LIBCXX_ASSERTION_SEMANTIC}'. Supported values are ${LIBCXX_SUPPORTED_ASSERTION_SEMANTICS}.")
+endif()
 set(LIBCXX_ASSERTION_HANDLER_FILE
   "vendor/llvm/default_assertion_handler.in"
   CACHE STRING
@@ -763,6 +776,17 @@ elseif (LIBCXX_HARDENING_MODE STREQUAL "extensive")
 elseif (LIBCXX_HARDENING_MODE STREQUAL "debug")
   config_define(8 _LIBCPP_HARDENING_MODE_DEFAULT)
 endif()
+if (LIBCXX_ASSERTION_SEMANTIC STREQUAL "hardening_dependent")
+  config_define(2 _LIBCPP_ASSERTION_SEMANTIC_DEFAULT)
+elseif (LIBCXX_ASSERTION_SEMANTIC STREQUAL "ignore")
+  config_define(4 _LIBCPP_ASSERTION_SEMANTIC_DEFAULT)
+elseif (LIBCXX_ASSERTION_SEMANTIC STREQUAL "observe")
+  config_define(8 _LIBCPP_ASSERTION_SEMANTIC_DEFAULT)
+elseif (LIBCXX_ASSERTION_SEMANTIC STREQUAL "quick_enforce")
+  config_define(16 _LIBCPP_ASSERTION_SEMANTIC_DEFAULT)
+elseif (LIBCXX_ASSERTION_SEMANTIC STREQUAL "enforce")
+  config_define(32 _LIBCPP_ASSERTION_SEMANTIC_DEFAULT)
+endif()
 
 if (LIBCXX_PSTL_BACKEND STREQUAL "serial")
   config_define(1 _LIBCPP_PSTL_BACKEND_SERIAL)
diff --git a/libcxx/docs/Hardening.rst b/libcxx/docs/Hardening.rst
@@ -328,6 +328,20 @@ following options to the compiler:
 All the :ref:`same notes <notes-for-users>` apply to setting this macro as for
 setting ``_LIBCPP_HARDENING_MODE``.
 
+Notes for vendors
+-----------------
+
+Similarly to hardening modes, vendors can set the default assertion semantic by
+providing ``LIBCXX_ASSERTION_SEMANTIC`` as a configuration option, with the
+possible values of ``hardening_dependent``, ``ignore``, ``observe``,
+``quick_enforce`` and ``enforce``. The default value is ``hardening_dependent``
+which is a special value that instructs the library to select the semantic based
+on the hardening mode in effect (the mapping is described in
+:ref:`the main section on assertion semantics <assertion-semantics>`).
+
+This option controls both the assertion semantic that the precompiled library is
+built with and the default assertion semantic that users will build with.
+
 .. _override-assertion-handler:
 
 Overriding the assertion failure handler
@@ -447,6 +461,13 @@ The first character of an ABI tag encodes the hardening mode:
 - ``d`` -- [d]ebug mode;
 - ``n`` -- [n]one mode.
 
+The second character of an ABI tag encodes the assertion semantic:
+
+- ``i`` -- [i]gnore semantic;
+- ``o`` -- [o]bserve semantic;
+- ``q`` -- [q]uick-enforce semantic;
+- ``e`` -- [e]nforce semantic.
+
 Hardened containers status
 ==========================
 
diff --git a/libcxx/include/__config b/libcxx/include/__config
@@ -291,14 +291,26 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_HARDENING_SIG n // "none"
 #  endif
 
+#  if _LIBCPP_ASSERTION_SEMANTIC == _LIBCPP_ASSERTION_SEMANTIC_OBSERVE
+#    define _LIBCPP_ASSERTION_SEMANTIC_SIG o
+#  elif _LIBCPP_ASSERTION_SEMANTIC == _LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE
+#    define _LIBCPP_ASSERTION_SEMANTIC_SIG q
+#  elif _LIBCPP_ASSERTION_SEMANTIC == _LIBCPP_ASSERTION_SEMANTIC_ENFORCE
+#    define _LIBCPP_ASSERTION_SEMANTIC_SIG e
+#  else
+#    define _LIBCPP_ASSERTION_SEMANTIC_SIG i // `ignore`
+#  endif
+
 #  if !_LIBCPP_HAS_EXCEPTIONS
 #    define _LIBCPP_EXCEPTIONS_SIG n
 #  else
 #    define _LIBCPP_EXCEPTIONS_SIG e
 #  endif
 
 #  define _LIBCPP_ODR_SIGNATURE                                                                                        \
-    _LIBCPP_CONCAT(_LIBCPP_CONCAT(_LIBCPP_HARDENING_SIG, _LIBCPP_EXCEPTIONS_SIG), _LIBCPP_VERSION)
+    _LIBCPP_CONCAT(                                                                                                    \
+        _LIBCPP_CONCAT(_LIBCPP_CONCAT(_LIBCPP_HARDENING_SIG, _LIBCPP_ASSERTION_SEMANTIC_SIG), _LIBCPP_EXCEPTIONS_SIG), \
+        _LIBCPP_VERSION)
 
 // This macro marks a symbol as being hidden from libc++'s ABI. This is achieved
 // on two levels:
diff --git a/libcxx/include/__config_site.in b/libcxx/include/__config_site.in
@@ -40,6 +40,7 @@
 
 // Hardening.
 #cmakedefine _LIBCPP_HARDENING_MODE_DEFAULT @_LIBCPP_HARDENING_MODE_DEFAULT@
+#cmakedefine _LIBCPP_ASSERTION_SEMANTIC_DEFAULT @_LIBCPP_ASSERTION_SEMANTIC_DEFAULT@
 
 // __USE_MINGW_ANSI_STDIO gets redefined on MinGW
 #ifdef __clang__
diff --git a/libcxx/include/__configuration/hardening.h b/libcxx/include/__configuration/hardening.h
@@ -135,13 +135,26 @@ _LIBCPP_HARDENING_MODE_EXTENSIVE, \
 _LIBCPP_HARDENING_MODE_DEBUG
 #endif
 
-// Hardening assertion semantics generally mirror the evaluation semantics of C++26 Contracts:
+// The library provides the macro `_LIBCPP_ASSERTION_SEMANTIC` for configuring the assertion semantic used by hardening;
+// it can be set to one of the following values:
+//
+// - `_LIBCPP_ASSERTION_SEMANTIC_IGNORE`;
+// - `_LIBCPP_ASSERTION_SEMANTIC_OBSERVE`;
+// - `_LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE`;
+// - `_LIBCPP_ASSERTION_SEMANTIC_ENFORCE`.
+//
+// libc++ assertion semantics generally mirror the evaluation semantics of C++26 Contracts:
 // - `ignore` evaluates the assertion but doesn't do anything if it fails (note that it differs from the Contracts
 //   `ignore` semantic which wouldn't evaluate the assertion at all);
 // - `observe` logs an error (indicating, if possible, that the error is fatal) and continues execution;
 // - `quick-enforce` terminates the program as fast as possible (via trapping);
 // - `enforce` logs an error and then terminates the program.
 //
+// Additionally, a special `hardening-dependent` value selects the assertion semantic based on the hardening mode in
+// effect: the production-capable modes (`fast` and `extensive`) map to `quick_enforce` and the `debug` mode maps to
+// `enforce`. The `hardening-dependent` semantic cannot be selected explicitly, it is only used when no assertion
+// semantic is provided by the user _and_ the library's default semantic is configured to be dependent on hardening.
+//
 // Notes:
 // - Continuing execution after a hardening check fails results in undefined behavior; the `observe` semantic is meant
 //   to make adopting hardening easier but should not be used outside of this scenario;
@@ -150,32 +163,53 @@ _LIBCPP_HARDENING_MODE_DEBUG
 //   hardened preconditions, however, be aware that using `ignore` does not produce a conforming "Hardened"
 //   implementation, unlike the other semantics above.
 // clang-format off
-#  define _LIBCPP_ASSERTION_SEMANTIC_IGNORE        (1 << 1)
-#  define _LIBCPP_ASSERTION_SEMANTIC_OBSERVE       (1 << 2)
-#  define _LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE (1 << 3)
-#  define _LIBCPP_ASSERTION_SEMANTIC_ENFORCE       (1 << 4)
+#  define _LIBCPP_ASSERTION_SEMANTIC_HARDENING_DEPENDENT (1 << 1)
+#  define _LIBCPP_ASSERTION_SEMANTIC_IGNORE              (1 << 2)
+#  define _LIBCPP_ASSERTION_SEMANTIC_OBSERVE             (1 << 3)
+#  define _LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE       (1 << 4)
+#  define _LIBCPP_ASSERTION_SEMANTIC_ENFORCE             (1 << 5)
 // clang-format on
 
-// Allow users to define an arbitrary assertion semantic; otherwise, use the default mapping from modes to semantics.
-// The default is for production-capable modes to use `quick-enforce` (i.e., trap) and for the `debug` mode to use
-// `enforce` (i.e., log and abort).
-#ifndef _LIBCPP_ASSERTION_SEMANTIC
-
-#  if _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
-#    define _LIBCPP_ASSERTION_SEMANTIC _LIBCPP_ASSERTION_SEMANTIC_ENFORCE
-#  else
-#    define _LIBCPP_ASSERTION_SEMANTIC _LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE
-#  endif
-
-#else
-
+// If the user attempts to configure the assertion semantic, check that it is allowed in the current environment.
+#if defined(_LIBCPP_ASSERTION_SEMANTIC)
 #  if !_LIBCPP_HAS_EXPERIMENTAL_LIBRARY
 #    error "Assertion semantics are an experimental feature."
 #  endif
 #  if defined(_LIBCPP_CXX03_LANG)
 #    error "Assertion semantics are not available in the C++03 mode."
 #  endif
+#endif // defined(_LIBCPP_ASSERTION_SEMANTIC)
+
+// User-provided semantic takes top priority -- don't override if set.
+#ifndef _LIBCPP_ASSERTION_SEMANTIC
 
-#endif // _LIBCPP_ASSERTION_SEMANTIC
+#  ifndef _LIBCPP_ASSERTION_SEMANTIC_DEFAULT
+#    error _LIBCPP_ASSERTION_SEMANTIC_DEFAULT is not defined. This definition should be set at configuration time in \
+the `__config_site` header, please make sure your installation of libc++ is not broken.
+#  endif
+
+#  if _LIBCPP_ASSERTION_SEMANTIC_DEFAULT != _LIBCPP_ASSERTION_SEMANTIC_HARDENING_DEPENDENT
+#    define _LIBCPP_ASSERTION_SEMANTIC _LIBCPP_ASSERTION_SEMANTIC_DEFAULT
+#  else
+#    if _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
+#      define _LIBCPP_ASSERTION_SEMANTIC _LIBCPP_ASSERTION_SEMANTIC_ENFORCE
+#    else
+#      define _LIBCPP_ASSERTION_SEMANTIC _LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE
+#    endif
+#  endif // _LIBCPP_ASSERTION_SEMANTIC_DEFAULT != _LIBCPP_ASSERTION_SEMANTIC_HARDENING_DEPENDENT
+
+#endif // #ifndef _LIBCPP_ASSERTION_SEMANTIC
+
+// Finally, validate the selected semantic (in case the user tries setting it to an incorrect value):
+#if _LIBCPP_ASSERTION_SEMANTIC != _LIBCPP_ASSERTION_SEMANTIC_IGNORE &&                                                 \
+    _LIBCPP_ASSERTION_SEMANTIC != _LIBCPP_ASSERTION_SEMANTIC_OBSERVE &&                                                \
+    _LIBCPP_ASSERTION_SEMANTIC != _LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE &&                                          \
+    _LIBCPP_ASSERTION_SEMANTIC != _LIBCPP_ASSERTION_SEMANTIC_ENFORCE
+#  error _LIBCPP_ASSERTION_SEMANTIC must be set to one of the following values: \
+_LIBCPP_ASSERTION_SEMANTIC_IGNORE, \
+_LIBCPP_ASSERTION_SEMANTIC_OBSERVE, \
+_LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE, \
+_LIBCPP_ASSERTION_SEMANTIC_ENFORCE
+#endif
 
 #endif // _LIBCPP___CONFIGURATION_HARDENING_H
diff --git a/libcxx/test/extensions/libcxx/odr_signature.assertion_semantics.sh.cpp b/libcxx/test/extensions/libcxx/odr_signature.assertion_semantics.sh.cpp
@@ -0,0 +1,71 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// ABI tags have no effect in MSVC mode.
+// XFAIL: msvc
+
+// Assertion semantics are not supported in C++03 mode and currently are experimental.
+// UNSUPPORTED: c++03, libcpp-has-no-experimental-hardening-observe-semantic
+
+// Test that we encode the assertion semantic in an ABI tag to avoid ODR violations when linking TUs that have different
+// values for it.
+
+// RUN: %{cxx} %s %{flags} %{compile_flags} -c -DTU1  -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_IGNORE        -o %t.tu1.o
+// RUN: %{cxx} %s %{flags} %{compile_flags} -c -DTU2  -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_OBSERVE       -o %t.tu2.o
+// RUN: %{cxx} %s %{flags} %{compile_flags} -c -DTU3  -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE -o %t.tu3.o
+// RUN: %{cxx} %s %{flags} %{compile_flags} -c -DTU4  -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_ENFORCE       -o %t.tu4.o
+// RUN: %{cxx} %s %{flags} %{compile_flags} -c -DMAIN                                                                                                    -o %t.main.o
+// RUN: %{cxx} %t.tu1.o %t.tu2.o %t.tu3.o %t.tu4.o %t.main.o %{flags} %{link_flags} -o %t.exe
+// RUN: %{exec} %t.exe
+
+#include "test_macros.h"
+
+// `ignore` assertion semantic.
+#ifdef TU1
+#  include <__config>
+_LIBCPP_HIDE_FROM_ABI TEST_NOINLINE inline int f() { return 1; }
+int tu1() { return f(); }
+#endif // TU1
+
+// `observe` assertion semantic.
+#ifdef TU2
+#  include <__config>
+_LIBCPP_HIDE_FROM_ABI TEST_NOINLINE inline int f() { return 2; }
+int tu2() { return f(); }
+#endif // TU2
+
+// `quick-enforce` assertion semantic.
+#ifdef TU3
+#  include <__config>
+_LIBCPP_HIDE_FROM_ABI TEST_NOINLINE inline int f() { return 3; }
+int tu3() { return f(); }
+#endif // TU3
+
+// `enforce` assertion semantic.
+#ifdef TU4
+#  include <__config>
+_LIBCPP_HIDE_FROM_ABI TEST_NOINLINE inline int f() { return 4; }
+int tu4() { return f(); }
+#endif // TU4
+
+#ifdef MAIN
+#  include <cassert>
+
+int tu1();
+int tu2();
+int tu3();
+int tu4();
+
+int main(int, char**) {
+  assert(tu1() == 1);
+  assert(tu2() == 2);
+  assert(tu3() == 3);
+  assert(tu4() == 4);
+  return 0;
+}
+#endif // MAIN
diff --git a/libcxx/test/libcxx/assertions/semantics/assertion_semantic_incorrect_value.sh.cpp b/libcxx/test/libcxx/assertions/semantics/assertion_semantic_incorrect_value.sh.cpp
@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// This test verifies that setting the assertion semantic to a value that's not part of the predefined constants
+// triggers a compile-time error.
+
+// Modules build produces a different error ("Could not build module 'std'").
+// UNSUPPORTED: clang-modules-build
+// UNSUPPORTED: c++03, libcpp-has-no-experimental-hardening-observe-semantic
+// REQUIRES: verify-support
+
+// RUN: %{verify} -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC=42
+// `hardening-dependent` cannot be set as the semantic (it's only an indicator to use hardening-related logic to pick
+// the final semantic).
+// RUN: %{verify} -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_HARDENING_DEPENDENT
+// Make sure that common cases of misuse produce readable errors. We deliberately disallow setting the assertion
+// semantic as if it were a boolean flag.
+// RUN: %{verify} -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC=0
+// RUN: %{verify} -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC=1
+// RUN: %{verify} -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC
+
+#include <cassert>
+
+// expected-error@*:* {{_LIBCPP_ASSERTION_SEMANTIC must be set to one of the following values: _LIBCPP_ASSERTION_SEMANTIC_IGNORE, _LIBCPP_ASSERTION_SEMANTIC_OBSERVE, _LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE, _LIBCPP_ASSERTION_SEMANTIC_ENFORCE}}
diff --git a/libcxx/test/libcxx/assertions/semantics/override_with_enforce_semantic.pass.cpp b/libcxx/test/libcxx/assertions/semantics/override_with_enforce_semantic.pass.cpp
@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// This test ensures that we can override the assertion semantic used by any checked hardening mode with `enforce` on
+// a per-TU basis (this is valid for the `debug` mode as well, though a no-op).
+
+// `check_assertion.h` is only available starting from C++11 and requires Unix headers and regex support.
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03, no-localization
+// UNSUPPORTED: libcpp-hardening-mode=none, libcpp-has-no-experimental-hardening-observe-semantic
+// The ability to set a custom abort message is required to compare the assertion message.
+// XFAIL: availability-verbose_abort-missing
+// ADDITIONAL_COMPILE_FLAGS: -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_ENFORCE
+
+#include <cassert>
+#include "check_assertion.h"
+
+int main(int, char**) {
+  _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(true, "Should not fire");
+  TEST_LIBCPP_ASSERT_FAILURE([] { _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(false, "Should fire and log a message"); }(),
+                             "Should fire and log a message");
+
+  return 0;
+}
diff --git a/libcxx/test/libcxx/assertions/semantics/override_with_ignore_semantic.pass.cpp b/libcxx/test/libcxx/assertions/semantics/override_with_ignore_semantic.pass.cpp
@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// This test ensures that we can override the assertion semantic used by any hardening mode with `ignore` on a per-TU
+// basis (this is valid for the `none` mode as well, though a no-op).
+
+// `check_assertion.h` is only available starting from C++11 and requires Unix headers and regex support.
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03, no-localization
+// UNSUPPORTED: libcpp-has-no-experimental-hardening-observe-semantic
+// ADDITIONAL_COMPILE_FLAGS: -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_IGNORE
+
+#include <cassert>
+#include "check_assertion.h"
+
+int main(int, char**) {
+  _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(true, "Should not fire");
+  _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(false, "Also should not fire");
+
+  return 0;
+}
diff --git a/libcxx/test/libcxx/assertions/semantics/override_with_observe_semantic.pass.cpp b/libcxx/test/libcxx/assertions/semantics/override_with_observe_semantic.pass.cpp
@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// This test ensures that we can override the assertion semantic used by any checked hardening mode with `observe` on
+// a per-TU basis.
+
+// `check_assertion.h` is only available starting from C++11 and requires Unix headers and regex support.
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03, no-localization
+// UNSUPPORTED: libcpp-hardening-mode=none, libcpp-has-no-experimental-hardening-observe-semantic
+// ADDITIONAL_COMPILE_FLAGS: -U_LIBCPP_ASSERTION_SEMANTIC -D_LIBCPP_ASSERTION_SEMANTIC=_LIBCPP_ASSERTION_SEMANTIC_OBSERVE
+
+#include <cassert>
+#include "check_assertion.h"
+
+int main(int, char**) {
+  _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(true, "Should not fire");
+  _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(false, "Also should not fire");
+  // TODO(hardening): check that a message is logged.
+
+  return 0;
+}
diff --git a/libcxx/test/libcxx/assertions/semantics/override_with_quick_enforce_semantic.pass.cpp b/libcxx/test/libcxx/assertions/semantics/override_with_quick_enforce_semantic.pass.cpp
diff --git a/libcxx/test/support/test.support/test_check_assertion.pass.cpp b/libcxx/test/support/test.support/test_check_assertion.pass.cpp
