diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -581,7 +581,7 @@
   endif()
   target_add_compile_flags_if_supported(${target} PRIVATE -Wextra -W -Wwrite-strings
                                                           -Wno-unused-parameter -Wno-long-long
-                                                          -Werror=return-type -Wextra-semi)
+                                                          -Werror=return-type -Wextra-semi -Wundef)
   if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
     target_add_compile_flags_if_supported(${target} PRIVATE
       -Wno-user-defined-literals
diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -348,7 +348,7 @@
 #  define _LIBCPP_NO_CFI
 #endif
 
-#if __ISO_C_VISIBLE >= 2011 || __cplusplus >= 201103L
+#if (defined(__ISO_C_VISIBLE) && (__ISO_C_VISIBLE >= 2011)) || __cplusplus >= 201103L
 #  if defined(__FreeBSD__)
 #    define _LIBCPP_HAS_ALIGNED_ALLOC
 #    define _LIBCPP_HAS_QUICK_EXIT
diff --git a/libcxx/include/ext/hash_map b/libcxx/include/ext/hash_map
--- a/libcxx/include/ext/hash_map
+++ b/libcxx/include/ext/hash_map
@@ -208,7 +208,7 @@
 #include <type_traits>
 #include <ext/__hash>
 
-#if __DEPRECATED
+#if defined(__DEPRECATED) && __DEPRECATED
 #if defined(_LIBCPP_WARNING)
     _LIBCPP_WARNING("Use of the header <ext/hash_map> is deprecated.  Migrate to <unordered_map>")
 #else
diff --git a/libcxx/include/ext/hash_set b/libcxx/include/ext/hash_set
--- a/libcxx/include/ext/hash_set
+++ b/libcxx/include/ext/hash_set
@@ -197,7 +197,7 @@
 #include <functional>
 #include <ext/__hash>
 
-#if __DEPRECATED
+#if defined(__DEPRECATED) && __DEPRECATED
 #if defined(_LIBCPP_WARNING)
     _LIBCPP_WARNING("Use of the header <ext/hash_set> is deprecated.  Migrate to <unordered_set>")
 #else
diff --git a/libcxx/include/future b/libcxx/include/future
--- a/libcxx/include/future
+++ b/libcxx/include/future
@@ -501,9 +501,7 @@
     error_code __ec_;
 public:
     future_error(error_code __ec);
-#if _LIBCPP_STD_VERS > 14
-    explicit future_error(future_errc _Ev) : logic_error(), __ec_(make_error_code(_Ev)) {}
-#endif
+
     _LIBCPP_INLINE_VISIBILITY
     const error_code& code() const _NOEXCEPT {return __ec_;}
 
diff --git a/libcxx/include/type_traits b/libcxx/include/type_traits
--- a/libcxx/include/type_traits
+++ b/libcxx/include/type_traits
@@ -835,7 +835,9 @@
 // is_pointer
 
 // Before Clang 11, __is_pointer didn't work for Objective-C types.
-#if __has_keyword(__is_pointer) && !(defined(_LIBCPP_COMPILER_CLANG) && _LIBCPP_CLANG_VER < 1100)
+#if __has_keyword(__is_pointer) &&                                             \
+    !(defined(_LIBCPP_COMPILER_CLANG) && defined(_LIBCPP_CLANG_VER) &&         \
+      _LIBCPP_CLANG_VER < 1100)
 
 template<class _Tp>
 struct _LIBCPP_TEMPLATE_VIS is_pointer : _BoolConstant<__is_pointer(_Tp)> { };
@@ -1131,7 +1133,10 @@
 
 // Before Clang 10, __is_fundamental didn't work for nullptr_t.
 // In C++03 nullptr_t is library-provided but must still count as "fundamental."
-#if __has_keyword(__is_fundamental) && !(defined(_LIBCPP_COMPILER_CLANG) && _LIBCPP_CLANG_VER < 1000) && !defined(_LIBCPP_CXX03_LANG)
+#if __has_keyword(__is_fundamental) &&                                         \
+    !(defined(_LIBCPP_COMPILER_CLANG) && defined(_LIBCPP_CLANG_VER) &&         \
+      _LIBCPP_CLANG_VER < 1000) &&                                             \
+    !defined(_LIBCPP_CXX03_LANG)
 
 template<class _Tp>
 struct _LIBCPP_TEMPLATE_VIS is_fundamental : _BoolConstant<__is_fundamental(_Tp)> { };
@@ -1416,7 +1421,9 @@
 // is_signed
 
 // Before Clang 10, __is_signed didn't work for floating-point types or enums.
-#if __has_keyword(__is_signed) && !(defined(_LIBCPP_COMPILER_CLANG) && _LIBCPP_CLANG_VER < 1000)
+#if __has_keyword(__is_signed) &&                                              \
+    !(defined(_LIBCPP_COMPILER_CLANG) && defined(_LIBCPP_CLANG_VER) &&         \
+      _LIBCPP_CLANG_VER < 1000)
 
 template<class _Tp>
 struct _LIBCPP_TEMPLATE_VIS is_signed : _BoolConstant<__is_signed(_Tp)> { };
@@ -1452,7 +1459,9 @@
 // is_unsigned
 
 // Before Clang 13, __is_unsigned returned true for enums with signed underlying type.
-#if __has_keyword(__is_unsigned) && !(defined(_LIBCPP_COMPILER_CLANG) && _LIBCPP_CLANG_VER < 1300)
+#if __has_keyword(__is_unsigned) &&                                            \
+    !(defined(_LIBCPP_COMPILER_CLANG) && defined(_LIBCPP_CLANG_VER) &&         \
+      _LIBCPP_CLANG_VER < 1300)
 
 template<class _Tp>
 struct _LIBCPP_TEMPLATE_VIS is_unsigned : _BoolConstant<__is_unsigned(_Tp)> { };
diff --git a/libcxx/test/std/atomics/types.pass.cpp b/libcxx/test/std/atomics/types.pass.cpp
--- a/libcxx/test/std/atomics/types.pass.cpp
+++ b/libcxx/test/std/atomics/types.pass.cpp
@@ -24,12 +24,13 @@
 
 #include <thread>
 #include <chrono>
+
+#include "test_macros.h"
+
 #if TEST_STD_VER >= 20
 # include <memory>
 #endif
 
-#include "test_macros.h"
-
 template <class A, bool Integral>
 struct test_atomic
 {
diff --git a/libcxx/test/std/numerics/rand/rand.eng/rand.eng.mers/ctor_sseq_all_zero.pass.cpp b/libcxx/test/std/numerics/rand/rand.eng/rand.eng.mers/ctor_sseq_all_zero.pass.cpp
--- a/libcxx/test/std/numerics/rand/rand.eng/rand.eng.mers/ctor_sseq_all_zero.pass.cpp
+++ b/libcxx/test/std/numerics/rand/rand.eng/rand.eng.mers/ctor_sseq_all_zero.pass.cpp
@@ -25,11 +25,8 @@
 #include <algorithm>
 #include <cassert>
 #include <cstddef>
-#if TEST_STD_VER >= 11
-#include <initializer_list>
 
 #include "test_macros.h"
-#endif
 
 struct all_zero_seed_seq {
   typedef unsigned int result_type;
diff --git a/libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.hash/hash_shared_ptr.pass.cpp b/libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.hash/hash_shared_ptr.pass.cpp
--- a/libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.hash/hash_shared_ptr.pass.cpp
+++ b/libcxx/test/std/utilities/memory/util.smartptr/util.smartptr.hash/hash_shared_ptr.pass.cpp
@@ -19,11 +19,11 @@
 #include <memory>
 #include <cassert>
 
+#include "test_macros.h"
+
 #if TEST_STD_VER >= 11
 #include "poisoned_hash_helper.h"
 
-#include "test_macros.h"
-
 struct A {};
 #endif
 
diff --git a/libcxx/test/support/test_macros.h b/libcxx/test/support/test_macros.h
--- a/libcxx/test/support/test_macros.h
+++ b/libcxx/test/support/test_macros.h
@@ -165,7 +165,8 @@
 
 // Sniff out to see if the underlying C library has C11 features
 // This is cribbed from __config; but lives here as well because we can't assume libc++
-#if __ISO_C_VISIBLE >= 2011 || TEST_STD_VER >= 11
+#if (defined(__ISO_C_VISIBLE) && (__ISO_C_VISIBLE >= 2011)) ||                 \
+    TEST_STD_VER >= 11
 #  if defined(__FreeBSD__)
 #    if __FreeBSD_version >= 1300064 || \
        (__FreeBSD_version >= 1201504 && __FreeBSD_version < 1300000)
diff --git a/libcxx/utils/libcxx/test/params.py b/libcxx/utils/libcxx/test/params.py
--- a/libcxx/utils/libcxx/test/params.py
+++ b/libcxx/utils/libcxx/test/params.py
@@ -14,6 +14,7 @@
   '-Wall',
   '-Wextra',
   '-Wshadow',
+  '-Wundef',
   '-Wno-unused-command-line-argument',
   '-Wno-attributes',
   '-Wno-pessimizing-move',