diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -577,7 +577,7 @@
     # -Wall is equivalent to -Weverything in GCC style compiler drivers.)
     target_add_compile_flags_if_supported(${target} PRIVATE -W4)
   else()
-    target_add_compile_flags_if_supported(${target} PRIVATE -Wall)
+    target_add_compile_flags_if_supported(${target} PRIVATE -Wall -Wundef)
   endif()
   target_add_compile_flags_if_supported(${target} PRIVATE -Wextra -W -Wwrite-strings
                                                           -Wno-unused-parameter -Wno-long-long
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
@@ -501,8 +501,8 @@
     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)) {}
+#if _LIBCPP_STD_VER > 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/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
@@ -25,10 +25,11 @@
 #include <algorithm>
 #include <cassert>
 #include <cstddef>
-#if TEST_STD_VER >= 11
-#include <initializer_list>
 
 #include "test_macros.h"
+
+#if TEST_STD_VER >= 11
+#include <initializer_list>
 #endif
 
 struct all_zero_seed_seq {
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',