diff --git a/libcxx/include/cstddef b/libcxx/include/cstddef
--- a/libcxx/include/cstddef
+++ b/libcxx/include/cstddef
@@ -25,7 +25,7 @@
 
     ptrdiff_t
     size_t
-    max_align_t
+    max_align_t // C++11
     nullptr_t
     byte // C++17
 
@@ -49,12 +49,8 @@
 using ::ptrdiff_t;
 using ::size_t;
 
-#if defined(__CLANG_MAX_ALIGN_T_DEFINED) || defined(_GCC_MAX_ALIGN_T) || \
-    defined(__DEFINED_max_align_t) || defined(__NetBSD__)
-// Re-use the compiler's <stddef.h> max_align_t where possible.
+#if !defined(_LIBCPP_CXX03_LANG)
 using ::max_align_t;
-#else
-typedef long double max_align_t;
 #endif
 
 template <class _Tp> struct __libcpp_is_integral                     { enum { value = 0 }; };
diff --git a/libcxx/include/stddef.h b/libcxx/include/stddef.h
--- a/libcxx/include/stddef.h
+++ b/libcxx/include/stddef.h
@@ -31,7 +31,7 @@
 
     ptrdiff_t
     size_t
-    max_align_t
+    max_align_t // C++11
     nullptr_t
 
 */
@@ -51,12 +51,6 @@
 using std::nullptr_t;
 }
 
-// Re-use the compiler's <stddef.h> max_align_t where possible.
-#if !defined(__CLANG_MAX_ALIGN_T_DEFINED) && !defined(_GCC_MAX_ALIGN_T) && \
-    !defined(__DEFINED_max_align_t) && !defined(__NetBSD__)
-typedef long double max_align_t;
-#endif
-
 #endif
 
 #endif  // _LIBCPP_STDDEF_H
diff --git a/libcxx/test/libcxx/experimental/memory/memory.resource.adaptor/memory.resource.adaptor.mem/db_deallocate.pass.cpp b/libcxx/test/libcxx/experimental/memory/memory.resource.adaptor/memory.resource.adaptor.mem/db_deallocate.pass.cpp
--- a/libcxx/test/libcxx/experimental/memory/memory.resource.adaptor/memory.resource.adaptor.mem/db_deallocate.pass.cpp
+++ b/libcxx/test/libcxx/experimental/memory/memory.resource.adaptor/memory.resource.adaptor.mem/db_deallocate.pass.cpp
@@ -36,8 +36,13 @@
     ex::resource_adaptor<Alloc> r(Alloc{P});
     ex::memory_resource & m1 = r;
 
+#ifdef __STDCPP_DEFAULT_NEW_ALIGNMENT__
+    std::size_t maxSize = std::numeric_limits<std::size_t>::max()
+                            - __STDCPP_DEFAULT_NEW_ALIGNMENT__;
+#else
     std::size_t maxSize = std::numeric_limits<std::size_t>::max()
                             - alignof(std::max_align_t);
+#endif
 
     m1.deallocate(nullptr, maxSize);
     assert(AssertCount == 0);
diff --git a/libcxx/test/libcxx/language.support/support.dynamic/libcpp_deallocate.sh.cpp b/libcxx/test/libcxx/language.support/support.dynamic/libcpp_deallocate.sh.cpp
--- a/libcxx/test/libcxx/language.support/support.dynamic/libcpp_deallocate.sh.cpp
+++ b/libcxx/test/libcxx/language.support/support.dynamic/libcpp_deallocate.sh.cpp
@@ -142,7 +142,11 @@
 
 void test_libcpp_dealloc() {
   void* p = nullptr;
+#ifdef __STDCPP_DEFAULT_NEW_ALIGNMENT__
+  size_t over_align_val = __STDCPP_DEFAULT_NEW_ALIGNMENT__ * 2;
+#else
   size_t over_align_val = TEST_ALIGNOF(std::max_align_t) * 2;
+#endif
   size_t under_align_val = TEST_ALIGNOF(int);
   size_t with_size_val = 2;
 
diff --git a/libcxx/test/std/containers/sequences/array/array.data/data.pass.cpp b/libcxx/test/std/containers/sequences/array/array.data/data.pass.cpp
--- a/libcxx/test/std/containers/sequences/array/array.data/data.pass.cpp
+++ b/libcxx/test/std/containers/sequences/array/array.data/data.pass.cpp
@@ -24,6 +24,14 @@
   NoDefault(int) {}
 };
 
+#if TEST_STD_VER < 11
+struct natural_alignment {
+    long t1;
+    long long t2;
+    double t3;
+    long double t4;
+};
+#endif
 
 int main(int, char**)
 {
@@ -52,13 +60,17 @@
       LIBCPP_ASSERT(p != nullptr);
     }
   {
+#if TEST_STD_VER < 11
+      typedef natural_alignment T;
+#else
       typedef std::max_align_t T;
+#endif
       typedef std::array<T, 0> C;
       const C c = {};
       const T* p = c.data();
       LIBCPP_ASSERT(p != nullptr);
       std::uintptr_t pint = reinterpret_cast<std::uintptr_t>(p);
-      assert(pint % TEST_ALIGNOF(std::max_align_t) == 0);
+      assert(pint % TEST_ALIGNOF(T) == 0);
     }
     {
       typedef NoDefault T;
diff --git a/libcxx/test/std/containers/sequences/array/array.data/data_const.pass.cpp b/libcxx/test/std/containers/sequences/array/array.data/data_const.pass.cpp
--- a/libcxx/test/std/containers/sequences/array/array.data/data_const.pass.cpp
+++ b/libcxx/test/std/containers/sequences/array/array.data/data_const.pass.cpp
@@ -24,6 +24,15 @@
   NoDefault(int) {}
 };
 
+#if TEST_STD_VER < 11
+struct natural_alignment {
+    long t1;
+    long long t2;
+    double t3;
+    long double t4;
+};
+#endif
+
 int main(int, char**)
 {
     {
@@ -50,13 +59,17 @@
       LIBCPP_ASSERT(p != nullptr);
     }
     {
+#if TEST_STD_VER < 11
+      typedef natural_alignment T;
+#else
       typedef std::max_align_t T;
+#endif
       typedef std::array<T, 0> C;
       const C c = {};
       const T* p = c.data();
       LIBCPP_ASSERT(p != nullptr);
       std::uintptr_t pint = reinterpret_cast<std::uintptr_t>(p);
-      assert(pint % TEST_ALIGNOF(std::max_align_t) == 0);
+      assert(pint % TEST_ALIGNOF(T) == 0);
     }
 #if TEST_STD_VER > 14
     {
diff --git a/libcxx/test/std/containers/sequences/array/size_and_alignment.pass.cpp b/libcxx/test/std/containers/sequences/array/size_and_alignment.pass.cpp
--- a/libcxx/test/std/containers/sequences/array/size_and_alignment.pass.cpp
+++ b/libcxx/test/std/containers/sequences/array/size_and_alignment.pass.cpp
@@ -49,6 +49,15 @@
   test<T, 0>();
 }
 
+#ifdef __STDCPP_DEFAULT_NEW_ALIGNMENT__
+struct TEST_ALIGNAS(__STDCPP_DEFAULT_NEW_ALIGNMENT__ * 2) TestType1 {
+
+};
+
+struct TEST_ALIGNAS(__STDCPP_DEFAULT_NEW_ALIGNMENT__ * 2) TestType2 {
+  char data[1000];
+};
+#else
 struct TEST_ALIGNAS(TEST_ALIGNOF(std::max_align_t) * 2) TestType1 {
 
 };
@@ -56,6 +65,7 @@
 struct TEST_ALIGNAS(TEST_ALIGNOF(std::max_align_t) * 2) TestType2 {
   char data[1000];
 };
+#endif
 
 //static_assert(sizeof(void*) == 4, "");
 
@@ -64,7 +74,10 @@
   test_type<int>();
   test_type<double>();
   test_type<long double>();
+
+#if TEST_STD_VER >= 11
   test_type<std::max_align_t>();
+#endif
   test_type<TestType1>();
   test_type<TestType2>();
 
diff --git a/libcxx/test/std/depr/depr.c.headers/stddef_h.pass.cpp b/libcxx/test/std/depr/depr.c.headers/stddef_h.pass.cpp
--- a/libcxx/test/std/depr/depr.c.headers/stddef_h.pass.cpp
+++ b/libcxx/test/std/depr/depr.c.headers/stddef_h.pass.cpp
@@ -43,6 +43,7 @@
                   "decltype(nullptr) == nullptr_t");
     static_assert(sizeof(nullptr_t) == sizeof(void*),
                   "sizeof(nullptr_t) == sizeof(void*)");
+#if TEST_STD_VER >= 11
 #if TEST_STD_VER > 17
 //   P0767
     static_assert(std::is_trivial<max_align_t>::value,
@@ -65,6 +66,7 @@
                   std::alignment_of<void*>::value,
                   "std::alignment_of<max_align_t>::value >= "
                   "std::alignment_of<void*>::value");
+#endif
 
   return 0;
 }
diff --git a/libcxx/test/std/language.support/support.types/max_align_t.pass.cpp b/libcxx/test/std/language.support/support.types/max_align_t.pass.cpp
--- a/libcxx/test/std/language.support/support.types/max_align_t.pass.cpp
+++ b/libcxx/test/std/language.support/support.types/max_align_t.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: c++98, c++03
+
 #include <cstddef>
 #include <type_traits>
 
@@ -41,5 +43,11 @@
                   "std::alignment_of<std::max_align_t>::value >= "
                   "std::alignment_of<void*>::value");
 
+#ifdef __STDCPP_DEFAULT_NEW_ALIGNMENT__
+    static_assert(std::alignment_of<std::max_align_t>::value <=
+                  __STDCPP_DEFAULT_NEW_ALIGNMENT__,
+                  "max_align_t alignment is no larger than new alignment");
+#endif
+
   return 0;
 }
diff --git a/libcxx/test/std/utilities/meta/meta.trans/meta.trans.other/aligned_storage.pass.cpp b/libcxx/test/std/utilities/meta/meta.trans/meta.trans.other/aligned_storage.pass.cpp
--- a/libcxx/test/std/utilities/meta/meta.trans/meta.trans.other/aligned_storage.pass.cpp
+++ b/libcxx/test/std/utilities/meta/meta.trans/meta.trans.other/aligned_storage.pass.cpp
@@ -17,6 +17,20 @@
 #include <cstddef>       // for std::max_align_t
 #include "test_macros.h"
 
+// The following tests assume naturally aligned types exist
+// up to 64bit (double). For larger types, max_align_t should
+// give the correct alignment. For pre-C++11 testing, only
+// the lower bound is checked.
+
+#if TEST_STD_VER < 11
+struct natural_alignment {
+    long t1;
+    long long t2;
+    double t3;
+    long double t4;
+};
+#endif
+
 int main(int, char**)
 {
     {
@@ -250,9 +264,6 @@
     static_assert(std::alignment_of<T1>::value == 8, "");
     static_assert(sizeof(T1) == 16, "");
     }
-    // Use alignof(std::max_align_t) below to find the max alignment instead of
-    // hardcoding it, because it's different on different platforms.
-    // (For example 8 on arm and 16 on x86.)
     {
     typedef std::aligned_storage<16>::type T1;
 #if TEST_STD_VER > 11
@@ -260,8 +271,15 @@
 #endif
     static_assert(std::is_trivial<T1>::value, "");
     static_assert(std::is_standard_layout<T1>::value, "");
-    static_assert(std::alignment_of<T1>::value == TEST_ALIGNOF(std::max_align_t),
-                  "");
+#if TEST_STD_VER >= 11
+    const size_t alignment = TEST_ALIGNOF(std::max_align_t) > 16 ?
+        16 : TEST_ALIGNOF(std::max_align_t);
+    static_assert(std::alignment_of<T1>::value == alignment, "");
+#else
+    static_assert(std::alignment_of<T1>::value >=
+                  TEST_ALIGNOF(natural_alignment), "");
+    static_assert(std::alignment_of<T1>::value <= 16, "");
+#endif
     static_assert(sizeof(T1) == 16, "");
     }
     {
@@ -271,9 +289,17 @@
 #endif
     static_assert(std::is_trivial<T1>::value, "");
     static_assert(std::is_standard_layout<T1>::value, "");
-    static_assert(std::alignment_of<T1>::value == TEST_ALIGNOF(std::max_align_t),
-                  "");
-    static_assert(sizeof(T1) == 16 + TEST_ALIGNOF(std::max_align_t), "");
+#if TEST_STD_VER >= 11
+    const size_t alignment = TEST_ALIGNOF(std::max_align_t) > 16 ?
+        16 : TEST_ALIGNOF(std::max_align_t);
+    static_assert(std::alignment_of<T1>::value == alignment, "");
+    static_assert(sizeof(T1) == 16 + alignment, "");
+#else
+    static_assert(std::alignment_of<T1>::value >=
+                  TEST_ALIGNOF(natural_alignment), "");
+    static_assert(std::alignment_of<T1>::value <= 16);
+    static_assert(sizeof(T1) % TEST_ALIGNOF(natural_alignment) == 0, "");
+#endif
     }
     {
     typedef std::aligned_storage<10>::type T1;