diff --git a/libcxx/include/version b/libcxx/include/version
--- a/libcxx/include/version
+++ b/libcxx/include/version
@@ -251,7 +251,9 @@
 #   define __cpp_lib_char8_t                            201811L
 # endif
 // # define __cpp_lib_concepts                             201806L
-# define __cpp_lib_constexpr_dynamic_alloc              201907L
+# if defined(__cpp_constexpr_dynamic_alloc)
+#   define __cpp_lib_constexpr_dynamic_alloc            201907L
+# endif
 // # define __cpp_lib_constexpr_misc                       201811L
 // # define __cpp_lib_constexpr_swap_algorithms            201806L
 # define __cpp_lib_constexpr_utility                    201811L
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/memory.version.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/memory.version.pass.cpp
--- a/libcxx/test/std/language.support/support.limits/support.limits.general/memory.version.pass.cpp
+++ b/libcxx/test/std/language.support/support.limits/support.limits.general/memory.version.pass.cpp
@@ -226,11 +226,17 @@
 #   endif
 # endif
 
-# ifndef __cpp_lib_constexpr_dynamic_alloc
-#   error "__cpp_lib_constexpr_dynamic_alloc should be defined in c++2a"
-# endif
-# if __cpp_lib_constexpr_dynamic_alloc != 201907L
-#   error "__cpp_lib_constexpr_dynamic_alloc should have the value 201907L in c++2a"
+# if !defined(__cpp_constexpr_dynamic_alloc)
+#   ifdef __cpp_lib_constexpr_dynamic_alloc
+#     error "__cpp_lib_constexpr_dynamic_alloc should be not be defined without compiler support"
+#   endif
+# else
+#   ifndef __cpp_lib_constexpr_dynamic_alloc
+#     error "__cpp_lib_constexpr_dynamic_alloc should be defined in c++2a"
+#   endif
+#   if __cpp_lib_constexpr_dynamic_alloc != 201907L
+#     error "__cpp_lib_constexpr_dynamic_alloc should have the value 201907L in c++2a"
+#   endif
 # endif
 
 # ifndef __cpp_lib_enable_shared_from_this
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.pass.cpp
--- a/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.pass.cpp
+++ b/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.pass.cpp
@@ -1877,11 +1877,13 @@
 #   endif
 # endif
 
-# ifndef __cpp_lib_constexpr_dynamic_alloc
-#   error "__cpp_lib_constexpr_dynamic_alloc should be defined in c++2a"
-# endif
-# if __cpp_lib_constexpr_dynamic_alloc != 201907L
-#   error "__cpp_lib_constexpr_dynamic_alloc should have the value 201907L in c++2a"
+# if TEST_STD_VER > 17 && defined(__cpp_constexpr_dynamic_alloc)
+#   ifndef __cpp_lib_constexpr_dynamic_alloc
+#     error "__cpp_lib_constexpr_dynamic_alloc should be defined in c++2a"
+#   endif
+#   if __cpp_lib_constexpr_dynamic_alloc != 201907L
+#     error "__cpp_lib_constexpr_dynamic_alloc should have the value 201907L in c++2a"
+#   endif
 # endif
 
 # if !defined(_LIBCPP_VERSION)
diff --git a/libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/allocate_hint.pass.cpp b/libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/allocate_hint.pass.cpp
--- a/libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/allocate_hint.pass.cpp
+++ b/libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/allocate_hint.pass.cpp
@@ -15,6 +15,10 @@
 //     ...
 // };
 
+// constexpr bool test() results in "error: constexpr function never produces a
+// constant expression" before clang 11
+// UNSUPPORTED: clang-5, clang-6, clang-7, clang-8, clang-9
+
 #include <memory>
 #include <cstdint>
 #include <cassert>
@@ -83,7 +87,7 @@
 int main(int, char**)
 {
     test();
-#if TEST_STD_VER > 17
+#if defined(__cpp_constexpr_dynamic_alloc)
     static_assert(test());
 #endif
     return 0;
diff --git a/libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/construct.pass.cpp b/libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/construct.pass.cpp
--- a/libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/construct.pass.cpp
+++ b/libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/construct.pass.cpp
@@ -169,7 +169,7 @@
 {
     test();
 
-#if TEST_STD_VER > 17
+#if defined(__cpp_constexpr_dynamic_alloc)
     static_assert(test());
 #endif
 
diff --git a/libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/destroy.pass.cpp b/libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/destroy.pass.cpp
--- a/libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/destroy.pass.cpp
+++ b/libcxx/test/std/utilities/memory/allocator.traits/allocator.traits.members/destroy.pass.cpp
@@ -16,6 +16,9 @@
 //     ...
 // };
 
+// constexpr destructors are only supported starting with clang 10
+// UNSUPPORTED: clang-5, clang-6, clang-7, clang-8, clang-9
+
 #include <memory>
 #include <cassert>
 #include <cstddef>
@@ -127,7 +130,7 @@
 int main(int, char**)
 {
     test();
-#if TEST_STD_VER > 17
+#if defined(__cpp_constexpr_dynamic_alloc)
     static_assert(test());
 #endif
     return 0;
diff --git a/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.pass.cpp b/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.pass.cpp
--- a/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.pass.cpp
+++ b/libcxx/test/std/utilities/memory/default.allocator/allocator.members/allocate.pass.cpp
@@ -77,7 +77,7 @@
   }
 }
 
-#if TEST_STD_VER > 17
+#if defined(__cpp_constexpr_dynamic_alloc)
 template <size_t Align>
 constexpr bool test_aligned_constexpr() {
     typedef AlignedType<Align> T;
@@ -99,7 +99,7 @@
     test_aligned<OverAligned>();
     test_aligned<OverAligned * 2>();
 
-#if TEST_STD_VER > 17
+#if defined(__cpp_constexpr_dynamic_alloc)
     static_assert(test_aligned_constexpr<1>());
     static_assert(test_aligned_constexpr<2>());
     static_assert(test_aligned_constexpr<4>());