diff --git a/libcxx/include/__type_traits/promote.h b/libcxx/include/__type_traits/promote.h
--- a/libcxx/include/__type_traits/promote.h
+++ b/libcxx/include/__type_traits/promote.h
@@ -13,7 +13,6 @@
 #include <__type_traits/integral_constant.h>
 #include <__type_traits/is_same.h>
 #include <__utility/declval.h>
-#include <cstddef>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
diff --git a/libcxx/include/cmath b/libcxx/include/cmath
--- a/libcxx/include/cmath
+++ b/libcxx/include/cmath
@@ -311,6 +311,7 @@
 #include <__type_traits/is_constant_evaluated.h>
 #include <__type_traits/is_floating_point.h>
 #include <__type_traits/is_same.h>
+#include <__type_traits/promote.h>
 #include <__type_traits/remove_cv.h>
 #include <version>