diff --git a/libcxx/include/__support/xlocale/__nop_locale_mgmt.h b/libcxx/include/__support/xlocale/__nop_locale_mgmt.h
--- a/libcxx/include/__support/xlocale/__nop_locale_mgmt.h
+++ b/libcxx/include/__support/xlocale/__nop_locale_mgmt.h
@@ -10,6 +10,8 @@
 #ifndef _LIBCPP_SUPPORT_XLOCALE_NOP_LOCALE_MGMT_H
 #define _LIBCPP_SUPPORT_XLOCALE_NOP_LOCALE_MGMT_H
 
+#include <__config>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
diff --git a/libcxx/include/__support/xlocale/__posix_l_fallback.h b/libcxx/include/__support/xlocale/__posix_l_fallback.h
--- a/libcxx/include/__support/xlocale/__posix_l_fallback.h
+++ b/libcxx/include/__support/xlocale/__posix_l_fallback.h
@@ -15,6 +15,14 @@
 #ifndef _LIBCPP_SUPPORT_XLOCALE_POSIX_L_FALLBACK_H
 #define _LIBCPP_SUPPORT_XLOCALE_POSIX_L_FALLBACK_H
 
+#include <__config>
+#include <ctype.h>
+#include <time.h>
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+#  include <wctype.h>
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -67,6 +75,15 @@
   return ::isxdigit(__c);
 }
 
+inline _LIBCPP_HIDE_FROM_ABI int toupper_l(int __c, locale_t) {
+  return ::toupper(__c);
+}
+
+inline _LIBCPP_HIDE_FROM_ABI int tolower_l(int __c, locale_t) {
+  return ::tolower(__c);
+}
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 inline _LIBCPP_HIDE_FROM_ABI int iswalnum_l(wint_t __c, locale_t) {
   return ::iswalnum(__c);
 }
@@ -115,14 +132,6 @@
   return ::iswxdigit(__c);
 }
 
-inline _LIBCPP_HIDE_FROM_ABI int toupper_l(int __c, locale_t) {
-  return ::toupper(__c);
-}
-
-inline _LIBCPP_HIDE_FROM_ABI int tolower_l(int __c, locale_t) {
-  return ::tolower(__c);
-}
-
 inline _LIBCPP_HIDE_FROM_ABI wint_t towupper_l(wint_t __c, locale_t) {
   return ::towupper(__c);
 }
@@ -130,6 +139,7 @@
 inline _LIBCPP_HIDE_FROM_ABI wint_t towlower_l(wint_t __c, locale_t) {
   return ::towlower(__c);
 }
+#endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
 
 inline _LIBCPP_HIDE_FROM_ABI int
 strcoll_l(const char *__s1, const char *__s2, locale_t) {
@@ -147,6 +157,7 @@
   return ::strftime(__s, __max, __format, __tm);
 }
 
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 inline _LIBCPP_HIDE_FROM_ABI int
 wcscoll_l(const wchar_t *__ws1, const wchar_t *__ws2, locale_t) {
   return ::wcscoll(__ws1, __ws2);
@@ -156,6 +167,7 @@
 wcsxfrm_l(wchar_t *__dest, const wchar_t *__src, size_t __n, locale_t) {
   return ::wcsxfrm(__dest, __src, __n);
 }
+#endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
 
 #ifdef __cplusplus
 }
diff --git a/libcxx/include/__support/xlocale/__strtonum_fallback.h b/libcxx/include/__support/xlocale/__strtonum_fallback.h
--- a/libcxx/include/__support/xlocale/__strtonum_fallback.h
+++ b/libcxx/include/__support/xlocale/__strtonum_fallback.h
@@ -15,6 +15,13 @@
 #ifndef _LIBCPP_SUPPORT_XLOCALE_STRTONUM_FALLBACK_H
 #define _LIBCPP_SUPPORT_XLOCALE_STRTONUM_FALLBACK_H
 
+#include <__config>
+#include <stdlib.h>
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+#  include <wchar.h>
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -44,6 +51,7 @@
   return ::strtoull(__nptr, __endptr, __base);
 }
 
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
 inline _LIBCPP_HIDE_FROM_ABI long long
 wcstoll_l(const wchar_t *__nptr, wchar_t **__endptr, int __base, locale_t) {
   return ::wcstoll(__nptr, __endptr, __base);
@@ -58,6 +66,7 @@
 wcstold_l(const wchar_t *__nptr, wchar_t **__endptr, locale_t) {
   return ::wcstold(__nptr, __endptr);
 }
+#endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
 
 #ifdef __cplusplus
 }