Index: libcxx/include/__locale
===================================================================
--- libcxx/include/__locale
+++ libcxx/include/__locale
@@ -14,7 +14,9 @@
 #include <__config>
 #include <cctype>
 #include <cstdint>
+#if !defined(_LIBCPP_HAS_NO_LOCALIZATION)
 #include <locale.h>
+#endif
 #include <memory>
 #include <mutex>
 #include <string>
Index: libcxx/include/charconv
===================================================================
--- libcxx/include/charconv
+++ libcxx/include/charconv
@@ -93,6 +93,12 @@
 
 #include <__debug>
 
+#if defined(_LIBCPP_HAS_NO_LOCALIZATION)
+#include <cctype>
+#else
+#include <locale>
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
@@ -426,6 +432,20 @@
     return {__r.ptr, errc::result_out_of_range};
 }
 
+template <typename _Tp>
+inline _LIBCPP_INLINE_VISIBILITY _Tp
+__letter_value(_Tp __c)
+{
+#if defined(__MVS__) && !defined(__NATIVE_ASCII_F)
+  if ('a' <= __c && __c <= 'i') return __c - 'a';
+  else if ('j' <= __c && __c <= 'r') return __c - 'j' + 9;
+  else if ('s' <= __c && __c <= 'z') return __c - 's' + 18;
+  else return 27;
+#else
+  return __c - 'a';
+#endif
+}
+
 template <typename _Tp>
 inline _LIBCPP_INLINE_VISIBILITY bool
 __in_pattern(_Tp __c)
@@ -449,10 +469,8 @@
         return {'0' <= __c && __c < '0' + __base, __c - '0'};
     else if (__in_pattern(__c))
         return {true, __c - '0'};
-    else if ('a' <= __c && __c < 'a' + __base - 10)
-        return {true, __c - 'a' + 10};
-    else
-        return {'A' <= __c && __c < 'A' + __base - 10, __c - 'A' + 10};
+    __c =  _VSTD::tolower(__c);
+    return {'a' <= __c && __letter_value(__c) < __base - 10, __letter_value(__c) + 10};
 }
 
 template <typename _It, typename _Tp, typename _Fn, typename... _Ts>