Index: libcxx/include/__locale
===================================================================
--- libcxx/include/__locale
+++ libcxx/include/__locale
@@ -568,6 +568,16 @@
     explicit ctype(size_t __refs = 0)
         : locale::facet(__refs) {}
 
+    _LIBCPP_HIDE_FROM_ABI
+    static bool __is_valid_in_encoding(char_type __c)
+    {
+#if defined(__MVS__)
+        return (__c >=0 && __c < 256);
+#else
+        return isascii(__c);
+#endif
+    }
+
     _LIBCPP_INLINE_VISIBILITY
     bool is(mask __m, char_type __c) const
     {
@@ -670,17 +680,28 @@
 
     explicit ctype(const mask* __tab = nullptr, bool __del = false, size_t __refs = 0);
 
+    _LIBCPP_HIDE_FROM_ABI
+    static bool __is_valid_in_encoding(char_type __c)
+    {
+#if defined(__MVS__)
+        (void)__c;
+        return true;
+#else
+        return isascii(__c);
+#endif
+    }
+
     _LIBCPP_INLINE_VISIBILITY
     bool is(mask __m, char_type __c) const
     {
-        return isascii(__c) ? (__tab_[static_cast<int>(__c)] & __m) !=0 : false;
+        return __is_valid_in_encoding(__c) ? (__tab_[static_cast<int>(__c)] & __m) !=0 : false;
     }
 
     _LIBCPP_INLINE_VISIBILITY
     const char_type* is(const char_type* __low, const char_type* __high, mask* __vec) const
     {
         for (; __low != __high; ++__low, ++__vec)
-            *__vec = isascii(*__low) ? __tab_[static_cast<int>(*__low)] : 0;
+            *__vec = __is_valid_in_encoding(*__low) ? __tab_[static_cast<int>(*__low)] : 0;
         return __low;
     }
 
@@ -688,7 +709,7 @@
     const char_type* scan_is (mask __m, const char_type* __low, const char_type* __high) const
     {
         for (; __low != __high; ++__low)
-            if (isascii(*__low) && (__tab_[static_cast<int>(*__low)] & __m))
+            if (__is_valid_in_encoding(*__low) && (__tab_[static_cast<int>(*__low)] & __m))
                 break;
         return __low;
     }
@@ -697,7 +718,7 @@
     const char_type* scan_not(mask __m, const char_type* __low, const char_type* __high) const
     {
         for (; __low != __high; ++__low)
-            if (!(isascii(*__low) && (__tab_[static_cast<int>(*__low)] & __m)))
+            if (!(__is_valid_in_encoding(*__low) && (__tab_[static_cast<int>(*__low)] & __m)))
                 break;
         return __low;
     }
Index: libcxx/src/locale.cpp
===================================================================
--- libcxx/src/locale.cpp
+++ libcxx/src/locale.cpp
@@ -862,15 +862,14 @@
 bool
 ctype<wchar_t>::do_is(mask m, char_type c) const
 {
-    return isascii(c) ? (ctype<char>::classic_table()[c] & m) != 0 : false;
+  return __is_valid_in_encoding(c) ? (ctype<char>::classic_table()[c] & m) != 0 : false;
 }
 
 const wchar_t*
 ctype<wchar_t>::do_is(const char_type* low, const char_type* high, mask* vec) const
 {
     for (; low != high; ++low, ++vec)
-        *vec = static_cast<mask>(isascii(*low) ?
-                                   ctype<char>::classic_table()[*low] : 0);
+      *vec = static_cast<mask>(__is_valid_in_encoding(*low) ? ctype<char>::classic_table()[*low] : 0);
     return low;
 }
 
@@ -878,8 +877,8 @@
 ctype<wchar_t>::do_scan_is(mask m, const char_type* low, const char_type* high) const
 {
     for (; low != high; ++low)
-        if (isascii(*low) && (ctype<char>::classic_table()[*low] & m))
-            break;
+      if (__is_valid_in_encoding(*low) && (ctype<char>::classic_table()[*low] & m))
+        break;
     return low;
 }
 
@@ -887,8 +886,8 @@
 ctype<wchar_t>::do_scan_not(mask m, const char_type* low, const char_type* high) const
 {
     for (; low != high; ++low)
-        if (!(isascii(*low) && (ctype<char>::classic_table()[*low] & m)))
-            break;
+      if (!(__is_valid_in_encoding(*low) && (ctype<char>::classic_table()[*low] & m)))
+        break;
     return low;
 }
 
@@ -896,13 +895,12 @@
 ctype<wchar_t>::do_toupper(char_type c) const
 {
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-    return isascii(c) ? _DefaultRuneLocale.__mapupper[c] : c;
-#elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || \
-      defined(__NetBSD__) || defined(__MVS__)
-    return isascii(c) ? ctype<char>::__classic_upper_table()[c] : c;
-#else
-    return (isascii(c) && iswlower_l(c, _LIBCPP_GET_C_LOCALE)) ? c-L'a'+L'A' : c;
-#endif
+  return __is_valid_in_encoding(c) ? _DefaultRuneLocale.__mapupper[c] : c;
+#  elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__NetBSD__) || defined(__MVS__)
+  return __is_valid_in_encoding(c) ? ctype<char>::__classic_upper_table()[c] : c;
+#  else
+  return (__is_valid_in_encoding(c) && iswlower_l(c, _LIBCPP_GET_C_LOCALE)) ? c - L'a' + L'A' : c;
+#  endif
 }
 
 const wchar_t*
@@ -910,13 +908,12 @@
 {
     for (; low != high; ++low)
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-        *low = isascii(*low) ? _DefaultRuneLocale.__mapupper[*low] : *low;
+      *low = __is_valid_in_encoding(*low) ? _DefaultRuneLocale.__mapupper[*low] : *low;
 #elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || \
       defined(__NetBSD__) || defined(__MVS__)
-        *low = isascii(*low) ? ctype<char>::__classic_upper_table()[*low]
-                             : *low;
+      *low = __is_valid_in_encoding(*low) ? ctype<char>::__classic_upper_table()[*low] : *low;
 #else
-        *low = (isascii(*low) && islower_l(*low, _LIBCPP_GET_C_LOCALE)) ? (*low-L'a'+L'A') : *low;
+      *low = (__is_valid_in_encoding(*low) && islower_l(*low, _LIBCPP_GET_C_LOCALE)) ? (*low - L'a' + L'A') : *low;
 #endif
     return low;
 }
@@ -925,13 +922,12 @@
 ctype<wchar_t>::do_tolower(char_type c) const
 {
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-    return isascii(c) ? _DefaultRuneLocale.__maplower[c] : c;
-#elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || \
-      defined(__NetBSD__) || defined(__MVS__)
-    return isascii(c) ? ctype<char>::__classic_lower_table()[c] : c;
-#else
-    return (isascii(c) && isupper_l(c, _LIBCPP_GET_C_LOCALE)) ? c-L'A'+'a' : c;
-#endif
+  return __is_valid_in_encoding(c) ? _DefaultRuneLocale.__maplower[c] : c;
+#  elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__NetBSD__) || defined(__MVS__)
+  return __is_valid_in_encoding(c) ? ctype<char>::__classic_lower_table()[c] : c;
+#  else
+  return (__is_valid_in_encoding(c) && isupper_l(c, _LIBCPP_GET_C_LOCALE)) ? c - L'A' + 'a' : c;
+#  endif
 }
 
 const wchar_t*
@@ -939,13 +935,12 @@
 {
     for (; low != high; ++low)
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-        *low = isascii(*low) ? _DefaultRuneLocale.__maplower[*low] : *low;
+      *low = __is_valid_in_encoding(*low) ? _DefaultRuneLocale.__maplower[*low] : *low;
 #elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || \
       defined(__NetBSD__) || defined(__MVS__)
-        *low = isascii(*low) ? ctype<char>::__classic_lower_table()[*low]
-                             : *low;
+      *low = __is_valid_in_encoding(*low) ? ctype<char>::__classic_lower_table()[*low] : *low;
 #else
-        *low = (isascii(*low) && isupper_l(*low, _LIBCPP_GET_C_LOCALE)) ? *low-L'A'+L'a' : *low;
+      *low = (__is_valid_in_encoding(*low) && isupper_l(*low, _LIBCPP_GET_C_LOCALE)) ? *low - L'A' + L'a' : *low;
 #endif
     return low;
 }
@@ -967,19 +962,19 @@
 char
 ctype<wchar_t>::do_narrow(char_type c, char dfault) const
 {
-    if (isascii(c))
-        return static_cast<char>(c);
-    return dfault;
+  if (__is_valid_in_encoding(c))
+    return static_cast<char>(c);
+  return dfault;
 }
 
 const wchar_t*
 ctype<wchar_t>::do_narrow(const char_type* low, const char_type* high, char dfault, char* dest) const
 {
     for (; low != high; ++low, ++dest)
-        if (isascii(*low))
-            *dest = static_cast<char>(*low);
-        else
-            *dest = dfault;
+      if (__is_valid_in_encoding(*low))
+        *dest = static_cast<char>(*low);
+      else
+        *dest = dfault;
     return low;
 }
 #endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
@@ -1009,15 +1004,13 @@
 ctype<char>::do_toupper(char_type c) const
 {
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-    return isascii(c) ?
-      static_cast<char>(_DefaultRuneLocale.__mapupper[static_cast<ptrdiff_t>(c)]) : c;
+  return __is_valid_in_encoding(c) ? static_cast<char>(_DefaultRuneLocale.__mapupper[static_cast<ptrdiff_t>(c)]) : c;
 #elif defined(__NetBSD__)
-    return static_cast<char>(__classic_upper_table()[static_cast<unsigned char>(c)]);
+  return static_cast<char>(__classic_upper_table()[static_cast<unsigned char>(c)]);
 #elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__MVS__)
-    return isascii(c) ?
-      static_cast<char>(__classic_upper_table()[static_cast<unsigned char>(c)]) : c;
+  return __is_valid_in_encoding(c) ? static_cast<char>(__classic_upper_table()[static_cast<unsigned char>(c)]) : c;
 #else
-    return (isascii(c) && islower_l(c, _LIBCPP_GET_C_LOCALE)) ? c-'a'+'A' : c;
+  return (__is_valid_in_encoding(c) && islower_l(c, _LIBCPP_GET_C_LOCALE)) ? c - 'a' + 'A' : c;
 #endif
 }
 
@@ -1026,15 +1019,16 @@
 {
     for (; low != high; ++low)
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-        *low = isascii(*low) ?
-          static_cast<char>(_DefaultRuneLocale.__mapupper[static_cast<ptrdiff_t>(*low)]) : *low;
+      *low = __is_valid_in_encoding(*low)
+                 ? static_cast<char>(_DefaultRuneLocale.__mapupper[static_cast<ptrdiff_t>(*low)])
+                 : *low;
 #elif defined(__NetBSD__)
-        *low = static_cast<char>(__classic_upper_table()[static_cast<unsigned char>(*low)]);
+      *low = static_cast<char>(__classic_upper_table()[static_cast<unsigned char>(*low)]);
 #elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__MVS__)
-        *low = isascii(*low) ?
-          static_cast<char>(__classic_upper_table()[static_cast<size_t>(*low)]) : *low;
+      *low =
+          __is_valid_in_encoding(*low) ? static_cast<char>(__classic_upper_table()[static_cast<size_t>(*low)]) : *low;
 #else
-        *low = (isascii(*low) && islower_l(*low, _LIBCPP_GET_C_LOCALE)) ? *low-'a'+'A' : *low;
+      *low = (__is_valid_in_encoding(*low) && islower_l(*low, _LIBCPP_GET_C_LOCALE)) ? *low - 'a' + 'A' : *low;
 #endif
     return low;
 }
@@ -1043,15 +1037,13 @@
 ctype<char>::do_tolower(char_type c) const
 {
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-    return isascii(c) ?
-      static_cast<char>(_DefaultRuneLocale.__maplower[static_cast<ptrdiff_t>(c)]) : c;
+  return __is_valid_in_encoding(c) ? static_cast<char>(_DefaultRuneLocale.__maplower[static_cast<ptrdiff_t>(c)]) : c;
 #elif defined(__NetBSD__)
-    return static_cast<char>(__classic_lower_table()[static_cast<unsigned char>(c)]);
+  return static_cast<char>(__classic_lower_table()[static_cast<unsigned char>(c)]);
 #elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__MVS__)
-    return isascii(c) ?
-      static_cast<char>(__classic_lower_table()[static_cast<size_t>(c)]) : c;
+  return __is_valid_in_encoding(c) ? static_cast<char>(__classic_lower_table()[static_cast<size_t>(c)]) : c;
 #else
-    return (isascii(c) && isupper_l(c, _LIBCPP_GET_C_LOCALE)) ? c-'A'+'a' : c;
+  return (__is_valid_in_encoding(c) && isupper_l(c, _LIBCPP_GET_C_LOCALE)) ? c - 'A' + 'a' : c;
 #endif
 }
 
@@ -1060,13 +1052,16 @@
 {
     for (; low != high; ++low)
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-        *low = isascii(*low) ? static_cast<char>(_DefaultRuneLocale.__maplower[static_cast<ptrdiff_t>(*low)]) : *low;
+      *low = __is_valid_in_encoding(*low)
+                 ? static_cast<char>(_DefaultRuneLocale.__maplower[static_cast<ptrdiff_t>(*low)])
+                 : *low;
 #elif defined(__NetBSD__)
-        *low = static_cast<char>(__classic_lower_table()[static_cast<unsigned char>(*low)]);
+      *low = static_cast<char>(__classic_lower_table()[static_cast<unsigned char>(*low)]);
 #elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__MVS__)
-        *low = isascii(*low) ? static_cast<char>(__classic_lower_table()[static_cast<size_t>(*low)]) : *low;
+      *low =
+          __is_valid_in_encoding(*low) ? static_cast<char>(__classic_lower_table()[static_cast<size_t>(*low)]) : *low;
 #else
-        *low = (isascii(*low) && isupper_l(*low, _LIBCPP_GET_C_LOCALE)) ? *low-'A'+'a' : *low;
+      *low = (__is_valid_in_encoding(*low) && isupper_l(*low, _LIBCPP_GET_C_LOCALE)) ? *low - 'A' + 'a' : *low;
 #endif
     return low;
 }
@@ -1088,19 +1083,19 @@
 char
 ctype<char>::do_narrow(char_type c, char dfault) const
 {
-    if (isascii(c))
-        return static_cast<char>(c);
-    return dfault;
+  if (__is_valid_in_encoding(c))
+    return static_cast<char>(c);
+  return dfault;
 }
 
 const char*
 ctype<char>::do_narrow(const char_type* low, const char_type* high, char dfault, char* dest) const
 {
     for (; low != high; ++low, ++dest)
-        if (isascii(*low))
-            *dest = *low;
-        else
-            *dest = dfault;
+      if (__is_valid_in_encoding(*low))
+        *dest = *low;
+      else
+        *dest = dfault;
     return low;
 }
 
@@ -1392,14 +1387,13 @@
 {
     for (; low != high; ++low, ++vec)
     {
-        if (isascii(*low))
-            *vec = static_cast<mask>(ctype<char>::classic_table()[*low]);
-        else
-        {
-            *vec = 0;
-            wint_t ch = static_cast<wint_t>(*low);
-            if (iswspace_l(ch, __l))
-                *vec |= space;
+      if (__is_valid_in_encoding(*low))
+        *vec = static_cast<mask>(ctype<char>::classic_table()[*low]);
+      else {
+        *vec = 0;
+        wint_t ch = static_cast<wint_t>(*low);
+        if (iswspace_l(ch, __l))
+          *vec |= space;
 #ifndef _LIBCPP_CTYPE_MASK_IS_COMPOSITE_PRINT
             if (iswprint_l(ch, __l))
                 *vec |= print;