Index: include/__locale
===================================================================
--- include/__locale
+++ include/__locale
@@ -38,6 +38,19 @@
 #pragma GCC system_header
 #endif
 
+extern "C" {
+// Strict C++ isn't supposed to provide isascii, but it turns out that several
+// libc's do, even under __cplusplus. To unify them all, it is wrapped in a shim
+// that has an implementation reserved name. Unfortunately, because glibc has
+// `#define __isascii ...` we can't name it that.
+static inline int __libcpp_isascii(int c) {
+    // Here we assume that the libc exports isascii
+    // (which is a non-strict extension)
+    int isascii(int);
+    return isascii(c);
+}
+}
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 class _LIBCPP_TYPE_VIS locale;
@@ -526,14 +539,14 @@
     _LIBCPP_ALWAYS_INLINE
     bool is(mask __m, char_type __c) const
     {
-        return isascii(__c) ? (__tab_[static_cast<int>(__c)] & __m) !=0 : false;
+        return __libcpp_isascii(__c) ? (__tab_[static_cast<int>(__c)] & __m) !=0 : false;
     }
 
     _LIBCPP_ALWAYS_INLINE
     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 = __libcpp_isascii(*__low) ? __tab_[static_cast<int>(*__low)] : 0;
         return __low;
     }
 
@@ -541,7 +554,7 @@
     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 (__libcpp_isascii(*__low) && (__tab_[static_cast<int>(*__low)] & __m))
                 break;
         return __low;
     }
@@ -550,7 +563,7 @@
     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 (!(__libcpp_isascii(*__low) && (__tab_[static_cast<int>(*__low)] & __m)))
                 break;
         return __low;
     }
Index: include/locale
===================================================================
--- include/locale
+++ include/locale
@@ -193,6 +193,11 @@
 #include <ctime>
 #if defined(_LIBCPP_MSVCRT) || defined(__MINGW32__)
 #include <support/win32/locale_win32.h>
+#elif defined(_NEWLIB_VERSION)
+// FIXME: replace all the uses of _NEWLIB_VERSION with __NEWLIB__ preceded by an
+// include of <sys/cdefs.h> once https://sourceware.org/ml/newlib-cvs/2014-q3/msg00038.html
+// has had a chance to bake for a bit
+#include <support/newlib/xlocale.h>
 #elif !defined(__ANDROID__)
 #include <nl_types.h>
 #endif
@@ -229,7 +234,8 @@
 // OSX has nice foo_l() functions that let you turn off use of the global
 // locale.  Linux, not so much.  The following functions avoid the locale when
 // that's possible and otherwise do the wrong thing.  FIXME.
-#if defined(__linux__) || defined(__EMSCRIPTEN__) || defined(_AIX)
+#if defined(__linux__) || defined(__EMSCRIPTEN__) || defined(_AIX) || \
+    defined(_NEWLIB_VERSION)
 
 #ifdef _LIBCPP_LOCALE__L_EXTENSIONS
 decltype(MB_CUR_MAX_L(_VSTD::declval<locale_t>()))
@@ -3673,7 +3679,7 @@
 typename messages<_CharT>::catalog
 messages<_CharT>::do_open(const basic_string<char>& __nm, const locale&) const
 {
-#if defined(_WIN32) || defined(__ANDROID__)
+#if defined(_WIN32) || defined(__ANDROID__) || defined(_NEWLIB_VERSION)
     return -1;
 #else // _WIN32 || __ANDROID__
     catalog __cat = (catalog)catopen(__nm.c_str(), NL_CAT_LOCALE);
@@ -3688,7 +3694,7 @@
 messages<_CharT>::do_get(catalog __c, int __set, int __msgid,
                          const string_type& __dflt) const
 {
-#if defined(_WIN32) || defined(__ANDROID__)
+#if defined(_WIN32) || defined(__ANDROID__) || defined(_NEWLIB_VERSION)
     return __dflt;
 #else // _WIN32
     string __ndflt;
@@ -3710,7 +3716,7 @@
 void
 messages<_CharT>::do_close(catalog __c) const
 {
-#if !defined(_WIN32) && !defined(__ANDROID__)
+#if !defined(_WIN32) && !defined(__ANDROID__) && !defined(_NEWLIB_VERSION)
     if (__c != -1)
         __c <<= 1;
     nl_catd __cat = (nl_catd)__c;
Index: include/support/android/locale_bionic.h
===================================================================
--- include/support/android/locale_bionic.h
+++ include/support/android/locale_bionic.h
@@ -20,173 +20,12 @@
 #include <stdlib.h>
 #include <xlocale.h>
 
-static inline int isalnum_l(int c, locale_t) {
-  return isalnum(c);
-}
-
-static inline int isalpha_l(int c, locale_t) {
-  return isalpha(c);
-}
-
-static inline int isblank_l(int c, locale_t) {
-  return isblank(c);
-}
-
-static inline int iscntrl_l(int c, locale_t) {
-  return iscntrl(c);
-}
-
-static inline int isdigit_l(int c, locale_t) {
-  return isdigit(c);
-}
-
-static inline int isgraph_l(int c, locale_t) {
-  return isgraph(c);
-}
-
-static inline int islower_l(int c, locale_t) {
-  return islower(c);
-}
-
-static inline int isprint_l(int c, locale_t) {
-  return isprint(c);
-}
-
-static inline int ispunct_l(int c, locale_t) {
-  return ispunct(c);
-}
-
-static inline int isspace_l(int c, locale_t) {
-  return isspace(c);
-}
-
-static inline int isupper_l(int c, locale_t) {
-  return isupper(c);
-}
-
-static inline int isxdigit_l(int c, locale_t) {
-  return isxdigit(c);
-}
-
-static inline int iswalnum_l(wint_t c, locale_t) {
-  return iswalnum(c);
-}
-
-static inline int iswalpha_l(wint_t c, locale_t) {
-  return iswalpha(c);
-}
-
-static inline int iswblank_l(wint_t c, locale_t) {
-  return iswblank(c);
-}
-
-static inline int iswcntrl_l(wint_t c, locale_t) {
-  return iswcntrl(c);
-}
-
-static inline int iswdigit_l(wint_t c, locale_t) {
-  return iswdigit(c);
-}
-
-static inline int iswgraph_l(wint_t c, locale_t) {
-  return iswgraph(c);
-}
-
-static inline int iswlower_l(wint_t c, locale_t) {
-  return iswlower(c);
-}
-
-static inline int iswprint_l(wint_t c, locale_t) {
-  return iswprint(c);
-}
-
-static inline int iswpunct_l(wint_t c, locale_t) {
-  return iswpunct(c);
-}
-
-static inline int iswspace_l(wint_t c, locale_t) {
-  return iswspace(c);
-}
-
-static inline int iswupper_l(wint_t c, locale_t) {
-  return iswupper(c);
-}
-
-static inline int iswxdigit_l(wint_t c, locale_t) {
-  return iswxdigit(c);
-}
-
-static inline int toupper_l(int c, locale_t) {
-  return toupper(c);
-}
-
-static inline int tolower_l(int c, locale_t) {
-  return tolower(c);
-}
-
-static inline int towupper_l(int c, locale_t) {
-  return towupper(c);
-}
-
-static inline int towlower_l(int c, locale_t) {
-  return towlower(c);
-}
-
-static inline int strcoll_l(const char *s1, const char *s2, locale_t) {
-  return strcoll(s1, s2);
-}
-
-static inline size_t strxfrm_l(char *dest, const char *src, size_t n,
-                               locale_t) {
-  return strxfrm(dest, src, n);
-}
-
-static inline size_t strftime_l(char *s, size_t max, const char *format,
-                                const struct tm *tm, locale_t) {
-  return strftime(s, max, format, tm);
-}
-
-static inline int wcscoll_l(const wchar_t *ws1, const wchar_t *ws2, locale_t) {
-  return wcscoll(ws1, ws2);
-}
-
-static inline size_t wcsxfrm_l(wchar_t *dest, const wchar_t *src, size_t n,
-                               locale_t) {
-  return wcsxfrm(dest, src, n);
-}
-
-static inline long double strtold_l(const char *nptr, char **endptr, locale_t) {
-  return strtold(nptr, endptr);
-}
-
-static inline long long strtoll_l(const char *nptr, char **endptr, size_t base,
-                                  locale_t) {
-  return strtoll(nptr, endptr, base);
-}
-
-static inline unsigned long long strtoull_l(const char *nptr, char **endptr,
-                                            size_t base, locale_t) {
-  return strtoull(nptr, endptr, base);
-}
-
-static inline long long wcstoll_l(const wchar_t *nptr, wchar_t **endptr,
-                                  size_t base, locale_t) {
-  return wcstoll(nptr, endptr, base);
-}
-
-static inline unsigned long long wcstoull_l(const wchar_t *nptr,
-                                            wchar_t **endptr, size_t base,
-                                            locale_t) {
-  return wcstoull(nptr, endptr, base);
-}
-
-static inline long double wcstold_l(const wchar_t *nptr, wchar_t **endptr,
-                                    locale_t) {
-  return wcstold(nptr, endptr);
-}
-
 #ifdef __cplusplus
 }
 #endif
+
+// Share implementation with Newlib
+#include <support/xlocale/xlocale.h>
+
 #endif // defined(__ANDROID__)
 #endif // _LIBCPP_SUPPORT_ANDROID_LOCALE_BIONIC_H
Index: include/support/newlib/xlocale.h
===================================================================
--- include/support/newlib/xlocale.h
+++ include/support/newlib/xlocale.h
@@ -0,0 +1,63 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_SUPPORT_NEWLIB_XLOCALE_H
+#define _LIBCPP_SUPPORT_NEWLIB_XLOCALE_H
+
+#if defined(_NEWLIB_VERSION)
+
+#include <cstdlib>
+#include <clocale>
+#include <cwctype>
+#include <ctype.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Patch over newlib's lack of extended locale support
+typedef void *locale_t;
+static inline locale_t duplocale(locale_t) {
+  return NULL;
+}
+
+static inline void freelocale(locale_t) {
+}
+
+static inline locale_t newlocale(int, const char *, locale_t) {
+  return NULL;
+}
+
+static inline locale_t uselocale(locale_t) {
+  return NULL;
+}
+
+#define LC_COLLATE_MASK (1 << LC_COLLATE)
+#define LC_CTYPE_MASK (1 << LC_CTYPE)
+#define LC_MESSAGES_MASK (1 << LC_MESSAGES)
+#define LC_MONETARY_MASK (1 << LC_MONETARY)
+#define LC_NUMERIC_MASK (1 << LC_NUMERIC)
+#define LC_TIME_MASK (1 << LC_TIME)
+#define LC_ALL_MASK (LC_COLLATE_MASK|\
+                     LC_CTYPE_MASK|\
+                     LC_MONETARY_MASK|\
+                     LC_NUMERIC_MASK|\
+                     LC_TIME_MASK|\
+                     LC_MESSAGES_MASK)
+
+// Share implementation with Android's Bionic
+#include <support/xlocale/xlocale.h>
+
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
+#endif // _NEWLIB_VERSION
+
+#endif
Index: include/support/xlocale/xlocale.h
===================================================================
--- include/support/xlocale/xlocale.h
+++ include/support/xlocale/xlocale.h
@@ -0,0 +1,194 @@
+// -*- C++ -*-
+//===------------------- support/xlocale/xlocale.h ------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// This is a shared implementation of a shim to provide extended locale support
+// on top of libc's that don't support it (like Android's bionic, and Newlib).
+//
+// The 'illusion' only works when the specified locale is "C" or "POSIX", but
+// that's about as good as we can do without implementing full xlocale support
+// in the underlying libc.
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_SUPPORT_XLOCALE_XLOCALE_H
+#define _LIBCPP_SUPPORT_XLOCALE_XLOCALE_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+static inline int isalnum_l(int c, locale_t) {
+  return isalnum(c);
+}
+
+static inline int isalpha_l(int c, locale_t) {
+  return isalpha(c);
+}
+
+static inline int isblank_l(int c, locale_t) {
+  return isblank(c);
+}
+
+static inline int iscntrl_l(int c, locale_t) {
+  return iscntrl(c);
+}
+
+static inline int isdigit_l(int c, locale_t) {
+  return isdigit(c);
+}
+
+static inline int isgraph_l(int c, locale_t) {
+  return isgraph(c);
+}
+
+static inline int islower_l(int c, locale_t) {
+  return islower(c);
+}
+
+static inline int isprint_l(int c, locale_t) {
+  return isprint(c);
+}
+
+static inline int ispunct_l(int c, locale_t) {
+  return ispunct(c);
+}
+
+static inline int isspace_l(int c, locale_t) {
+  return isspace(c);
+}
+
+static inline int isupper_l(int c, locale_t) {
+  return isupper(c);
+}
+
+static inline int isxdigit_l(int c, locale_t) {
+  return isxdigit(c);
+}
+
+static inline int iswalnum_l(wint_t c, locale_t) {
+  return iswalnum(c);
+}
+
+static inline int iswalpha_l(wint_t c, locale_t) {
+  return iswalpha(c);
+}
+
+static inline int iswblank_l(wint_t c, locale_t) {
+  return iswblank(c);
+}
+
+static inline int iswcntrl_l(wint_t c, locale_t) {
+  return iswcntrl(c);
+}
+
+static inline int iswdigit_l(wint_t c, locale_t) {
+  return iswdigit(c);
+}
+
+static inline int iswgraph_l(wint_t c, locale_t) {
+  return iswgraph(c);
+}
+
+static inline int iswlower_l(wint_t c, locale_t) {
+  return iswlower(c);
+}
+
+static inline int iswprint_l(wint_t c, locale_t) {
+  return iswprint(c);
+}
+
+static inline int iswpunct_l(wint_t c, locale_t) {
+  return iswpunct(c);
+}
+
+static inline int iswspace_l(wint_t c, locale_t) {
+  return iswspace(c);
+}
+
+static inline int iswupper_l(wint_t c, locale_t) {
+  return iswupper(c);
+}
+
+static inline int iswxdigit_l(wint_t c, locale_t) {
+  return iswxdigit(c);
+}
+
+static inline int toupper_l(int c, locale_t) {
+  return toupper(c);
+}
+
+static inline int tolower_l(int c, locale_t) {
+  return tolower(c);
+}
+
+static inline int towupper_l(int c, locale_t) {
+  return towupper(c);
+}
+
+static inline int towlower_l(int c, locale_t) {
+  return towlower(c);
+}
+
+static inline int strcoll_l(const char *s1, const char *s2, locale_t) {
+  return strcoll(s1, s2);
+}
+
+static inline size_t strxfrm_l(char *dest, const char *src, size_t n,
+                               locale_t) {
+  return strxfrm(dest, src, n);
+}
+
+static inline size_t strftime_l(char *s, size_t max, const char *format,
+                                const struct tm *tm, locale_t) {
+  return strftime(s, max, format, tm);
+}
+
+static inline int wcscoll_l(const wchar_t *ws1, const wchar_t *ws2, locale_t) {
+  return wcscoll(ws1, ws2);
+}
+
+static inline size_t wcsxfrm_l(wchar_t *dest, const wchar_t *src, size_t n,
+                               locale_t) {
+  return wcsxfrm(dest, src, n);
+}
+
+static inline long double strtold_l(const char *nptr, char **endptr, locale_t) {
+  return strtold(nptr, endptr);
+}
+
+static inline long long strtoll_l(const char *nptr, char **endptr, int base,
+                                  locale_t) {
+  return strtoll(nptr, endptr, base);
+}
+
+static inline unsigned long long strtoull_l(const char *nptr, char **endptr,
+                                            int base, locale_t) {
+  return strtoull(nptr, endptr, base);
+}
+
+static inline long long wcstoll_l(const wchar_t *nptr, wchar_t **endptr,
+                                  int base, locale_t) {
+  return wcstoll(nptr, endptr, base);
+}
+
+static inline unsigned long long wcstoull_l(const wchar_t *nptr,
+                                            wchar_t **endptr, int base,
+                                            locale_t) {
+  return wcstoull(nptr, endptr, base);
+}
+
+static inline long double wcstold_l(const wchar_t *nptr, wchar_t **endptr,
+                                    locale_t) {
+  return wcstold(nptr, endptr);
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // _LIBCPP_SUPPORT_XLOCALE_XLOCALE_H
Index: src/locale.cpp
===================================================================
--- src/locale.cpp
+++ src/locale.cpp
@@ -765,7 +765,7 @@
 const ctype_base::mask ctype_base::blank;
 const ctype_base::mask ctype_base::alnum;
 const ctype_base::mask ctype_base::graph;
-    
+
 locale::id ctype<wchar_t>::id;
 
 ctype<wchar_t>::~ctype()
@@ -775,14 +775,14 @@
 bool
 ctype<wchar_t>::do_is(mask m, char_type c) const
 {
-    return isascii(c) ? (ctype<char>::classic_table()[c] & m) != 0 : false;
+    return __libcpp_isascii(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) ?
+        *vec = static_cast<mask>(__libcpp_isascii(*low) ?
                                    ctype<char>::classic_table()[*low] : 0);
     return low;
 }
@@ -791,7 +791,7 @@
 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))
+        if (__libcpp_isascii(*low) && (ctype<char>::classic_table()[*low] & m))
             break;
     return low;
 }
@@ -800,7 +800,7 @@
 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)))
+        if (!(__libcpp_isascii(*low) && (ctype<char>::classic_table()[*low] & m)))
             break;
     return low;
 }
@@ -809,11 +809,11 @@
 ctype<wchar_t>::do_toupper(char_type c) const
 {
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-    return isascii(c) ? _DefaultRuneLocale.__mapupper[c] : c;
+    return __libcpp_isascii(c) ? _DefaultRuneLocale.__mapupper[c] : c;
 #elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__NetBSD__)
-    return isascii(c) ? ctype<char>::__classic_upper_table()[c] : c;
+    return __libcpp_isascii(c) ? ctype<char>::__classic_upper_table()[c] : c;
 #else
-    return (isascii(c) && iswlower_l(c, __cloc())) ? c-L'a'+L'A' : c;
+    return (__libcpp_isascii(c) && iswlower_l(c, __cloc())) ? c-L'a'+L'A' : c;
 #endif
 }
 
@@ -822,12 +822,12 @@
 {
     for (; low != high; ++low)
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-        *low = isascii(*low) ? _DefaultRuneLocale.__mapupper[*low] : *low;
+        *low = __libcpp_isascii(*low) ? _DefaultRuneLocale.__mapupper[*low] : *low;
 #elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__NetBSD__)
-        *low = isascii(*low) ? ctype<char>::__classic_upper_table()[*low]
+        *low = __libcpp_isascii(*low) ? ctype<char>::__classic_upper_table()[*low]
                              : *low;
 #else
-        *low = (isascii(*low) && islower_l(*low, __cloc())) ? (*low-L'a'+L'A') : *low;
+        *low = (__libcpp_isascii(*low) && islower_l(*low, __cloc())) ? (*low-L'a'+L'A') : *low;
 #endif
     return low;
 }
@@ -836,11 +836,11 @@
 ctype<wchar_t>::do_tolower(char_type c) const
 {
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-    return isascii(c) ? _DefaultRuneLocale.__maplower[c] : c;
+    return __libcpp_isascii(c) ? _DefaultRuneLocale.__maplower[c] : c;
 #elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__NetBSD__)
-    return isascii(c) ? ctype<char>::__classic_lower_table()[c] : c;
+    return __libcpp_isascii(c) ? ctype<char>::__classic_lower_table()[c] : c;
 #else
-    return (isascii(c) && isupper_l(c, __cloc())) ? c-L'A'+'a' : c;
+    return (__libcpp_isascii(c) && isupper_l(c, __cloc())) ? c-L'A'+'a' : c;
 #endif
 }
 
@@ -849,12 +849,12 @@
 {
     for (; low != high; ++low)
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-        *low = isascii(*low) ? _DefaultRuneLocale.__maplower[*low] : *low;
+        *low = __libcpp_isascii(*low) ? _DefaultRuneLocale.__maplower[*low] : *low;
 #elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__NetBSD__)
-        *low = isascii(*low) ? ctype<char>::__classic_lower_table()[*low]
+        *low = __libcpp_isascii(*low) ? ctype<char>::__classic_lower_table()[*low]
                              : *low;
 #else
-        *low = (isascii(*low) && isupper_l(*low, __cloc())) ? *low-L'A'+L'a' : *low;
+        *low = (__libcpp_isascii(*low) && isupper_l(*low, __cloc())) ? *low-L'A'+L'a' : *low;
 #endif
     return low;
 }
@@ -876,7 +876,7 @@
 char
 ctype<wchar_t>::do_narrow(char_type c, char dfault) const
 {
-    if (isascii(c))
+    if (__libcpp_isascii(c))
         return static_cast<char>(c);
     return dfault;
 }
@@ -885,7 +885,7 @@
 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))
+        if (__libcpp_isascii(*low))
             *dest = static_cast<char>(*low);
         else
             *dest = dfault;
@@ -915,15 +915,15 @@
 ctype<char>::do_toupper(char_type c) const
 {
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-    return isascii(c) ?
+    return __libcpp_isascii(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)]);
 #elif defined(__GLIBC__) || defined(__EMSCRIPTEN__)
-    return isascii(c) ? 
+    return __libcpp_isascii(c) ? 
       static_cast<char>(__classic_upper_table()[static_cast<unsigned char>(c)]) : c;
 #else
-    return (isascii(c) && islower_l(c, __cloc())) ? c-'a'+'A' : c;
+    return (__libcpp_isascii(c) && islower_l(c, __cloc())) ? c-'a'+'A' : c;
 #endif
 }
 
@@ -932,15 +932,15 @@
 {
     for (; low != high; ++low)
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-        *low = isascii(*low) ?
+        *low = __libcpp_isascii(*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)]);
 #elif defined(__GLIBC__) || defined(__EMSCRIPTEN__)
-        *low = isascii(*low) ?
+        *low = __libcpp_isascii(*low) ?
           static_cast<char>(__classic_upper_table()[static_cast<size_t>(*low)]) : *low;
 #else
-        *low = (isascii(*low) && islower_l(*low, __cloc())) ? *low-'a'+'A' : *low;
+        *low = (__libcpp_isascii(*low) && islower_l(*low, __cloc())) ? *low-'a'+'A' : *low;
 #endif
     return low;
 }
@@ -949,15 +949,15 @@
 ctype<char>::do_tolower(char_type c) const
 {
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-    return isascii(c) ?
+    return __libcpp_isascii(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)]);
 #elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__NetBSD__)
-    return isascii(c) ?
+    return __libcpp_isascii(c) ?
       static_cast<char>(__classic_lower_table()[static_cast<size_t>(c)]) : c;
 #else
-    return (isascii(c) && isupper_l(c, __cloc())) ? c-'A'+'a' : c;
+    return (__libcpp_isascii(c) && isupper_l(c, __cloc())) ? c-'A'+'a' : c;
 #endif
 }
 
@@ -966,13 +966,13 @@
 {
     for (; low != high; ++low)
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
-        *low = isascii(*low) ? static_cast<char>(_DefaultRuneLocale.__maplower[static_cast<ptrdiff_t>(*low)]) : *low;
+        *low = __libcpp_isascii(*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)]);
 #elif defined(__GLIBC__) || defined(__EMSCRIPTEN__)
-        *low = isascii(*low) ? static_cast<char>(__classic_lower_table()[static_cast<size_t>(*low)]) : *low;
+        *low = __libcpp_isascii(*low) ? static_cast<char>(__classic_lower_table()[static_cast<size_t>(*low)]) : *low;
 #else
-        *low = (isascii(*low) && isupper_l(*low, __cloc())) ? *low-'A'+'a' : *low;
+        *low = (__libcpp_isascii(*low) && isupper_l(*low, __cloc())) ? *low-'A'+'a' : *low;
 #endif
     return low;
 }
@@ -994,7 +994,7 @@
 char
 ctype<char>::do_narrow(char_type c, char dfault) const
 {
-    if (isascii(c))
+    if (__libcpp_isascii(c))
         return static_cast<char>(c);
     return dfault;
 }
@@ -1003,7 +1003,7 @@
 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))
+        if (__libcpp_isascii(*low))
             *dest = *low;
         else
             *dest = dfault;
@@ -1199,7 +1199,7 @@
 {
     for (; low != high; ++low, ++vec)
     {
-        if (isascii(*low))
+        if (__libcpp_isascii(*low))
             *vec = static_cast<mask>(ctype<char>::classic_table()[*low]);
         else
         {