Index: libcxx/trunk/CMakeLists.txt
===================================================================
--- libcxx/trunk/CMakeLists.txt
+++ libcxx/trunk/CMakeLists.txt
@@ -121,6 +121,7 @@
 option(LIBCXX_ENABLE_MONOTONIC_CLOCK
   "Build libc++ with support for a monotonic clock.
    This option may only be set to OFF when LIBCXX_ENABLE_THREADS=OFF." ON)
+option(LIBCXX_HAS_MUSL_LIBC "Build libc++ with support for the Musl C library" OFF)
 
 # Misc options ----------------------------------------------------------------
 # FIXME: Turn -pedantic back ON. It is currently off because it warns
@@ -200,6 +201,11 @@
         LIBCXX_ENABLE_ABI_LINKER_SCRIPT")
 endif()
 
+if (LIBCXX_HAS_MUSL_LIBC AND NOT LIBCXX_INSTALL_SUPPORT_HEADERS)
+  message(FATAL_ERROR "LIBCXX_INSTALL_SUPPORT_HEADERS can not be turned off"
+                      "when building for Musl with LIBCXX_HAS_MUSL_LIBC.")
+endif()
+
 #===============================================================================
 # Configure System
 #===============================================================================
@@ -364,6 +370,8 @@
 config_define_if_not(LIBCXX_ENABLE_MONOTONIC_CLOCK _LIBCPP_HAS_NO_MONOTONIC_CLOCK)
 config_define_if_not(LIBCXX_ENABLE_THREAD_UNSAFE_C_FUNCTIONS _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS)
 
+config_define_if(LIBCXX_HAS_MUSL_LIBC _LIBCPP_HAS_MUSL_LIBC)
+
 if (LIBCXX_NEEDS_SITE_CONFIG)
   configure_file(
     include/__config_site.in
Index: libcxx/trunk/include/__config
===================================================================
--- libcxx/trunk/include/__config
+++ libcxx/trunk/include/__config
@@ -386,14 +386,19 @@
 #elif defined(__ANDROID__)
 #define _LIBCPP_HAS_QUICK_EXIT
 #elif defined(__linux__)
-#include <features.h>
+#if !defined(_LIBCPP_HAS_MUSL_LIBC)
+# include <features.h>
 #if __GLIBC_PREREQ(2, 15)
 #define _LIBCPP_HAS_QUICK_EXIT
 #endif
 #if __GLIBC_PREREQ(2, 17)
 #define _LIBCPP_HAS_C11_FEATURES
 #endif
+#else // defined(_LIBCPP_HAS_MUSL_LIBC)
+#define _LIBCPP_HAS_QUICK_EXIT
+#define _LIBCPP_HAS_C11_FEATURES
 #endif
+#endif // __linux__
 #endif
 
 #if (__has_feature(cxx_noexcept))
Index: libcxx/trunk/include/__config_site.in
===================================================================
--- libcxx/trunk/include/__config_site.in
+++ libcxx/trunk/include/__config_site.in
@@ -18,5 +18,6 @@
 #cmakedefine _LIBCPP_HAS_NO_THREADS
 #cmakedefine _LIBCPP_HAS_NO_MONOTONIC_CLOCK
 #cmakedefine _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
+#cmakedefine _LIBCPP_HAS_MUSL_LIBC
 
-#endif
+#endif // _LIBCPP_CONFIG_SITE
Index: libcxx/trunk/include/__locale
===================================================================
--- libcxx/trunk/include/__locale
+++ libcxx/trunk/include/__locale
@@ -37,6 +37,8 @@
 #elif (defined(__GLIBC__) || defined(__APPLE__)      || defined(__FreeBSD__) \
     || defined(__EMSCRIPTEN__) || defined(__IBMCPP__))
 # include <xlocale.h>
+#elif defined(_LIBCPP_HAS_MUSL_LIBC)
+# include <support/musl/xlocale.h>
 #endif // __GLIBC__ || __APPLE__ || __FreeBSD__ || __sun__ || __EMSCRIPTEN__ || __IBMCPP__
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -329,7 +331,7 @@
 class _LIBCPP_TYPE_VIS ctype_base
 {
 public:
-#ifdef __GLIBC__
+#if defined(__GLIBC__)
     typedef unsigned short mask;
     static const mask space  = _ISspace;
     static const mask print  = _ISprint;
@@ -359,7 +361,7 @@
     typedef __uint32_t mask;
 # elif defined(__FreeBSD__)
     typedef unsigned long mask;
-# elif defined(__EMSCRIPTEN__) ||  defined(__NetBSD__)
+# elif defined(__EMSCRIPTEN__) || defined(__NetBSD__) || defined(_LIBCPP_HAS_MUSL_LIBC)
     typedef unsigned short mask;
 # endif
     static const mask space  = _CTYPE_S;
@@ -406,7 +408,11 @@
 # define _LIBCPP_CTYPE_MASK_IS_COMPOSITE_ALPHA
 # define _LIBCPP_CTYPE_MASK_IS_COMPOSITE_XDIGIT
 #else
+#if defined(_LIBCPP_HAS_MUSL_LIBC)
+    typedef unsigned short mask;
+#else
     typedef unsigned long mask;
+#endif
     static const mask space  = 1<<0;
     static const mask print  = 1<<1;
     static const mask cntrl  = 1<<2;
@@ -628,7 +634,7 @@
 #endif
     _LIBCPP_ALWAYS_INLINE const mask* table() const  _NOEXCEPT {return __tab_;}
     static const mask* classic_table()  _NOEXCEPT;
-#if defined(__GLIBC__) || defined(__EMSCRIPTEN__)
+#if defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(_LIBCPP_HAS_MUSL_LIBC)
     static const int* __classic_upper_table() _NOEXCEPT;
     static const int* __classic_lower_table() _NOEXCEPT;
 #endif
Index: libcxx/trunk/include/support/musl/xlocale.h
===================================================================
--- libcxx/trunk/include/support/musl/xlocale.h
+++ libcxx/trunk/include/support/musl/xlocale.h
@@ -0,0 +1,58 @@
+// -*- C++ -*-
+//===------------------- support/musl/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 adds support for the extended locale functions that are currently
+// missing from the Musl C library.
+//
+// This 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 Musl.
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_SUPPORT_MUSL_XLOCALE_H
+#define _LIBCPP_SUPPORT_MUSL_XLOCALE_H
+
+#include <cstdlib>
+#include <cwchar>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+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_MUSL_XLOCALE_H
Index: libcxx/trunk/src/locale.cpp
===================================================================
--- libcxx/trunk/src/locale.cpp
+++ libcxx/trunk/src/locale.cpp
@@ -812,7 +812,8 @@
 {
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
     return isascii(c) ? _DefaultRuneLocale.__mapupper[c] : c;
-#elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__NetBSD__)
+#elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || \
+      defined(__NetBSD__) || defined(_LIBCPP_HAS_MUSL_LIBC)
     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;
@@ -825,7 +826,8 @@
     for (; low != high; ++low)
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
         *low = isascii(*low) ? _DefaultRuneLocale.__mapupper[*low] : *low;
-#elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__NetBSD__)
+#elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || \
+      defined(__NetBSD__) || defined(_LIBCPP_HAS_MUSL_LIBC)
         *low = isascii(*low) ? ctype<char>::__classic_upper_table()[*low]
                              : *low;
 #else
@@ -839,7 +841,8 @@
 {
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
     return isascii(c) ? _DefaultRuneLocale.__maplower[c] : c;
-#elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__NetBSD__)
+#elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || \
+      defined(__NetBSD__) || defined(_LIBCPP_HAS_MUSL_LIBC)
     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;
@@ -852,7 +855,8 @@
     for (; low != high; ++low)
 #ifdef _LIBCPP_HAS_DEFAULTRUNELOCALE
         *low = isascii(*low) ? _DefaultRuneLocale.__maplower[*low] : *low;
-#elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(__NetBSD__)
+#elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || \
+      defined(__NetBSD__) || defined(_LIBCPP_HAS_MUSL_LIBC)
         *low = isascii(*low) ? ctype<char>::__classic_lower_table()[*low]
                              : *low;
 #else
@@ -921,8 +925,8 @@
       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) ? 
+#elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(_LIBCPP_HAS_MUSL_LIBC)
+    return isascii(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;
@@ -938,7 +942,7 @@
           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__)
+#elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(_LIBCPP_HAS_MUSL_LIBC)
         *low = isascii(*low) ?
           static_cast<char>(__classic_upper_table()[static_cast<size_t>(*low)]) : *low;
 #else
@@ -955,7 +959,7 @@
       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__)
+#elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(_LIBCPP_HAS_MUSL_LIBC)
     return isascii(c) ?
       static_cast<char>(__classic_lower_table()[static_cast<size_t>(c)]) : c;
 #else
@@ -971,7 +975,7 @@
         *low = 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__)
+#elif defined(__GLIBC__) || defined(__EMSCRIPTEN__) || defined(_LIBCPP_HAS_MUSL_LIBC)
         *low = isascii(*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;
@@ -1012,7 +1016,7 @@
     return low;
 }
 
-#ifdef __EMSCRIPTEN__
+#if defined(__EMSCRIPTEN__) || defined(_LIBCPP_HAS_MUSL_LIBC)
 extern "C" const unsigned short ** __ctype_b_loc();
 extern "C" const int ** __ctype_tolower_loc();
 extern "C" const int ** __ctype_toupper_loc();
@@ -1114,7 +1118,7 @@
     return _ctype+1; // internal ctype mask table defined in msvcrt.dll
 // This is assumed to be safe, which is a nonsense assumption because we're
 // going to end up dereferencing it later...
-#elif defined(__EMSCRIPTEN__)
+#elif defined(__EMSCRIPTEN__) || defined(_LIBCPP_HAS_MUSL_LIBC)
     return *__ctype_b_loc();
 #elif defined(_NEWLIB_VERSION)
     // Newlib has a 257-entry table in ctype_.c, where (char)0 starts at [1].
@@ -1157,7 +1161,7 @@
     return _C_toupper_tab_ + 1;
 }
 
-#elif defined(__EMSCRIPTEN__)
+#elif defined(__EMSCRIPTEN__) || defined(_LIBCPP_HAS_MUSL_LIBC)
 const int*
 ctype<char>::__classic_lower_table() _NOEXCEPT
 {
@@ -1169,7 +1173,7 @@
 {
     return *__ctype_toupper_loc();
 }
-#endif // __GLIBC__ || __EMSCRIPTEN__ || __NETBSD__
+#endif // __GLIBC__ || __NETBSD__ || __EMSCRIPTEN__ || defined(_LIBCPP_HAS_MUSL_LIBC)
 
 // template <> class ctype_byname<char>