Cherry-pick upstream r252457

include/locale

 // -*- C++ -*-
 //===-------------------------- locale ------------------------------------===//
 //
 //                     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.
 //
 //===----------------------------------------------------------------------===//
 
@@ skipping 217 matching lines @@
 typedef _VSTD::remove_pointer<locale_t>::type __locale_struct;
 typedef _VSTD::unique_ptr<__locale_struct, decltype(&freelocale)> __locale_unique_ptr;
 #ifndef _LIBCPP_LOCALE__L_EXTENSIONS
 typedef _VSTD::unique_ptr<__locale_struct, decltype(&uselocale)> __locale_raii;
 #endif
 
 // 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) || \
-    defined(_NEWLIB_VERSION)
+    defined(_NEWLIB_VERSION) || defined(_LIBCPP_HAS_MUSL_LIBC)
 
 #ifdef _LIBCPP_LOCALE__L_EXTENSIONS
 decltype(MB_CUR_MAX_L(_VSTD::declval<locale_t>()))
 inline _LIBCPP_INLINE_VISIBILITY
 __mb_cur_max_l(locale_t __l)
 {
   return MB_CUR_MAX_L(__l);
 }
 #else  // _LIBCPP_LOCALE__L_EXTENSIONS
 inline _LIBCPP_ALWAYS_INLINE
@@ skipping 4215 matching lines @@
         __rt = this;
         if ((__cm_ & ios_base::out) && sync())
             __rt = 0;
     }
     return __rt;
 }
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_LOCALE