commit c5bbc9258a7182e14eb731e5251842bc417b5822
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Mar 10 20:12:09 2017 +0000

    PR libstdc++/79511 fix endianness of UTF-16 data
    
    	PR libstdc++/79511
    	* src/c++11/codecvt.cc (write_utf16_code_point): Don't write 0xffff
    	as a surrogate pair.
    	(__codecvt_utf8_utf16_base<char32_t>::do_in): Use native endianness
    	for internal representation.
    	(__codecvt_utf8_utf16_base<wchar_t>::do_in): Likewise.
    	* testsuite/22_locale/codecvt/codecvt_utf8_utf16/79511.cc: New test.

diff --git a/libstdc++-v3/src/c++11/codecvt.cc b/libstdc++-v3/src/c++11/codecvt.cc
index 12a4d4f..9b63e2b 100644
--- a/libstdc++-v3/src/c++11/codecvt.cc
+++ b/libstdc++-v3/src/c++11/codecvt.cc
@@ -315,7 +315,7 @@ namespace
   {
     static_assert(sizeof(C) >= 2, "a code unit must be at least 16-bit");
 
-    if (codepoint < max_single_utf16_unit)
+    if (codepoint <= max_single_utf16_unit)
       {
 	if (to.size() > 0)
 	  {
@@ -1341,7 +1341,11 @@ do_in(state_type&, const extern_type* __from, const extern_type* __from_end,
 {
   range<const char> from{ __from, __from_end };
   range<char32_t> to{ __to, __to_end };
-  auto res = utf16_in(from, to, _M_maxcode, _M_mode);
+  codecvt_mode mode = codecvt_mode(_M_mode & (consume_header|generate_header));
+#if __BYTE_ORDER__ != __ORDER_BIG_ENDIAN__
+  mode = codecvt_mode(mode | little_endian);
+#endif
+  auto res = utf16_in(from, to, _M_maxcode, mode);
   __from_next = from.next;
   __to_next = to.next;
   return res;
@@ -1411,7 +1415,11 @@ do_in(state_type&, const extern_type* __from, const extern_type* __from_end,
 {
   range<const char> from{ __from, __from_end };
   range<wchar_t> to{ __to, __to_end };
-  auto res = utf16_in(from, to, _M_maxcode, _M_mode);
+  codecvt_mode mode = codecvt_mode(_M_mode & (consume_header|generate_header));
+#if __BYTE_ORDER__ != __ORDER_BIG_ENDIAN__
+  mode = codecvt_mode(mode | little_endian);
+#endif
+  auto res = utf16_in(from, to, _M_maxcode, mode);
   __from_next = from.next;
   __to_next = to.next;
   return res;
diff --git a/libstdc++-v3/testsuite/22_locale/codecvt/codecvt_utf8_utf16/79511.cc b/libstdc++-v3/testsuite/22_locale/codecvt/codecvt_utf8_utf16/79511.cc
new file mode 100644
index 0000000..5555bcb
--- /dev/null
+++ b/libstdc++-v3/testsuite/22_locale/codecvt/codecvt_utf8_utf16/79511.cc
@@ -0,0 +1,60 @@
+// Copyright (C) 2017 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-do run { target c++11 } }
+
+#include <locale>
+#include <codecvt>
+#include <testsuite_hooks.h>
+
+// PR libstdc++/79511
+
+template<typename ElemT>
+  std::basic_string<ElemT> conv(const char* src)
+  {
+    std::wstring_convert<std::codecvt_utf8_utf16<ElemT>, ElemT> conv;
+    return conv.from_bytes(src);
+  }
+
+void
+test01()
+{
+  static char const src[] = "\xEF\xBF\xBF";
+  VERIFY( conv<char16_t>(src) == u"\xffff" );
+  VERIFY( conv<char32_t>(src) == U"\xffff" );
+#ifdef _GLIBCXX_USE_WCHAR_T
+  VERIFY( conv<wchar_t>(src) == L"\xffff" );
+#endif
+}
+
+void
+test02()
+{
+  static char const src[] = "\xE2\x82\xAC";
+  VERIFY( conv<char16_t>(src) == u"\x20ac" );
+  VERIFY( conv<char32_t>(src) == U"\x20ac" );
+#ifdef _GLIBCXX_USE_WCHAR_T
+  VERIFY( conv<wchar_t>(src) == L"\x20ac" );
+#endif
+}
+
+int
+main()
+{
+  test01();
+  test02();
+}