Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -567,6 +567,15 @@
 
 #endif // __clang__ || __GNUC__ || _MSC_VER || __IBMCPP__
 
+#define _LIBCPP_BEGIN_NAMESPACE_INTERNAL namespace __libcpp_internal _LIBCPP_HIDDEN {
+#define _LIBCPP_END_NAMESPACE_INTERNAL }
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+  _LIBCPP_BEGIN_NAMESPACE_INTERNAL
+  _LIBCPP_END_NAMESPACE_INTERNAL
+  using namespace __libcpp_internal;
+_LIBCPP_END_NAMESPACE_STD
+
 #ifdef _LIBCPP_HAS_NO_UNICODE_CHARS
 typedef unsigned short char16_t;
 typedef unsigned int   char32_t;
Index: include/locale
===================================================================
--- include/locale
+++ include/locale
@@ -368,6 +368,8 @@
 #endif
 }
 
+_LIBCPP_BEGIN_NAMESPACE_INTERNAL
+
 inline
 int __snprintf_l(char *__s, size_t __n, locale_t __l, const char *__format, ...) {
   va_list __va;
@@ -410,6 +412,8 @@
   return __res;
 }
 
+_LIBCPP_END_NAMESPACE_INTERNAL
+
 #endif  // __linux__
 
 // __scan_keyword
Index: test/libcxx/symbols/inline_namespace.sh.cpp
===================================================================
--- /dev/null
+++ test/libcxx/symbols/inline_namespace.sh.cpp
@@ -0,0 +1,16 @@
+// Test that symbols withing the __libcpp_internal namespace are given hidden
+// visibility and do not appear in the dylib.
+
+// Check that these example symbols do not appear.
+// RUN: %env not sym_match.py %libcxx \
+// RUN:  [_]+ZNSt3110sscanf_lEPKcP15__locale_structS1_z \
+// RUN:  [_]+ZNSt3__117__libcpp_internal10__sscanf_lEPKcP15__locale_structS2_z \
+// RUN:  [_]+ZNSt3112asprintf_lEPPcP15__locale_structPKcz \
+// RUN:  [_]+ZNSt3__117__libcpp_internal12__asprintf_lEPPcP15__locale_structPKcz \
+// RUN:  [_]+ZNSt3112snprintf_lEPcmP15__locale_structPKcz \
+// RUN:  [_]+ZNSt3__117__libcpp_internal12__snprintf_lEPcmP15__locale_structPKcz
+
+// Check that no symbols in the __libcpp_internal namespace have been emitted
+// into the dylib.
+// RUN: %env not sym_match.py %libcxx .*__libcpp_internal
+
Index: test/libcxx/test/config.py
===================================================================
--- test/libcxx/test/config.py
+++ test/libcxx/test/config.py
@@ -631,6 +631,21 @@
         sub = self.config.substitutions
         # Configure compiler substitions
         sub.append(('%cxx', self.cxx.path))
+        # Add %libcxx as a substition for the full path to the created libc++
+        # library.
+        libcxx_root = self.cxx_library_root
+        assert libcxx_root
+        libcxx_lib = os.path.join(libcxx_root, 'libc++.a')
+        if os.path.isfile(libcxx_lib):
+            pass # Use libc++.a as libcxx_lib.
+        elif self.target_info.platform() == 'darwin':
+            libcxx_lib = os.path.join(libcxx_root, 'libc++.dylib')
+        else:
+            libcxx_lib = os.path.join(libcxx_root, 'libc++.so')
+        if not os.path.isfile(libcxx_lib):
+            self.lit_config.error("Failed to find libc++ library in %s" %
+                                  libcxx_root)
+        sub.append(('%libcxx', str(libcxx_lib)))
         # Configure flags substitutions
         flags_str = ' '.join(self.cxx.flags)
         compile_flags_str = ' '.join(self.cxx.compile_flags)
@@ -650,10 +665,22 @@
         sub.append(('%compile', compile_str))
         sub.append(('%link', link_str))
         sub.append(('%build', build_str))
+        # Build the PATH variable from sys.path and self.lit_config.path
+        # and self.env['PATH'].  Also add libcxx/utils/sym_check to the path.
+        path_list = []
+        if 'PATH' in os.environ:
+            path_list += os.environ['PATH'].split(':')
+        path_list += self.lit_config.path
+        if 'PATH' in self.env:
+            path_list += self.env['PATH'].split(':')
+        path_list += [os.path.join(self.libcxx_src_root, 'utils', 'sym_check')]
+        env_cpy = dict(self.env)
+        env_cpy['PATH'] = ':'.join(path_list)
         # Configure exec prefix substitutions.
-        exec_env_str = 'env ' if len(self.env) != 0 else ''
-        for k, v in self.env.items():
+        exec_env_str = 'env ' if len(env_cpy) != 0 else ''
+        for k, v in env_cpy.items():
             exec_env_str += ' %s=%s' % (k, v)
+        sub.append(('%env', exec_env_str))
         # Configure run env substitution.
         exec_str = ''
         if self.lit_config.useValgrind:
Index: utils/sym_check/sym_match.py
===================================================================
--- utils/sym_check/sym_match.py
+++ utils/sym_check/sym_match.py
@@ -36,6 +36,9 @@
     sys.stdout.write(report)
     if matching_count != 0:
         print('%d matching symbols found...' % matching_count)
+        sys.exit(0)
+    else:
+        sys.exit(1)
 
 
 if __name__ == '__main__':