Index: include/clang/Basic/LangOptions.def
===================================================================
--- include/clang/Basic/LangOptions.def
+++ include/clang/Basic/LangOptions.def
@@ -47,6 +47,7 @@
 LANGOPT(MicrosoftExt      , 1, 0, "Microsoft C++ extensions")
 LANGOPT(AsmBlocks         , 1, 0, "Microsoft inline asm blocks")
 LANGOPT(Borland           , 1, 0, "Borland extensions")
+LANGOPT(Plan9Ext          , 1, 0, "Plan 9 extensions")
 LANGOPT(CPlusPlus         , 1, 0, "C++")
 LANGOPT(CPlusPlus11       , 1, 0, "C++11")
 LANGOPT(CPlusPlus1y       , 1, 0, "C++1y")
Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -775,6 +775,8 @@
 def fno_pic : Flag<["-"], "fno-pic">, Group<f_Group>;
 def fpie : Flag<["-"], "fpie">, Group<f_Group>;
 def fno_pie : Flag<["-"], "fno-pie">, Group<f_Group>;
+def fplan9_extensions : Flag<["-"], "fplan9-extensions">, Group<f_Group>, Flags<[CC1Option]>;
+def fno_plan9_extensions : Flag<["-"], "fno-plan9-extensions">, Group<f_Group>;
 def fprofile_arcs : Flag<["-"], "fprofile-arcs">, Group<f_Group>;
 def fprofile_generate : Flag<["-"], "fprofile-generate">, Group<f_Group>;
 def framework : Separate<["-"], "framework">, Flags<[LinkerInput]>;
Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -3678,6 +3678,11 @@
                    options::OPT_fno_borland_extensions, false))
     CmdArgs.push_back("-fborland-extensions");
 
+  // -fno-plan9-extensions is default.
+  if (Args.hasFlag(options::OPT_fplan9_extensions,
+                   options::OPT_fno_plan9_extensions, false))
+    CmdArgs.push_back("-fplan9-extensions");
+
   // -fno-delayed-template-parsing is default, except for Windows where MSVC STL
   // needs it.
   if (Args.hasFlag(options::OPT_fdelayed_template_parsing,
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -1351,6 +1351,7 @@
   Opts.MSCVersion = getLastArgIntValue(Args, OPT_fmsc_version, 0, Diags);
   Opts.VtorDispMode = getLastArgIntValue(Args, OPT_vtordisp_mode_EQ, 1, Diags);
   Opts.Borland = Args.hasArg(OPT_fborland_extensions);
+  Opts.Plan9Ext = Args.hasArg(OPT_fplan9_extensions);
   Opts.WritableStrings = Args.hasArg(OPT_fwritable_strings);
   Opts.ConstStrings = Args.hasFlag(OPT_fconst_strings, OPT_fno_const_strings,
                                    Opts.ConstStrings);
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -1769,8 +1769,8 @@
     return New->setInvalidDecl();
   }
 
-  // Modules always permit redefinition of typedefs, as does C11.
-  if (getLangOpts().Modules || getLangOpts().C11)
+  // Modules always permit redefinition of typedefs, as does C11 and Plan 9.
+  if (getLangOpts().Modules || getLangOpts().C11 || getLangOpts().Plan9Ext)
     return;
   
   // If we have a redefinition of a typedef in C, emit a warning.  This warning
Index: test/Sema/c11-typedef-redef.c
===================================================================
--- test/Sema/c11-typedef-redef.c
+++ test/Sema/c11-typedef-redef.c
@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -std=c11 %s -verify
+// RUN: %clang_cc1 -fplan9-extensions %s -verify
 
 typedef int type;
 typedef type type;