Index: clang/include/clang/Basic/LangOptions.def
===================================================================
--- clang/include/clang/Basic/LangOptions.def
+++ clang/include/clang/Basic/LangOptions.def
@@ -92,6 +92,7 @@
 LANGOPT(MicrosoftExt      , 1, 0, "Microsoft C++ extensions")
 LANGOPT(AsmBlocks         , 1, 0, "Microsoft inline asm blocks")
 LANGOPT(Borland           , 1, 0, "Borland extensions")
+LANGOPT(Plan9             , 1, 0, "Plan 9 extensions")
 LANGOPT(CPlusPlus         , 1, 0, "C++")
 LANGOPT(CPlusPlus11       , 1, 0, "C++11")
 LANGOPT(CPlusPlus14       , 1, 0, "C++14")
Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -1343,6 +1343,10 @@
   LangOpts<"Borland">, DefaultFalse,
   PosFlag<SetTrue, [CC1Option], "Accept non-standard constructs supported by the Borland compiler">,
   NegFlag<SetFalse>>;
+defm plan9_extensions : BoolFOption<"plan9-extensions",
+  LangOpts<"Plan9">, DefaultFalse,
+  PosFlag<SetTrue, [CC1Option], "Accept non-standard constructs supported by the Plan 9 compiler">,
+  NegFlag<SetFalse>>;
 def fbuiltin : Flag<["-"], "fbuiltin">, Group<f_Group>, Flags<[CoreOption]>;
 def fbuiltin_module_map : Flag <["-"], "fbuiltin-module-map">, Group<f_Group>,
   Flags<[NoXarchOption]>, HelpText<"Load the clang builtins module map file.">;
Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -6495,6 +6495,9 @@
   Args.addOptInFlag(CmdArgs, options::OPT_fborland_extensions,
                     options::OPT_fno_borland_extensions);
 
+  Args.addOptInFlag(CmdArgs, options::OPT_fplan9_extensions,
+                    options::OPT_fno_plan9_extensions);
+
   // -fno-declspec is default, except for PS4/PS5.
   if (Args.hasFlag(options::OPT_fdeclspec, options::OPT_fno_declspec,
                    RawTriple.isPS()))
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -2560,8 +2560,8 @@
     return New->setInvalidDecl();
   }
 
-  // Modules always permit redefinition of typedefs, as does C11.
-  if (getLangOpts().Modules || getLangOpts().C11)
+  // Modules always permit redefinition of typedefs, as do C11 and Plan 9.
+  if (getLangOpts().Modules || getLangOpts().C11 || getLangOpts().Plan9)
     return;
 
   // If we have a redefinition of a typedef in C, emit a warning.  This warning
Index: clang/test/Sema/c11-typedef-redef.c
===================================================================
--- clang/test/Sema/c11-typedef-redef.c
+++ clang/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;