Index: clang/include/clang/Basic/DiagnosticGroups.td
===================================================================
--- clang/include/clang/Basic/DiagnosticGroups.td
+++ clang/include/clang/Basic/DiagnosticGroups.td
@@ -1133,6 +1133,12 @@
 // A warning group for warnings about code that may be incompatible on AIX.
 def AIXCompat : DiagGroup<"aix-compat">;
 
+// Warnings for Plan 9 extensions.
+def Plan9AnonTag : DiagGroup<"plan9-anon-tag">;
+
+// Warnings group for warnings about Plan 9 extensions.
+def Plan9 : DiagGroup<"plan9", [Plan9AnonTag]>;
+
 // Warnings for Microsoft extensions.
 def MicrosoftCharize : DiagGroup<"microsoft-charize">;
 def MicrosoftDrectveSection : DiagGroup<"microsoft-drectve-section">;
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -9011,6 +9011,9 @@
 def ext_ms_anonymous_record : ExtWarn<
   "anonymous %select{structs|unions}0 are a Microsoft extension">,
   InGroup<MicrosoftAnonTag>;
+def ext_plan9_anonymous_record : ExtWarn<
+  "anonymous %select{structs|unions}0 are a Plan 9 extension">,
+  InGroup<Plan9AnonTag>;
 
 // C++ local classes
 def err_reference_to_local_in_enclosing_context : Error<
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -4989,8 +4989,10 @@
       else if (const RecordType *UT = DS.getRepAsType().get()->getAsUnionType())
         Record = UT->getDecl();
 
-      if (Record && getLangOpts().MicrosoftExt) {
-        Diag(DS.getBeginLoc(), diag::ext_ms_anonymous_record)
+      if (Record && (getLangOpts().MicrosoftExt || getLangOpts().Plan9)) {
+        Diag(DS.getBeginLoc(), getLangOpts().MicrosoftExt
+                                   ? diag::ext_ms_anonymous_record
+                                   : diag::ext_plan9_anonymous_record)
             << Record->isUnion() << DS.getSourceRange();
         return BuildMicrosoftCAnonymousStruct(S, DS, Record);
       }
Index: clang/test/Sema/2c-anon-record.c
===================================================================
--- /dev/null
+++ clang/test/Sema/2c-anon-record.c
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 %s -fsyntax-only -Wplan9 -verify -fplan9-extensions
+
+struct A {
+  int x;
+};
+
+struct B {
+  struct A; // expected-warning {{anonymous structs are a Plan 9 extension}}
+};