diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index 5c3f115..67f43d2 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/include/clang/Basic/DiagnosticSemaKinds.td
@@ -192,6 +192,9 @@ def warn_return_value_size: Warning<
 def warn_return_value_udt: Warning<
   "%0 has C-linkage specified, but returns user-defined type %1 which is "
   "incompatible with C">, InGroup<ReturnTypeCLinkage>;
+def warn_return_value_udt_incomplete: Warning<
+  "%0 has C-linkage specified, but returns incomplete type %1 which could be "
+  "incompatible with C">, InGroup<ReturnTypeCLinkage>;
 def warn_implicit_function_decl : Warning<
   "implicit declaration of function %0">,
   InGroup<ImplicitFunctionDeclare>, DefaultIgnore;
diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index 52aa5b7..12444e5 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -6086,10 +6086,11 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,
     // compatible, and if it does, warn the user.
     if (NewFD->isExternC()) {
       QualType R = NewFD->getResultType();
-      if (!R.isPODType(Context) && 
-          !R->isVoidType())
-        Diag( NewFD->getLocation(), diag::warn_return_value_udt ) 
-          << NewFD << R;
+      if (R->isIncompleteType() && !R->isVoidType())
+        Diag(NewFD->getLocation(), diag::warn_return_value_udt_incomplete)
+            << NewFD << R;
+      else if (!R.isPODType(Context) && !R->isVoidType())
+        Diag(NewFD->getLocation(), diag::warn_return_value_udt) << NewFD << R;
     }
   }
   return Redeclaration;
diff --git a/test/SemaCXX/function-extern-c.cpp b/test/SemaCXX/function-extern-c.cpp
index f20cd38..16dbbb2 100644
--- a/test/SemaCXX/function-extern-c.cpp
+++ b/test/SemaCXX/function-extern-c.cpp
@@ -36,3 +36,5 @@ extern "C" void f7( U u );
 extern "C" double f8(void);
 extern "C" long long f11( void );
 extern "C" A *f10( void );
+
+extern "C" struct mypodstruct f12(); // expected-warning {{'f12' has C-linkage specified, but returns incomplete type 'struct mypodstruct' which could be incompatible with C}}