diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/clang/include/clang/Basic/DiagnosticParseKinds.td
index 95522189e190..1f03a0777cf5 100644
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -56,6 +56,8 @@ def ext_c99_variable_decl_in_for_loop : Extension<
   "variable declaration in for loop is a C99-specific feature">, InGroup<C99>;
 def ext_c99_compound_literal : Extension<
   "compound literals are a C99-specific feature">, InGroup<C99>;
+def ext_c99_flexible_array_member : Extension<
+  "Flexible array members are a C99-specific feature">, InGroup<C99>;
 def ext_enumerator_list_comma : Extension<
   "commas at the end of enumerator lists are a %select{C99|C++11}0-specific "
   "feature">;
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index ab1a1b40251c..574c18c2e39d 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -9483,6 +9483,13 @@ void Sema::ActOnFields(Scope* S,
         else if (Fields.size() == 1) 
           Diag(FD->getLocation(), diag::ext_flexible_array_empty_aggregate_gnu)
             << FD->getDeclName() << Record->getTagKind();
+      } else if (!getLangOpts().C99) {
+      if (Record->isUnion())
+        Diag(FD->getLocation(), diag::ext_flexible_array_union_gnu)
+          << FD->getDeclName();
+      else
+        Diag(FD->getLocation(), diag::ext_c99_flexible_array_member)
+          << FD->getDeclName() << Record->getTagKind();
       } else if (NumNamedMembers < 1) {
         Diag(FD->getLocation(), diag::err_flexible_array_empty_struct)
           << FD->getDeclName();
diff --git a/clang/test/Sema/c89.c b/clang/test/Sema/c89.c
index ac8c6e0b7b74..25a10481e923 100644
--- a/clang/test/Sema/c89.c
+++ b/clang/test/Sema/c89.c
@@ -90,4 +90,6 @@ void test16() {
   printg("Hello, world!\n"); /* expected-warning {{implicit declaration of function 'printg'}} */
 }
 
+struct x { int x,y[]; }; /* expected-warning {{Flexible array members are a C99-specific feature}} */
+
 void main() {} /* expected-error {{'main' must return 'int'}} */