Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -3380,7 +3380,8 @@
     "candidate address cannot be taken because parameter %0 has "
     "pass_object_size attribute">;
 def err_diagnose_if_succeeded : Error<"%0">;
-def warn_diagnose_if_succeeded : Warning<"%0">, InGroup<UserDefinedWarnings>;
+def warn_diagnose_if_succeeded : Warning<"%0">, InGroup<UserDefinedWarnings>,
+    ShowInSystemHeader;
 def note_ovl_candidate_disabled_by_function_cond_attr : Note<
     "candidate disabled: %0">;
 def note_ovl_candidate_disabled_by_extension : Note<
Index: test/Sema/Inputs/diagnose-if-warn-system-header.h
===================================================================
--- /dev/null
+++ test/Sema/Inputs/diagnose-if-warn-system-header.h
@@ -0,0 +1,11 @@
+#pragma GCC system_header
+
+inline int system_header_func(int x)
+  __attribute__((diagnose_if(x == x, "system header warning", "warning"))) // expected-note {{from 'diagnose_if' attribute}}
+{
+  return 0;
+}
+
+void test_system_header() {
+  system_header_func(0); // expected-warning {{system header warning}}
+}
Index: test/Sema/diagnose_if.c
===================================================================
--- test/Sema/diagnose_if.c
+++ test/Sema/diagnose_if.c
@@ -150,3 +150,6 @@
 void runAlwaysWarnWithArg(int a) {
   alwaysWarnWithArg(a); // expected-warning{{alwaysWarn}}
 }
+
+// Test that diagnose_if warnings generated in system headers are not ignored.
+#include "Inputs/diagnose-if-warn-system-header.h"