Index: lib/Serialization/ASTReader.cpp
===================================================================
--- lib/Serialization/ASTReader.cpp
+++ lib/Serialization/ASTReader.cpp
@@ -1945,15 +1945,13 @@
   Module *PrevOwner = nullptr;
   if (SubmoduleID PrevModID = PrevMI->getOwningModuleID())
     PrevOwner = Reader.getSubmodule(PrevModID);
-  SourceManager &SrcMgr = Reader.getSourceManager();
-  bool PrevInSystem
-    = PrevOwner? PrevOwner->IsSystem
-               : SrcMgr.isInSystemHeader(PrevMI->getDefinitionLoc());
-  bool NewInSystem
-    = NewOwner? NewOwner->IsSystem
-              : SrcMgr.isInSystemHeader(NewMI->getDefinitionLoc());
   if (PrevOwner && PrevOwner == NewOwner)
     return false;
+  SourceManager &SrcMgr = Reader.getSourceManager();
+  bool PrevInSystem = (PrevOwner && PrevOwner->IsSystem) ||
+                      SrcMgr.isInSystemHeader(PrevMI->getDefinitionLoc());
+  bool NewInSystem = (NewOwner && NewOwner->IsSystem) ||
+                     SrcMgr.isInSystemHeader(NewMI->getDefinitionLoc());
   return PrevInSystem && NewInSystem;
 }
 
Index: test/Modules/Inputs/macro-ambiguity/a/foo.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/macro-ambiguity/a/foo.h
@@ -0,0 +1,6 @@
+#ifndef A_FOO_H
+#define A_FOO_H
+
+#include_next <foo.h>
+
+#endif // FOO_H
Index: test/Modules/Inputs/macro-ambiguity/b/foo.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/macro-ambiguity/b/foo.h
@@ -0,0 +1,6 @@
+#ifndef B_FOO_H
+#define B_FOO_H
+
+#include <bar.h>
+
+#endif // B_FOO_H
Index: test/Modules/Inputs/macro-ambiguity/c/bar.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/macro-ambiguity/c/bar.h
@@ -0,0 +1,10 @@
+#ifndef C_BAR_H
+#define C_BAR_H
+
+#define FOO(x) x + x
+
+#define MORE_META(x) (x)
+
+#define META_FOO(x) MORE_META(FOO(x))
+
+#endif
Index: test/Modules/Inputs/macro-ambiguity/c/d/baz.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/macro-ambiguity/c/d/baz.h
@@ -0,0 +1,6 @@
+#ifndef C_D_BAZ_H
+#define C_D_BAZ_H
+
+#define FOO(x) 2 * x
+
+#endif // C_D_BAZ_H
Index: test/Modules/Inputs/macro-ambiguity/module.modulemap
===================================================================
--- /dev/null
+++ test/Modules/Inputs/macro-ambiguity/module.modulemap
@@ -0,0 +1,6 @@
+module a { header "Inputs/macro-ambiguity/a/foo.h" export * use bc }
+module bc [system] {
+  textual header "Inputs/macro-ambiguity/b/foo.h"
+  textual header "Inputs/macro-ambiguity/c/bar.h"
+  textual header "Inputs/macro-ambiguity/c/d/baz.h"
+}
Index: test/Modules/macro-ambiguity.cpp
===================================================================
--- /dev/null
+++ test/Modules/macro-ambiguity.cpp
@@ -0,0 +1,35 @@
+// RUN: rm -rf %t
+// RUN: cd %S
+//
+// RUN: %clang_cc1 -fmodules -x c++ -fmodules-cache-path=%t \
+// RUN:   -v -iquote . \
+// RUN:   -isystem Inputs/macro-ambiguity/a \
+// RUN:   -isystem Inputs/macro-ambiguity/b \
+// RUN:   -isystem Inputs/macro-ambiguity/c \
+// RUN:   -fno-modules-implicit-maps -fmodule-map-file-home-is-cwd \
+// RUN:   -emit-module -fmodule-name=a -o %t/a.pcm \
+// RUN:   Inputs/macro-ambiguity/module.modulemap
+//
+// RUN: %clang_cc1 -fmodules -x c++ -fmodules-cache-path=%t \
+// RUN:   -v -iquote . \
+// RUN:   -isystem Inputs/macro-ambiguity/a \
+// RUN:   -isystem Inputs/macro-ambiguity/b \
+// RUN:   -isystem Inputs/macro-ambiguity/c \
+// RUN:   -fno-implicit-modules -fno-modules-implicit-maps \
+// RUN:   -fmodule-map-file-home-is-cwd \
+// RUN:   -fmodule-map-file=Inputs/macro-ambiguity/module.modulemap \
+// RUN:   -fmodule-file=%t/a.pcm \
+// RUN:   -Wambiguous-macro -verify macro-ambiguity.cpp
+
+// FIXME: We should add tests for cases which *will* issue the diagnostic as
+// well.
+// expected-no-diagnostics
+
+#include <d/baz.h>
+#include <foo.h>
+
+int test(int x) {
+  int foo = META_FOO(x) + FOO(x);
+
+  return foo;
+}