diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -5256,8 +5256,11 @@
     return FD->getDefinition();
   if (TagDecl *TD = dyn_cast<TagDecl>(D))
     return TD->getDefinition();
+  // The first definition for this ObjCInterfaceDecl might be in the TU
+  // and not associated with any module. Use the one we know to be complete
+  // and have just seen in a module.
   if (ObjCInterfaceDecl *ID = dyn_cast<ObjCInterfaceDecl>(D))
-    return ID->getDefinition();
+    return ID;
   if (ObjCProtocolDecl *PD = dyn_cast<ObjCProtocolDecl>(D))
     return PD->getDefinition();
   if (TemplateDecl *TD = dyn_cast<TemplateDecl>(D))
diff --git a/clang/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/Headers/Bar.h b/clang/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/Headers/Bar.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/Headers/Bar.h
@@ -0,0 +1 @@
+// interface-diagnose-missing-import/Foo.framework/Headers/Bar.h
diff --git a/clang/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/Headers/Foo.h b/clang/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/Headers/Foo.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/Headers/Foo.h
@@ -0,0 +1,2 @@
+#import <Foo/RandoPriv.h>
+#import <Foo/Bar.h>
diff --git a/clang/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/Modules/module.modulemap b/clang/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/Modules/module.modulemap
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/Modules/module.modulemap
@@ -0,0 +1,6 @@
+// interface-diagnose-missing-import/Foo.framework/Modules/module.modulemap
+framework module Foo {
+  umbrella header "Foo.h"
+  export *
+  module * { export * }
+}
diff --git a/clang/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/PrivateHeaders/RandoPriv.h b/clang/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/PrivateHeaders/RandoPriv.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/interface-diagnose-missing-import/Foo.framework/PrivateHeaders/RandoPriv.h
@@ -0,0 +1,4 @@
+@interface NSObject
+@end
+@interface Buggy : NSObject
+@end
diff --git a/clang/test/Modules/interface-diagnose-missing-import.m b/clang/test/Modules/interface-diagnose-missing-import.m
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/interface-diagnose-missing-import.m
@@ -0,0 +1,11 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 %s -fsyntax-only -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -F%S/Inputs/interface-diagnose-missing-import -verify
+@interface Buggy
+@end
+
+@import Foo.Bar;
+
+@interface Buggy (MyExt) // expected-error {{definition of 'Buggy' must be imported from module 'Foo' before it is required}}
+@end
+
+// expected-note@Foo/RandoPriv.h:3{{previous definition is here}}