Index: include/clang/Basic/LangOptions.def
===================================================================
--- include/clang/Basic/LangOptions.def
+++ include/clang/Basic/LangOptions.def
@@ -123,9 +123,9 @@
 LANGOPT(MathErrno         , 1, 1, "errno support for math functions")
 BENIGN_LANGOPT(HeinousExtensions , 1, 0, "Extensions that we really don't like and may be ripped out at any time")
 LANGOPT(Modules           , 1, 0, "modules extension to C")
-LANGOPT(ModulesDeclUse    , 1, 0, "require declaration of module uses")
+COMPATIBLE_LANGOPT(ModulesDeclUse    , 1, 0, "require declaration of module uses")
 LANGOPT(ModulesSearchAll  , 1, 1, "search even non-imported modules to find unresolved references")
-LANGOPT(ModulesStrictDeclUse, 1, 0, "require declaration of module uses and all headers to be in modules")
+COMPATIBLE_LANGOPT(ModulesStrictDeclUse, 1, 0, "require declaration of module uses and all headers to be in modules")
 LANGOPT(ModulesErrorRecovery, 1, 1, "automatically import modules as needed when performing error recovery")
 BENIGN_LANGOPT(ModulesImplicitMaps, 1, 1, "use files called module.modulemap implicitly as module maps")
 COMPATIBLE_LANGOPT(Optimize          , 1, 0, "__OPTIMIZE__ predefined macro")
Index: test/Modules/declare-use-compatible.cpp
===================================================================
--- /dev/null
+++ test/Modules/declare-use-compatible.cpp
@@ -0,0 +1,36 @@
+// Used module not built with -decluse.
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -x c++ -fmodules -fmodule-name=XB -emit-module \
+// RUN:   -I %S/Inputs/declare-use %S/Inputs/declare-use/module.map -o %t/b.pcm
+// RUN: %clang_cc1 -x c++ -fmodules -fmodules-cache-path=%t \
+// RUN:   -fmodules-decluse \
+// RUN:   -fmodule-file=%t/b.pcm -fmodule-name=XE -I %S/Inputs/declare-use %s
+//
+// Main module not built with -decluse.
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -x c++ -fmodules -fmodule-name=XB -emit-module \
+// RUN:   -fmodules-decluse \
+// RUN:   -I %S/Inputs/declare-use %S/Inputs/declare-use/module.map -o %t/b.pcm
+// RUN: %clang_cc1 -x c++ -fmodules -fmodules-cache-path=%t \
+// RUN:   -fmodule-file=%t/b.pcm -fmodule-name=XE -I %S/Inputs/declare-use %s
+//
+// Used module not built with -decluse.
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -x c++ -fmodules -fmodule-name=XB -emit-module \
+// RUN:   -I %S/Inputs/declare-use %S/Inputs/declare-use/module.map -o %t/b.pcm
+// RUN: %clang_cc1 -x c++ -fmodules -fmodules-cache-path=%t \
+// RUN:   -fmodules-strict-decluse \
+// RUN:   -fmodule-file=%t/b.pcm -fmodule-name=XE -I %S/Inputs/declare-use %s
+//
+// Main module not built with -decluse.
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -x c++ -fmodules -fmodule-name=XB -emit-module \
+// RUN:   -fmodules-strict-decluse \
+// RUN:   -I %S/Inputs/declare-use %S/Inputs/declare-use/module.map -o %t/b.pcm
+// RUN: %clang_cc1 -x c++ -fmodules -fmodules-cache-path=%t \
+// RUN:   -fmodule-file=%t/b.pcm -fmodule-name=XE -I %S/Inputs/declare-use %s
+
+#include "b.h"
+
+const int g = b;
+