diff --git a/include/clang/Basic/DiagnosticFrontendKinds.td b/include/clang/Basic/DiagnosticFrontendKinds.td
index 8b4cb47e545d939e019bb9f23b532c7439735bf5..57c24e9be73a7bccdd1ce9e9123e3d7cb3583ccc 100644
--- a/include/clang/Basic/DiagnosticFrontendKinds.td
+++ b/include/clang/Basic/DiagnosticFrontendKinds.td
@@ -179,6 +179,8 @@ def warn_incompatible_analyzer_plugin_api : Warning<
 def note_incompatible_analyzer_plugin_api : Note<
     "current API version is '%0', but plugin was compiled with version '%1'">;
 
+def err_module_build_requires_fmodules : Error<
+  "module compilation requires '-fmodules'">;
 def err_module_interface_requires_modules_ts : Error<
   "module interface compilation requires '-fmodules-ts'">;
 def warn_module_config_mismatch : Warning<
diff --git a/include/clang/Frontend/FrontendActions.h b/include/clang/Frontend/FrontendActions.h
index 84db293c46f3bacb46ab3145da6413166b89fefa..c45aeaa208c8caf217187cdf959e9f9e76002e96 100644
--- a/include/clang/Frontend/FrontendActions.h
+++ b/include/clang/Frontend/FrontendActions.h
@@ -111,6 +111,8 @@ protected:
 
 class GenerateModuleFromModuleMapAction : public GenerateModuleAction {
 private:
+  bool BeginSourceFileAction(CompilerInstance &CI) override;
+
   std::unique_ptr<raw_pwrite_stream>
   CreateOutputFile(CompilerInstance &CI, StringRef InFile) override;
 };
diff --git a/lib/Frontend/FrontendActions.cpp b/lib/Frontend/FrontendActions.cpp
index 0fbcc1c7399e8df90463cd5301d588dc6cd8186f..d42400183a433f2740a55722bbe8efafbb9a0245 100644
--- a/lib/Frontend/FrontendActions.cpp
+++ b/lib/Frontend/FrontendActions.cpp
@@ -163,6 +163,16 @@ GenerateModuleAction::CreateASTConsumer(CompilerInstance &CI,
   return llvm::make_unique<MultiplexConsumer>(std::move(Consumers));
 }
 
+bool GenerateModuleFromModuleMapAction::BeginSourceFileAction(
+    CompilerInstance &CI) {
+  if (!CI.getLangOpts().Modules) {
+    CI.getDiagnostics().Report(diag::err_module_build_requires_fmodules);
+    return false;
+  }
+
+  return GenerateModuleAction::BeginSourceFileAction(CI);
+}
+
 std::unique_ptr<raw_pwrite_stream>
 GenerateModuleFromModuleMapAction::CreateOutputFile(CompilerInstance &CI,
                                                     StringRef InFile) {
diff --git a/test/Modules/missing-flag.cpp b/test/Modules/missing-flag.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..560183be59e8a15b4fcebfbf7f718ec4098a7c75
--- /dev/null
+++ b/test/Modules/missing-flag.cpp
@@ -0,0 +1,4 @@
+// RUN: not %clang_cc1 -x c++-module-map %s -emit-module -fmodule-name=Foo -o %t 2>&1 | FileCheck %s
+// CHECK: module compilation requires '-fmodules'
+module Foo {}
+#pragma clang module contents
diff --git a/test/Modules/preprocess-build.cpp b/test/Modules/preprocess-build.cpp
index bf9f16162be24ff4f6a3db91e771c42e15957ac5..36a0740a0a76ed6964119fb96fc33eace3179232 100644
--- a/test/Modules/preprocess-build.cpp
+++ b/test/Modules/preprocess-build.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -std=c++1z %s -verify
+// RUN: %clang_cc1 -std=c++1z -fmodules %s -verify
 
 #pragma clang module build baz
   module baz {}
diff --git a/test/Modules/relative-dep-gen.cpp b/test/Modules/relative-dep-gen.cpp
index e58dd6eb359628720dc19afc9141fcf095b80cb9..bfa9471859b820985922f0fc3d4672a22e0e1a20 100644
--- a/test/Modules/relative-dep-gen.cpp
+++ b/test/Modules/relative-dep-gen.cpp
@@ -2,17 +2,17 @@
 // RUN: rm -rf %t
 // RUN: mkdir %t
 //
-// RUN: %clang_cc1 -cc1 -fno-implicit-modules -fmodule-name=relative-dep-gen -emit-module -x c++ Inputs/relative-dep-gen.modulemap -dependency-file %t/build.d -MT mod.pcm -o %t/mod.pcm
-// RUN: %clang_cc1 -cc1 -fno-implicit-modules -fmodule-map-file=Inputs/relative-dep-gen.modulemap -fmodule-file=%t/mod.pcm -dependency-file %t/use-explicit.d -MT use.o relative-dep-gen.cpp -fsyntax-only
-// RUN: %clang_cc1 -cc1 -fno-implicit-modules -fmodule-map-file=Inputs/relative-dep-gen.modulemap -dependency-file %t/use-implicit.d relative-dep-gen.cpp -MT use.o -fsyntax-only
+// RUN: %clang_cc1 -fmodules -fno-implicit-modules -fmodule-name=relative-dep-gen -emit-module -x c++ Inputs/relative-dep-gen.modulemap -dependency-file %t/build.d -MT mod.pcm -o %t/mod.pcm
+// RUN: %clang_cc1 -fmodules -fno-implicit-modules -fmodule-map-file=Inputs/relative-dep-gen.modulemap -fmodule-file=%t/mod.pcm -dependency-file %t/use-explicit.d -MT use.o relative-dep-gen.cpp -fsyntax-only
+// RUN: %clang_cc1 -fmodules -fmodules-cache-path=%t -fmodule-map-file=Inputs/relative-dep-gen.modulemap -dependency-file %t/use-implicit.d relative-dep-gen.cpp -MT use.o -fsyntax-only
 //
 // RUN: FileCheck --check-prefix=CHECK-BUILD %s < %t/build.d
-// RUN: FileCheck --check-prefix=CHECK-USE %s < %t/use-explicit.d
-// RUN: FileCheck --check-prefix=CHECK-USE %s < %t/use-implicit.d
+// RUN: FileCheck --check-prefix=CHECK-USE --check-prefix=CHECK-EXPLICIT %s < %t/use-explicit.d
+// RUN: FileCheck --check-prefix=CHECK-USE --check-prefix=CHECK-IMPLICIT %s < %t/use-implicit.d
 //
-// RUN: %clang_cc1 -cc1 -fno-implicit-modules -fmodule-name=relative-dep-gen -emit-module -x c++ Inputs/relative-dep-gen-cwd.modulemap -dependency-file %t/build-cwd.d -MT mod.pcm -o %t/mod-cwd.pcm -fmodule-map-file-home-is-cwd
-// RUN: %clang_cc1 -cc1 -fno-implicit-modules -fmodule-map-file=Inputs/relative-dep-gen-cwd.modulemap -fmodule-file=%t/mod-cwd.pcm -dependency-file %t/use-explicit-cwd.d -MT use.o relative-dep-gen.cpp -fsyntax-only -fmodule-map-file-home-is-cwd
-// RUN: %clang_cc1 -cc1 -fno-implicit-modules -fmodule-map-file=Inputs/relative-dep-gen-cwd.modulemap -dependency-file %t/use-implicit-cwd.d relative-dep-gen.cpp -MT use.o -fsyntax-only -fmodule-map-file-home-is-cwd
+// RUN: %clang_cc1 -fmodules -fno-implicit-modules -fmodule-name=relative-dep-gen -emit-module -x c++ Inputs/relative-dep-gen-cwd.modulemap -dependency-file %t/build-cwd.d -MT mod.pcm -o %t/mod-cwd.pcm -fmodule-map-file-home-is-cwd
+// RUN: %clang_cc1 -fmodules -fno-implicit-modules -fmodule-map-file=Inputs/relative-dep-gen-cwd.modulemap -fmodule-file=%t/mod-cwd.pcm -dependency-file %t/use-explicit-cwd.d -MT use.o relative-dep-gen.cpp -fsyntax-only -fmodule-map-file-home-is-cwd
+// RUN: %clang_cc1 -fmodules -fmodules-cache-path=%t -fmodule-map-file=Inputs/relative-dep-gen-cwd.modulemap -dependency-file %t/use-implicit-cwd.d relative-dep-gen.cpp -MT use.o -fsyntax-only -fmodule-map-file-home-is-cwd
 //
 // RUN: FileCheck --check-prefix=CHECK-BUILD %s < %t/build-cwd.d
 // RUN: FileCheck --check-prefix=CHECK-USE %s < %t/use-explicit-cwd.d
@@ -23,7 +23,7 @@
 // RUN: cp %S/Inputs/relative-dep-gen-1.h %t/Inputs
 // RUN: cp %s %t
 // RUN: cd %t
-// RUN: %clang_cc1 -cc1 -fno-implicit-modules -fmodule-file=%t/mod-cwd.pcm -dependency-file %t/use-explicit-no-map-cwd.d -MT use.o relative-dep-gen.cpp -fsyntax-only -fmodule-map-file-home-is-cwd
+// RUN: %clang_cc1 -fmodules -fno-implicit-modules -fmodule-file=%t/mod-cwd.pcm -dependency-file %t/use-explicit-no-map-cwd.d -MT use.o relative-dep-gen.cpp -fsyntax-only -fmodule-map-file-home-is-cwd
 // RUN: cat %t/use-explicit-no-map-cwd.d
 // RUN: FileCheck --check-prefix=CHECK-USE %s < %t/use-explicit-no-map-cwd.d
 
@@ -35,4 +35,5 @@
 // CHECK-BUILD:   {{[ \t]}}Inputs/relative-dep-gen-2.h
 // CHECK-USE: use.o:
 // CHECK-USE-DAG:   {{[ \t]}}relative-dep-gen.cpp
-// CHECK-USE-DAG:   {{[ \t]}}Inputs{{[/\\]}}relative-dep-gen-1.h
+// CHECK-EXPLICIT-DAG:   mod.pcm
+// CHECK-IMPLICIT-DAG:   {{[ \t]}}Inputs{{[/\\]}}relative-dep-gen-1.h