Index: include/clang/Basic/LangOptions.def
===================================================================
--- include/clang/Basic/LangOptions.def
+++ include/clang/Basic/LangOptions.def
@@ -96,6 +96,7 @@
 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")
+LANGOPT(ModulesStrictDeclUse, 1, 0, "require declaration of module uses and all headers to be in modules")
 LANGOPT(Optimize          , 1, 0, "__OPTIMIZE__ predefined macro")
 LANGOPT(OptimizeSize      , 1, 0, "__OPTIMIZE_SIZE__ predefined macro")
 LANGOPT(Static            , 1, 0, "__STATIC__ predefined macro (as opposed to __DYNAMIC__)")
Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -607,6 +607,9 @@
 def fmodules_decluse : Flag <["-"], "fmodules-decluse">, Group<f_Group>,
   Flags<[DriverOption,CC1Option]>,
   HelpText<"Require declaration of modules used within a module">;
+def fmodules_strict_decluse : Flag <["-"], "fmodules-strict-decluse">, Group<f_Group>,
+  Flags<[DriverOption,CC1Option]>,
+  HelpText<"Like -fmodules-decluse but requires all headers to be in modules">;
 def fretain_comments_from_system_headers : Flag<["-"], "fretain-comments-from-system-headers">, Group<f_Group>, Flags<[CC1Option]>;
 
 def fmudflapth : Flag<["-"], "fmudflapth">, Group<f_Group>;
@@ -666,6 +669,8 @@
   Flags<[DriverOption]>;
 def fno_modules_decluse : Flag <["-"], "fno-modules-decluse">, Group<f_Group>,
   Flags<[DriverOption]>;
+def fno_modules_strict_decluse : Flag <["-"], "fno-strict-modules-decluse">, Group<f_Group>,
+  Flags<[DriverOption]>;
 def fno_ms_extensions : Flag<["-"], "fno-ms-extensions">, Group<f_Group>;
 def fno_ms_compatibility : Flag<["-"], "fno-ms-compatibility">, Group<f_Group>;
 def fno_delayed_template_parsing : Flag<["-"], "fno-delayed-template-parsing">, Group<f_Group>;
Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -3469,6 +3469,14 @@
     CmdArgs.push_back("-fmodules-decluse");
   }
 
+  // -fmodules-strict-decluse is like -fmodule-decluse, but also checks that
+  // all #included headers are part of modules.
+  if (Args.hasFlag(options::OPT_fmodules_strict_decluse,
+                   options::OPT_fno_modules_strict_decluse,
+                   false)) {
+    CmdArgs.push_back("-fmodules-strict-decluse");
+  }
+
   // -fmodule-name specifies the module that is currently being built (or
   // used for header checking by -fmodule-maps).
   if (Arg *A = Args.getLastArg(options::OPT_fmodule_name)) {
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -1343,7 +1343,9 @@
   Opts.Blocks = Args.hasArg(OPT_fblocks);
   Opts.BlocksRuntimeOptional = Args.hasArg(OPT_fblocks_runtime_optional);
   Opts.Modules = Args.hasArg(OPT_fmodules);
-  Opts.ModulesDeclUse = Args.hasArg(OPT_fmodules_decluse);
+  Opts.ModulesStrictDeclUse = Args.hasArg(OPT_fmodules_strict_decluse);
+  Opts.ModulesDeclUse =
+      Args.hasArg(OPT_fmodules_decluse) || Opts.ModulesStrictDeclUse;
   Opts.CharIsSigned = Opts.OpenCL || !Args.hasArg(OPT_fno_signed_char);
   Opts.WChar = Opts.CPlusPlus && !Args.hasArg(OPT_fno_wchar);
   Opts.ShortWChar = Args.hasFlag(OPT_fshort_wchar, OPT_fno_short_wchar, false);
Index: lib/Lex/ModuleMap.cpp
===================================================================
--- lib/Lex/ModuleMap.cpp
+++ lib/Lex/ModuleMap.cpp
@@ -243,8 +243,12 @@
     resolveUses(RequestingModule, /*Complain=*/false);
 
   HeadersMap::iterator Known = findKnownHeader(File);
-  if (Known == Headers.end())
+  if (Known == Headers.end()) {
+    if (LangOpts.ModulesStrictDeclUse)
+      Diags.Report(FilenameLoc, diag::error_undeclared_use_of_module)
+          << RequestingModule->getFullModuleName() << Filename;
     return;
+  }
 
   Module *Private = NULL;
   Module *NotUsed = NULL;