Index: docs/Modules.rst
===================================================================
--- docs/Modules.rst
+++ docs/Modules.rst
@@ -210,6 +210,9 @@
 ``-fno-modules-implicit-maps``
   Suppresses the implicit search for files called ``module.modulemap`` and similar. Instead, module files need to be explicitly specified via ``-fmodule-map-file`` or transitively used.
 
+``-fno-modules-implicit-bulids``
+  All modules used by the build must be specified with ``-fmodule-file``.
+
 Module Semantics
 ================
 
Index: include/clang/Basic/DiagnosticCommonKinds.td
===================================================================
--- include/clang/Basic/DiagnosticCommonKinds.td
+++ include/clang/Basic/DiagnosticCommonKinds.td
@@ -81,6 +81,8 @@
   "only functions can have deleted definitions">;
 def err_module_not_found : Error<"module '%0' not found">, DefaultFatal;
 def err_module_not_built : Error<"could not build module '%0'">, DefaultFatal;
+def err_module_build_disabled: Error<
+  "module '%0' not found, but module build was disabled">, DefaultFatal;
 def err_module_lock_failure : Error<
   "could not acquire lock file for module '%0'">, DefaultFatal;
 def err_module_cycle : Error<"cyclic dependency in module '%0': %1">, 
Index: include/clang/Basic/LangOptions.def
===================================================================
--- include/clang/Basic/LangOptions.def
+++ include/clang/Basic/LangOptions.def
@@ -127,6 +127,7 @@
 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")
+BENIGN_LANGOPT(ModulesImplicitBuilds, 1, 1, "build modules that are not specified via -fmodule-file")
 COMPATIBLE_LANGOPT(Optimize          , 1, 0, "__OPTIMIZE__ predefined macro")
 COMPATIBLE_LANGOPT(OptimizeSize      , 1, 0, "__OPTIMIZE_SIZE__ predefined macro")
 LANGOPT(Static            , 1, 0, "__STATIC__ predefined macro (as opposed to __DYNAMIC__)")
@@ -228,4 +229,3 @@
 #undef COMPATIBLE_ENUM_LANGOPT
 #undef BENIGN_ENUM_LANGOPT
 #undef VALUE_LANGOPT
-
Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -681,6 +681,12 @@
 def fno_modules_implicit_maps :
   Flag <["-"], "fno-modules-implicit-maps">,
   Group<f_Group>, Flags<[DriverOption, CC1Option]>;
+def fmodules_implicit_builds :
+  Flag <["-"], "fmodules-implicit-builds">,
+  Group<f_Group>, Flags<[DriverOption, CC1Option]>;
+def fno_modules_implicit_builds :
+  Flag <["-"], "fno-modules-implicit-builds">,
+  Group<f_Group>, Flags<[DriverOption, CC1Option]>;
 def fretain_comments_from_system_headers : Flag<["-"], "fretain-comments-from-system-headers">, Group<f_Group>, Flags<[CC1Option]>;
 
 def fmudflapth : Flag<["-"], "fmudflapth">, Group<f_Group>;
Index: lib/Frontend/CompilerInstance.cpp
===================================================================
--- lib/Frontend/CompilerInstance.cpp
+++ lib/Frontend/CompilerInstance.cpp
@@ -1409,6 +1409,12 @@
         KnownModules[Path[0].first] = nullptr;
         return ModuleLoadResult();
       }
+      if (!getLangOpts().ModulesImplicitBuilds) {
+        getDiagnostics().Report(ModuleNameLoc, diag::err_module_build_disabled)
+            << ModuleName;
+        ModuleBuildFailed = true;
+        return ModuleLoadResult();
+      }
 
       // The module file is missing or out-of-date. Build it.
       assert(Module && "missing module file");
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -1476,6 +1476,8 @@
   Opts.ModulesErrorRecovery = !Args.hasArg(OPT_fno_modules_error_recovery);
   Opts.ModulesImplicitMaps = Args.hasFlag(OPT_fmodules_implicit_maps,
                                           OPT_fno_modules_implicit_maps, true);
+  Opts.ModulesImplicitBuilds = Args.hasFlag(
+      OPT_fmodules_implicit_builds, OPT_fno_modules_implicit_builds, true);
   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: test/Modules/no-implicit-builds.cpp
===================================================================
--- /dev/null
+++ test/Modules/no-implicit-builds.cpp
@@ -0,0 +1,3 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -x objective-c -fno-modules-implicit-builds -fmodules-cache-path=%t -fmodules -I %S/Inputs/private %s -verify
+@import libPrivate1;  // expected-error {{not found, but module build was disabled}}