Index: include/clang/Basic/Module.h
===================================================================
--- include/clang/Basic/Module.h
+++ include/clang/Basic/Module.h
@@ -541,6 +541,7 @@
   ///
   /// \returns The submodule if found, or NULL otherwise.
   Module *findSubmodule(StringRef Name) const;
+  Module *findOrCreateSubmodule(StringRef Name);
 
   /// Determine whether the specified module would be visible to
   /// a lookup at the end of this module.
Index: lib/Basic/Module.cpp
===================================================================
--- lib/Basic/Module.cpp
+++ lib/Basic/Module.cpp
@@ -321,6 +321,21 @@
   return SubModules[Pos->getValue()];
 }
 
+Module *Module::findOrCreateSubmodule(StringRef Name) {
+  llvm::StringMap<unsigned>::const_iterator Pos = SubModuleIndex.find(Name);
+  if (Pos != SubModuleIndex.end())
+    return SubModules[Pos->getValue()];
+  if (!InferSubmodules)
+    return nullptr;
+  Module *Result = new Module(Name, SourceLocation(), this, false, InferExplicitSubmodules, 0);
+  Result->InferExplicitSubmodules = InferExplicitSubmodules;
+  Result->InferSubmodules = InferSubmodules;
+  Result->InferExportWildcard = InferExportWildcard;
+  if (Result->InferExportWildcard)
+    Result->Exports.push_back(Module::ExportDecl(nullptr, true));
+  return Result;
+}
+
 void Module::getExportedModules(SmallVectorImpl<Module *> &Exported) const {
   // All non-explicit submodules are exported.
   for (std::vector<Module *>::const_iterator I = SubModules.begin(),
Index: lib/Lex/Pragma.cpp
===================================================================
--- lib/Lex/Pragma.cpp
+++ lib/Lex/Pragma.cpp
@@ -1586,14 +1586,15 @@
     // be loaded or implicitly loadable.
     // FIXME: We could create the submodule here. We'd need to know whether
     // it's supposed to be explicit, but not much else.
-    Module *M = PP.getHeaderSearchInfo().lookupModule(Current);
+    auto &HSI = PP.getHeaderSearchInfo();
+    Module *M = HSI.lookupModule(Current);
     if (!M) {
       PP.Diag(ModuleName.front().second,
               diag::err_pp_module_begin_no_module_map) << Current;
       return;
     }
     for (unsigned I = 1; I != ModuleName.size(); ++I) {
-      auto *NewM = M->findSubmodule(ModuleName[I].first->getName());
+      auto *NewM = M->findOrCreateSubmodule(ModuleName[I].first->getName());
       if (!NewM) {
         PP.Diag(ModuleName[I].second, diag::err_pp_module_begin_no_submodule)
           << M->getFullModuleName() << ModuleName[I].first;
Index: test/Modules/preprocess-umbrella.cpp
===================================================================
--- /dev/null
+++ test/Modules/preprocess-umbrella.cpp
@@ -0,0 +1,36 @@
+// FIXME: The standalone module still seems to cause clang to want to test for
+// the existence of a 'foo' directory:
+// RUN: mkdir %t
+// RUN: cp %s %t
+// RUN: mkdir %t/foo
+// RUN: cd %t
+// RUN: not %clang_cc1 -fmodules -fsyntax-only %s 2>&1 | FileCheck %s
+
+// CHECK: error: no matching function for call to 'foo'
+// CHECK: note: candidate function not viable: requires 0 arguments, but 1 was provided
+
+// FIXME: This should use -verify, but it seems it doesn't hook up the
+// SourceManager correctly or something, and the foo.h note gets attributed to
+// the synthetic module translation unit "foo.map Line 2:...".
+// %clang_cc1 -fmodules -verify %s
+
+#pragma clang module build foo
+module foo {
+  umbrella "foo"
+  module * {
+    export *
+  }
+}
+#pragma clang module contents
+#pragma clang module begin foo.foo
+# 1 "foo.h" 1
+#ifndef FOO_FOO_H
+void foo();
+#endif
+#pragma clang module end
+#pragma clang module endbuild
+#pragma clang module import foo.foo
+// expected-note@foo.h:2 {{candidate function not viable: requires 0 arguments, but 1 was provided}}
+int main() {
+  foo(1); // expected-error {{no matching function for call to 'foo'}}
+}