Index: lib/Frontend/CompilerInstance.cpp
===================================================================
--- lib/Frontend/CompilerInstance.cpp
+++ lib/Frontend/CompilerInstance.cpp
@@ -1595,7 +1595,22 @@
                              Module::NameVisibilityKind Visibility,
                              bool IsInclusionDirective) {
   // Determine what file we're searching from.
-  StringRef ModuleName = Path[0].first->getName();
+  // FIXME: Should we be deciding whether this is a submodule (here and
+  // below) based on -fmodules-ts or should we pass a flag and make the
+  // caller decide?
+  std::string ModuleName;
+  if (getLangOpts().ModulesTS) {
+    // FIXME: Same code as Sema::ActOnModuleDecl() so there is probably a
+    // better place/way to do this.
+    for (auto &Piece : Path) {
+      if (!ModuleName.empty())
+        ModuleName += ".";
+      ModuleName += Piece.first->getName();
+    }
+  }
+  else
+    ModuleName = Path[0].first->getName();
+
   SourceLocation ModuleNameLoc = Path[0].second;
 
   // If we've already handled this import, just return the cached result.
@@ -1810,7 +1825,7 @@
 
   // Verify that the rest of the module path actually corresponds to
   // a submodule.
-  if (Path.size() > 1) {
+  if (!getLangOpts().ModulesTS && Path.size() > 1) {
     for (unsigned I = 1, N = Path.size(); I != N; ++I) {
       StringRef Name = Path[I].first->getName();
       clang::Module *Sub = Module->findSubmodule(Name);