5 files changed, 23 insertions, 3 deletions
diff --git a/clang/lib/Basic/LangOptions.cpp b/clang/lib/Basic/LangOptions.cpp
index f8714b2389c..ae1c7992249 100644
--- a/clang/lib/Basic/LangOptions.cpp
+++ b/clang/lib/Basic/LangOptions.cpp
@@ -36,5 +36,6 @@ void LangOptions::resetNonModularOptions() {
   Sanitize = SanitizerOptions::Disabled;
 
   CurrentModule.clear();
+  ImplementationOfModule.clear();
 }
 
diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
index 6af920d9fd7..4a75058e6af 100644
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -1265,7 +1265,8 @@ CompilerInstance::loadModule(SourceLocation ImportLoc,
   // when both the preprocessor and parser see the same import declaration.
   if (!ImportLoc.isInvalid() && LastModuleImportLoc == ImportLoc) {
     // Make the named module visible.
-    if (LastModuleImportResult && ModuleName != getLangOpts().CurrentModule)
+    if (LastModuleImportResult && ModuleName != getLangOpts().CurrentModule &&
+        ModuleName != getLangOpts().ImplementationOfModule)
       ModuleManager->makeModuleVisible(LastModuleImportResult, Visibility,
                                        ImportLoc, /*Complain=*/false);
     return LastModuleImportResult;
@@ -1279,7 +1280,8 @@ CompilerInstance::loadModule(SourceLocation ImportLoc,
   if (Known != KnownModules.end()) {
     // Retrieve the cached top-level module.
     Module = Known->second;    
-  } else if (ModuleName == getLangOpts().CurrentModule) {
+  } else if (ModuleName == getLangOpts().CurrentModule ||
+             ModuleName == getLangOpts().ImplementationOfModule) {
     // This is the module we're building. 
     Module = PP->getHeaderSearchInfo().lookupModule(ModuleName);
     Known = KnownModules.insert(std::make_pair(Path[0].first, Module)).first;
@@ -1448,6 +1450,10 @@ CompilerInstance::loadModule(SourceLocation ImportLoc,
       Module = Sub;
     }
   }
+
+  // Don't make the module visible if we are in the implementation.
+  if (ModuleName == getLangOpts().ImplementationOfModule)
+    return ModuleLoadResult(Module, false);
   
   // Make the named module visible, if it's not already part of the module
   // we are parsing.
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 20258710de3..0c6d77d308c 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1495,6 +1495,14 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
   Opts.DebuggerObjCLiteral = Args.hasArg(OPT_fdebugger_objc_literal);
   Opts.ApplePragmaPack = Args.hasArg(OPT_fapple_pragma_pack);
   Opts.CurrentModule = Args.getLastArgValue(OPT_fmodule_name);
+  Opts.ImplementationOfModule =
+      Args.getLastArgValue(OPT_fmodule_implementation_of);
+
+  if (!Opts.CurrentModule.empty() && !Opts.ImplementationOfModule.empty() &&
+      Opts.CurrentModule != Opts.ImplementationOfModule) {
+    Diags.Report(diag::err_conflicting_module_names)
+        << Opts.CurrentModule << Opts.ImplementationOfModule;
+  }
 
   if (Arg *A = Args.getLastArg(OPT_faddress_space_map_mangling_EQ)) {
     switch (llvm::StringSwitch<unsigned>(A->getValue())
diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
index 1741c302f33..a8e27c71da8 100644
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -1539,7 +1539,9 @@ void Preprocessor::HandleIncludeDirective(SourceLocation HashLoc,
 
   // If we are supposed to import a module rather than including the header,
   // do so now.
-  if (SuggestedModule && getLangOpts().Modules) {
+  if (SuggestedModule && getLangOpts().Modules &&
+      SuggestedModule.getModule()->getTopLevelModuleName() !=
+      getLangOpts().ImplementationOfModule) {
     // Compute the module access path corresponding to this module.
     // FIXME: Should we have a second loadModule() overload to avoid this
     // extra lookup step?
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index bbe69306b64..9ce09288de1 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -13438,6 +13438,9 @@ DeclResult Sema::ActOnModuleImport(SourceLocation AtLoc,
   if (Mod->getTopLevelModuleName() == getLangOpts().CurrentModule)
     Diag(ImportLoc, diag::err_module_self_import)
         << Mod->getFullModuleName() << getLangOpts().CurrentModule;
+  else if (Mod->getTopLevelModuleName() == getLangOpts().ImplementationOfModule)
+    Diag(ImportLoc, diag::err_module_import_in_implementation)
+        << Mod->getFullModuleName() << getLangOpts().ImplementationOfModule;
 
   SmallVector<SourceLocation, 2> IdentifierLocs;
   Module *ModCheck = Mod;