diff --git a/include/clang/Basic/DiagnosticGroups.td b/include/clang/Basic/DiagnosticGroups.td
index 9af83b1..4ddf207 100644
--- a/include/clang/Basic/DiagnosticGroups.td
+++ b/include/clang/Basic/DiagnosticGroups.td
@@ -159,6 +159,7 @@
 def MismatchedReturnTypes : DiagGroup<"mismatched-return-types">;
 def MismatchedTags : DiagGroup<"mismatched-tags">;
 def MissingFieldInitializers : DiagGroup<"missing-field-initializers">;
+def ModuleConflict : DiagGroup<"module-conflict">;
 def NullArithmetic : DiagGroup<"null-arithmetic">;
 def NullCharacter : DiagGroup<"null-character">;
 def NullDereference : DiagGroup<"null-dereference">;
diff --git a/include/clang/Basic/DiagnosticLexKinds.td b/include/clang/Basic/DiagnosticLexKinds.td
index 80c5249..339788b 100644
--- a/include/clang/Basic/DiagnosticLexKinds.td
+++ b/include/clang/Basic/DiagnosticLexKinds.td
@@ -536,6 +536,10 @@
   "configuration macros are only allowed on top-level modules">;
 def err_mmap_expected_config_macro : Error<
   "expected configuration macro name after ','">;
+def err_mmap_expected_conflicts_comma : Error<
+  "expected ',' after conflicting module name">;
+def err_mmap_expected_conflicts_message : Error<
+  "expected a message describing the conflict with '%0'">;
 def err_mmap_missing_module_unqualified : Error<
   "no module named '%0' visible from '%1'">;
 def err_mmap_missing_module_qualified : Error<
diff --git a/include/clang/Basic/DiagnosticSerializationKinds.td b/include/clang/Basic/DiagnosticSerializationKinds.td
index bc5bd4e..7137404 100644
--- a/include/clang/Basic/DiagnosticSerializationKinds.td
+++ b/include/clang/Basic/DiagnosticSerializationKinds.td
@@ -44,7 +44,10 @@
 def err_pch_with_compiler_errors : Error<
     "PCH file contains compiler errors">;
     
-    
+def warn_module_conflict : Warning<
+    "module '%0' conflicts with already-imported module '%1': %2">, 
+    InGroup<ModuleConflict>;
+
 def err_pch_macro_def_undef : Error<
     "macro '%0' was %select{defined|undef'd}1 in the precompiled header but "
     "%select{undef'd|defined}1 on the command line">;
diff --git a/include/clang/Basic/Module.h b/include/clang/Basic/Module.h
index 2add580..d2a43f0 100644
--- a/include/clang/Basic/Module.h
+++ b/include/clang/Basic/Module.h
@@ -201,6 +201,31 @@
   /// (intentionally) change how this module is built.
   std::vector<std::string> ConfigMacros;
 
+  /// \brief An unresolved conflict with another module.
+  struct UnresolvedConflict {
+    /// \brief The (unresolved) module id.
+    ModuleId Id;
+
+    /// \brief The message provided to the user when there is a conflict.
+    std::string Message;
+  };
+
+  /// \brief The list of conflicts for which the module-id has not yet been
+  /// resolved.
+  std::vector<UnresolvedConflict> UnresolvedConflicts;
+
+  /// \brief A conflict between two modules.
+  struct Conflict {
+    /// \brief The module that this module conflicts with.
+    Module *Other;
+
+    /// \brief The message provided to the user when there is a conflict.
+    std::string Message;
+  };
+
+  /// \brief The list of conflicts.
+  std::vector<Conflict> Conflicts;
+
   /// \brief Construct a top-level module.
   explicit Module(StringRef Name, SourceLocation DefinitionLoc,
                   bool IsFramework)
diff --git a/include/clang/Frontend/ASTUnit.h b/include/clang/Frontend/ASTUnit.h
index 108114d..02c57d7 100644
--- a/include/clang/Frontend/ASTUnit.h
+++ b/include/clang/Frontend/ASTUnit.h
@@ -843,7 +843,8 @@
 
   virtual void makeModuleVisible(Module *Mod,
                                  Module::NameVisibilityKind Visibility,
-                                 SourceLocation ImportLoc) { }
+                                 SourceLocation ImportLoc,
+                                 bool Complain) { }
 
 };
 
diff --git a/include/clang/Frontend/CompilerInstance.h b/include/clang/Frontend/CompilerInstance.h
index 273fcc1..0d67462 100644
--- a/include/clang/Frontend/CompilerInstance.h
+++ b/include/clang/Frontend/CompilerInstance.h
@@ -663,7 +663,8 @@
 
   virtual void makeModuleVisible(Module *Mod,
                                  Module::NameVisibilityKind Visibility,
-                                 SourceLocation ImportLoc);
+                                 SourceLocation ImportLoc,
+                                 bool Complain);
 
 };
 
diff --git a/include/clang/Lex/ModuleLoader.h b/include/clang/Lex/ModuleLoader.h
index 93e69a6..3acf915 100644
--- a/include/clang/Lex/ModuleLoader.h
+++ b/include/clang/Lex/ModuleLoader.h
@@ -83,7 +83,8 @@
   /// \brief Make the given module visible.
   virtual void makeModuleVisible(Module *Mod,
                                  Module::NameVisibilityKind Visibility,
-                                 SourceLocation ImportLoc) = 0;
+                                 SourceLocation ImportLoc,
+                                 bool Complain) = 0;
 };
   
 }
diff --git a/include/clang/Lex/ModuleMap.h b/include/clang/Lex/ModuleMap.h
index cffa5b7b..33c92f5 100644
--- a/include/clang/Lex/ModuleMap.h
+++ b/include/clang/Lex/ModuleMap.h
@@ -134,7 +134,20 @@
   Module::ExportDecl 
   resolveExport(Module *Mod, const Module::UnresolvedExportDecl &Unresolved,
                 bool Complain) const;
-  
+
+  /// \brief Resolve the given module id to an actual module.
+  ///
+  /// \param Id The module-id to resolve.
+  ///
+  /// \param Mod The module in which we're resolving the module-id.
+  ///
+  /// \param Complain Whether this routine should complain about unresolvable
+  /// module-ids.
+  ///
+  /// \returns The resolved module, or null if the module-id could not be
+  /// resolved.
+  Module *resolveModuleId(const ModuleId &Id, Module *Mod, bool Complain) const;
+
 public:
   /// \brief Construct a new module map.
   ///
@@ -265,7 +278,17 @@
   /// false otherwise.
   bool resolveExports(Module *Mod, bool Complain);
 
-  /// \brief Infers the (sub)module based on the given source location and 
+  /// \brief Resolve all of the unresolved conflicts in the given module.
+  ///
+  /// \param Mod The module whose conflicts should be resolved.
+  ///
+  /// \param Complain Whether to emit diagnostics for failures.
+  ///
+  /// \returns true if any errors were encountered while resolving conflicts,
+  /// false otherwise.
+  bool resolveConflicts(Module *Mod, bool Complain);
+
+  /// \brief Infers the (sub)module based on the given source location and
   /// source manager.
   ///
   /// \param Loc The location within the source that we are querying, along
diff --git a/include/clang/Serialization/ASTBitCodes.h b/include/clang/Serialization/ASTBitCodes.h
index 2669d8e..85f88ad 100644
--- a/include/clang/Serialization/ASTBitCodes.h
+++ b/include/clang/Serialization/ASTBitCodes.h
@@ -611,7 +611,9 @@
       /// \brief Specifies a library or framework to link against.
       SUBMODULE_LINK_LIBRARY = 10,
       /// \brief Specifies a configuration macro for this module.
-      SUBMODULE_CONFIG_MACRO = 11
+      SUBMODULE_CONFIG_MACRO = 11,
+      /// \brief Specifies a conflict with another module.
+      SUBMODULE_CONFLICT = 12
     };
 
     /// \brief Record types used within a comments block.
diff --git a/include/clang/Serialization/ASTReader.h b/include/clang/Serialization/ASTReader.h
index 61a0cbe..9b4a5c0 100644
--- a/include/clang/Serialization/ASTReader.h
+++ b/include/clang/Serialization/ASTReader.h
@@ -520,27 +520,30 @@
   HiddenNamesMapType HiddenNamesMap;
   
   
-  /// \brief A module import or export that hasn't yet been resolved.
-  struct UnresolvedModuleImportExport {
+  /// \brief A module import, export, or conflict that hasn't yet been resolved.
+  struct UnresolvedModuleRef {
     /// \brief The file in which this module resides.
     ModuleFile *File;
     
     /// \brief The module that is importing or exporting.
     Module *Mod;
-    
+
+    /// \brief The kind of module reference.
+    enum { Import, Export, Conflict } Kind;
+
     /// \brief The local ID of the module that is being exported.
     unsigned ID;
-    
-    /// \brief Whether this is an import (vs. an export).
-    unsigned IsImport : 1;
-    
+
     /// \brief Whether this is a wildcard export.
     unsigned IsWildcard : 1;
+
+    /// \brief String data.
+    StringRef String;
   };
   
   /// \brief The set of module imports and exports that still need to be 
   /// resolved.
-  SmallVector<UnresolvedModuleImportExport, 2> UnresolvedModuleImportExports;
+  SmallVector<UnresolvedModuleRef, 2> UnresolvedModuleRefs;
   
   /// \brief A vector containing selectors that have already been loaded.
   ///
@@ -1188,9 +1191,14 @@
   ///
   /// \param NameVisibility The level of visibility to give the names in the
   /// module.  Visibility can only be increased over time.
+  ///
+  /// \param ImportLoc The location at which the import occurs.
+  ///
+  /// \param Complain Whether to complain about conflicting module imports.
   void makeModuleVisible(Module *Mod, 
                          Module::NameVisibilityKind NameVisibility,
-                         SourceLocation ImportLoc);
+                         SourceLocation ImportLoc,
+                         bool Complain);
   
   /// \brief Make the names within this set of hidden names visible.
   void makeNamesVisible(const HiddenNames &Names);
diff --git a/lib/Basic/Module.cpp b/lib/Basic/Module.cpp
index 197c53f..13518cd 100644
--- a/lib/Basic/Module.cpp
+++ b/lib/Basic/Module.cpp
@@ -347,6 +347,24 @@
     OS << "\"";
   }
 
+  for (unsigned I = 0, N = UnresolvedConflicts.size(); I != N; ++I) {
+    OS.indent(Indent + 2);
+    OS << "conflict ";
+    printModuleId(OS, UnresolvedConflicts[I].Id);
+    OS << ", \"";
+    OS.write_escaped(UnresolvedConflicts[I].Message);
+    OS << "\"\n";
+  }
+
+  for (unsigned I = 0, N = Conflicts.size(); I != N; ++I) {
+    OS.indent(Indent + 2);
+    OS << "conflict ";
+    OS << Conflicts[I].Other->getFullModuleName();
+    OS << ", \"";
+    OS.write_escaped(Conflicts[I].Message);
+    OS << "\"\n";
+  }
+
   if (InferSubmodules) {
     OS.indent(Indent + 2);
     if (InferExplicitSubmodules)
diff --git a/lib/Frontend/CompilerInstance.cpp b/lib/Frontend/CompilerInstance.cpp
index a20e7d7..633f3c5 100644
--- a/lib/Frontend/CompilerInstance.cpp
+++ b/lib/Frontend/CompilerInstance.cpp
@@ -1007,7 +1007,7 @@
     // Make the named module visible.
     if (LastModuleImportResult)
       ModuleManager->makeModuleVisible(LastModuleImportResult, Visibility,
-                                       ImportLoc);
+                                       ImportLoc, /*Complain=*/false);
     return LastModuleImportResult;
   }
   
@@ -1265,7 +1265,8 @@
       return ModuleLoadResult();
     }
 
-    ModuleManager->makeModuleVisible(Module, Visibility, ImportLoc);
+    ModuleManager->makeModuleVisible(Module, Visibility, ImportLoc,
+                                     /*Complain=*/true);
   }
 
   // Check for any configuration macros that have changed.
@@ -1294,7 +1295,8 @@
 
 void CompilerInstance::makeModuleVisible(Module *Mod,
                                          Module::NameVisibilityKind Visibility,
-                                         SourceLocation ImportLoc){
-  ModuleManager->makeModuleVisible(Mod, Visibility, ImportLoc);
+                                         SourceLocation ImportLoc,
+                                         bool Complain){
+  ModuleManager->makeModuleVisible(Mod, Visibility, ImportLoc, Complain);
 }
 
diff --git a/lib/Lex/ModuleMap.cpp b/lib/Lex/ModuleMap.cpp
index 9cea5aa..0c03201 100644
--- a/lib/Lex/ModuleMap.cpp
+++ b/lib/Lex/ModuleMap.cpp
@@ -45,35 +45,42 @@
     return Module::ExportDecl(0, true);
   }
   
+  // Resolve the module-id.
+  Module *Context = resolveModuleId(Unresolved.Id, Mod, Complain);
+  if (!Context)
+    return Module::ExportDecl();
+
+  return Module::ExportDecl(Context, Unresolved.Wildcard);
+}
+
+Module *ModuleMap::resolveModuleId(const ModuleId &Id, Module *Mod,
+                                   bool Complain) const {
   // Find the starting module.
-  Module *Context = lookupModuleUnqualified(Unresolved.Id[0].first, Mod);
+  Module *Context = lookupModuleUnqualified(Id[0].first, Mod);
   if (!Context) {
     if (Complain)
-      Diags->Report(Unresolved.Id[0].second, 
-                    diag::err_mmap_missing_module_unqualified)
-        << Unresolved.Id[0].first << Mod->getFullModuleName();
-    
-    return Module::ExportDecl();
+      Diags->Report(Id[0].second, diag::err_mmap_missing_module_unqualified)
+      << Id[0].first << Mod->getFullModuleName();
+
+    return 0;
   }
 
   // Dig into the module path.
-  for (unsigned I = 1, N = Unresolved.Id.size(); I != N; ++I) {
-    Module *Sub = lookupModuleQualified(Unresolved.Id[I].first,
-                                        Context);
+  for (unsigned I = 1, N = Id.size(); I != N; ++I) {
+    Module *Sub = lookupModuleQualified(Id[I].first, Context);
     if (!Sub) {
       if (Complain)
-        Diags->Report(Unresolved.Id[I].second, 
-                      diag::err_mmap_missing_module_qualified)
-          << Unresolved.Id[I].first << Context->getFullModuleName()
-          << SourceRange(Unresolved.Id[0].second, Unresolved.Id[I-1].second);
-      
-      return Module::ExportDecl();      
+        Diags->Report(Id[I].second, diag::err_mmap_missing_module_qualified)
+        << Id[I].first << Context->getFullModuleName()
+        << SourceRange(Id[0].second, Id[I-1].second);
+
+      return 0;
     }
-    
+
     Context = Sub;
   }
-  
-  return Module::ExportDecl(Context, Unresolved.Wildcard);
+
+  return Context;
 }
 
 ModuleMap::ModuleMap(FileManager &FileMgr, const DiagnosticConsumer &DC,
@@ -603,6 +610,25 @@
   return HadError;
 }
 
+bool ModuleMap::resolveConflicts(Module *Mod, bool Complain) {
+  bool HadError = false;
+  for (unsigned I = 0, N = Mod->UnresolvedConflicts.size(); I != N; ++I) {
+    Module *OtherMod = resolveModuleId(Mod->UnresolvedConflicts[I].Id,
+                                       Mod, Complain);
+    if (!OtherMod) {
+      HadError = true;
+      continue;
+    }
+
+    Module::Conflict Conflict;
+    Conflict.Other = OtherMod;
+    Conflict.Message = Mod->UnresolvedConflicts[I].Message;
+    Mod->Conflicts.push_back(Conflict);
+  }
+  Mod->UnresolvedConflicts.clear();
+  return HadError;
+}
+
 Module *ModuleMap::inferModuleFromLocation(FullSourceLoc Loc) {
   if (Loc.isInvalid())
     return 0;
@@ -644,6 +670,7 @@
     enum TokenKind {
       Comma,
       ConfigMacros,
+      Conflict,
       EndOfFile,
       HeaderKeyword,
       Identifier,
@@ -744,6 +771,7 @@
     void parseExportDecl();
     void parseLinkDecl();
     void parseConfigMacros();
+    void parseConflict();
     void parseInferredModuleDecl(bool Framework, bool Explicit);
     bool parseOptionalAttributes(Attributes &Attrs);
 
@@ -782,6 +810,7 @@
     Tok.StringLength = LToken.getLength();
     Tok.Kind = llvm::StringSwitch<MMToken::TokenKind>(Tok.getString())
                  .Case("config_macros", MMToken::ConfigMacros)
+                 .Case("conflict", MMToken::Conflict)
                  .Case("exclude", MMToken::ExcludeKeyword)
                  .Case("explicit", MMToken::ExplicitKeyword)
                  .Case("export", MMToken::ExportKeyword)
@@ -1107,6 +1136,10 @@
       parseConfigMacros();
       break;
 
+    case MMToken::Conflict:
+      parseConflict();
+      break;
+
     case MMToken::ExplicitKeyword:
     case MMToken::FrameworkKeyword:
     case MMToken::ModuleKeyword:
@@ -1554,6 +1587,56 @@
   } while (true);
 }
 
+/// \brief Format a module-id into a string.
+static std::string formatModuleId(const ModuleId &Id) {
+  std::string result;
+  {
+    llvm::raw_string_ostream OS(result);
+
+    for (unsigned I = 0, N = Id.size(); I != N; ++I) {
+      if (I)
+        OS << ".";
+      OS << Id[I].first;
+    }
+  }
+
+  return result;
+}
+
+/// \brief Parse a conflict declaration.
+///
+///   module-declaration:
+///     'conflict' module-id ',' string-literal
+void ModuleMapParser::parseConflict() {
+  assert(Tok.is(MMToken::Conflict));
+  SourceLocation ConflictLoc = consumeToken();
+  Module::UnresolvedConflict Conflict;
+
+  // Parse the module-id.
+  if (parseModuleId(Conflict.Id))
+    return;
+
+  // Parse the ','.
+  if (!Tok.is(MMToken::Comma)) {
+    Diags.Report(Tok.getLocation(), diag::err_mmap_expected_conflicts_comma)
+      << SourceRange(ConflictLoc);
+    return;
+  }
+  consumeToken();
+
+  // Parse the message.
+  if (!Tok.is(MMToken::StringLiteral)) {
+    Diags.Report(Tok.getLocation(), diag::err_mmap_expected_conflicts_message)
+      << formatModuleId(Conflict.Id);
+    return;
+  }
+  Conflict.Message = Tok.getString().str();
+  consumeToken();
+
+  // Add this unresolved conflict.
+  ActiveModule->UnresolvedConflicts.push_back(Conflict);
+}
+
 /// \brief Parse an inferred module declaration (wildcard modules).
 ///
 ///   module-declaration:
@@ -1801,6 +1884,7 @@
 
     case MMToken::Comma:
     case MMToken::ConfigMacros:
+    case MMToken::Conflict:
     case MMToken::ExcludeKeyword:
     case MMToken::ExportKeyword:
     case MMToken::HeaderKeyword:
diff --git a/lib/Sema/Sema.cpp b/lib/Sema/Sema.cpp
index 4aeb511..8b17c12 100644
--- a/lib/Sema/Sema.cpp
+++ b/lib/Sema/Sema.cpp
@@ -634,11 +634,12 @@
         Module *Mod = Stack.back();
         Stack.pop_back();
 
-        // Resolve the exported declarations.
+        // Resolve the exported declarations and conflicts.
         // FIXME: Actually complain, once we figure out how to teach the
-        // diagnostic client to deal with complains in the module map at this
+        // diagnostic client to deal with complaints in the module map at this
         // point.
         ModMap.resolveExports(Mod, /*Complain=*/false);
+        ModMap.resolveConflicts(Mod, /*Complain=*/false);
 
         // Queue the submodules, so their exports will also be resolved.
         for (Module::submodule_iterator Sub = Mod->submodule_begin(),
diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index 1cffce0..d046f64 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -11826,7 +11826,8 @@
   Consumer.HandleImplicitImportDecl(ImportD);
 
   // Make the module visible.
-  PP.getModuleLoader().makeModuleVisible(Mod, Module::AllVisible, Loc);
+  PP.getModuleLoader().makeModuleVisible(Mod, Module::AllVisible, Loc,
+                                         /*Complain=*/false);
 }
 
 void Sema::ActOnPragmaRedefineExtname(IdentifierInfo* Name,
diff --git a/lib/Serialization/ASTReader.cpp b/lib/Serialization/ASTReader.cpp
index 126770b..29538a1 100644
--- a/lib/Serialization/ASTReader.cpp
+++ b/lib/Serialization/ASTReader.cpp
@@ -2716,7 +2716,8 @@
 
 void ASTReader::makeModuleVisible(Module *Mod, 
                                   Module::NameVisibilityKind NameVisibility,
-                                  SourceLocation ImportLoc) {
+                                  SourceLocation ImportLoc,
+                                  bool Complain) {
   llvm::SmallPtrSet<Module *, 4> Visited;
   SmallVector<Module *, 4> Stack;
   Stack.push_back(Mod);  
@@ -2764,6 +2765,20 @@
       if (Visited.insert(Exported))
         Stack.push_back(Exported);
     }
+
+    // Detect any conflicts.
+    if (Complain) {
+      assert(ImportLoc.isValid() && "Missing import location");
+      for (unsigned I = 0, N = Mod->Conflicts.size(); I != N; ++I) {
+        if (Mod->Conflicts[I].Other->NameVisibility >= NameVisibility) {
+          Diag(ImportLoc, diag::warn_module_conflict)
+            << Mod->getFullModuleName()
+            << Mod->Conflicts[I].Other->getFullModuleName()
+            << Mod->Conflicts[I].Message;
+          // FIXME: Need note where the other module was imported.
+        }
+      }
+    }
   }
 }
 
@@ -2879,22 +2894,34 @@
     Id->second->setOutOfDate(true);
   
   // Resolve any unresolved module exports.
-  for (unsigned I = 0, N = UnresolvedModuleImportExports.size(); I != N; ++I) {
-    UnresolvedModuleImportExport &Unresolved = UnresolvedModuleImportExports[I];
+  for (unsigned I = 0, N = UnresolvedModuleRefs.size(); I != N; ++I) {
+    UnresolvedModuleRef &Unresolved = UnresolvedModuleRefs[I];
     SubmoduleID GlobalID = getGlobalSubmoduleID(*Unresolved.File,Unresolved.ID);
     Module *ResolvedMod = getSubmodule(GlobalID);
-    
-    if (Unresolved.IsImport) {
+
+    switch (Unresolved.Kind) {
+    case UnresolvedModuleRef::Conflict:
+      if (ResolvedMod) {
+        Module::Conflict Conflict;
+        Conflict.Other = ResolvedMod;
+        Conflict.Message = Unresolved.String.str();
+        Unresolved.Mod->Conflicts.push_back(Conflict);
+      }
+      continue;
+
+    case UnresolvedModuleRef::Import:
       if (ResolvedMod)
         Unresolved.Mod->Imports.push_back(ResolvedMod);
       continue;
-    }
 
-    if (ResolvedMod || Unresolved.IsWildcard)
-      Unresolved.Mod->Exports.push_back(
-        Module::ExportDecl(ResolvedMod, Unresolved.IsWildcard));
+    case UnresolvedModuleRef::Export:
+      if (ResolvedMod || Unresolved.IsWildcard)
+        Unresolved.Mod->Exports.push_back(
+          Module::ExportDecl(ResolvedMod, Unresolved.IsWildcard));
+      continue;
+    }
   }
-  UnresolvedModuleImportExports.clear();
+  UnresolvedModuleRefs.clear();
   
   InitializeContext();
 
@@ -3196,7 +3223,8 @@
   for (unsigned I = 0, N = ImportedModules.size(); I != N; ++I) {
     if (Module *Imported = getSubmodule(ImportedModules[I]))
       makeModuleVisible(Imported, Module::AllVisible,
-                        /*ImportLoc=*/SourceLocation());
+                        /*ImportLoc=*/SourceLocation(),
+                        /*Complain=*/false);
   }
   ImportedModules.clear();
 }
@@ -3534,9 +3562,11 @@
       
       SubmodulesLoaded[GlobalIndex] = CurrentModule;
 
-      // Clear out link libraries and config macros; the module file has them.
+      // Clear out data that will be replaced by what is the module file.
       CurrentModule->LinkLibraries.clear();
       CurrentModule->ConfigMacros.clear();
+      CurrentModule->UnresolvedConflicts.clear();
+      CurrentModule->Conflicts.clear();
       break;
     }
         
@@ -3660,13 +3690,13 @@
         break;
       
       for (unsigned Idx = 0; Idx != Record.size(); ++Idx) {
-        UnresolvedModuleImportExport Unresolved;
+        UnresolvedModuleRef Unresolved;
         Unresolved.File = &F;
         Unresolved.Mod = CurrentModule;
         Unresolved.ID = Record[Idx];
-        Unresolved.IsImport = true;
+        Unresolved.Kind = UnresolvedModuleRef::Import;
         Unresolved.IsWildcard = false;
-        UnresolvedModuleImportExports.push_back(Unresolved);
+        UnresolvedModuleRefs.push_back(Unresolved);
       }
       break;
     }
@@ -3681,13 +3711,13 @@
         break;
       
       for (unsigned Idx = 0; Idx + 1 < Record.size(); Idx += 2) {
-        UnresolvedModuleImportExport Unresolved;
+        UnresolvedModuleRef Unresolved;
         Unresolved.File = &F;
         Unresolved.Mod = CurrentModule;
         Unresolved.ID = Record[Idx];
-        Unresolved.IsImport = false;
+        Unresolved.Kind = UnresolvedModuleRef::Export;
         Unresolved.IsWildcard = Record[Idx + 1];
-        UnresolvedModuleImportExports.push_back(Unresolved);
+        UnresolvedModuleRefs.push_back(Unresolved);
       }
       
       // Once we've loaded the set of exports, there's no reason to keep 
@@ -3733,6 +3763,26 @@
 
       CurrentModule->ConfigMacros.push_back(Blob.str());
       break;
+
+    case SUBMODULE_CONFLICT: {
+      if (First) {
+        Error("missing submodule metadata record at beginning of block");
+        return true;
+      }
+
+      if (!CurrentModule)
+        break;
+
+      UnresolvedModuleRef Unresolved;
+      Unresolved.File = &F;
+      Unresolved.Mod = CurrentModule;
+      Unresolved.ID = Record[0];
+      Unresolved.Kind = UnresolvedModuleRef::Conflict;
+      Unresolved.IsWildcard = false;
+      Unresolved.String = Blob;
+      UnresolvedModuleRefs.push_back(Unresolved);
+      break;
+    }
     }
   }
 }
diff --git a/lib/Serialization/ASTWriter.cpp b/lib/Serialization/ASTWriter.cpp
index de5e6e0..f0b1200 100644
--- a/lib/Serialization/ASTWriter.cpp
+++ b/lib/Serialization/ASTWriter.cpp
@@ -2180,6 +2180,12 @@
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob));    // Macro name
   unsigned ConfigMacroAbbrev = Stream.EmitAbbrev(Abbrev);
 
+  Abbrev = new BitCodeAbbrev();
+  Abbrev->Add(BitCodeAbbrevOp(SUBMODULE_CONFLICT));
+  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6));  // Other module
+  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob));    // Message
+  unsigned ConflictAbbrev = Stream.EmitAbbrev(Abbrev);
+
   // Write the submodule metadata block.
   RecordData Record;
   Record.push_back(getNumberOfModules(WritingModule));
@@ -2295,6 +2301,17 @@
                                 Mod->LinkLibraries[I].Library);
     }
 
+    // Emit the conflicts.
+    for (unsigned I = 0, N = Mod->Conflicts.size(); I != N; ++I) {
+      Record.clear();
+      Record.push_back(SUBMODULE_CONFLICT);
+      unsigned OtherID = getSubmoduleID(Mod->Conflicts[I].Other);
+      assert(OtherID && "Unknown submodule!");
+      Record.push_back(OtherID);
+      Stream.EmitRecordWithBlob(ConflictAbbrev, Record,
+                                Mod->Conflicts[I].Message);
+    }
+
     // Emit the configuration macros.
     for (unsigned I = 0, N =  Mod->ConfigMacros.size(); I != N; ++I) {
       Record.clear();
diff --git a/test/Modules/Inputs/Conflicts/conflict_a.h b/test/Modules/Inputs/Conflicts/conflict_a.h
new file mode 100644
index 0000000..c16b5f5
--- /dev/null
+++ b/test/Modules/Inputs/Conflicts/conflict_a.h
@@ -0,0 +1 @@
+int conflict_a;
diff --git a/test/Modules/Inputs/Conflicts/conflict_b.h b/test/Modules/Inputs/Conflicts/conflict_b.h
new file mode 100644
index 0000000..4baf16f
--- /dev/null
+++ b/test/Modules/Inputs/Conflicts/conflict_b.h
@@ -0,0 +1 @@
+int conflict_b;
diff --git a/test/Modules/Inputs/Conflicts/module.map b/test/Modules/Inputs/Conflicts/module.map
new file mode 100644
index 0000000..e6aafac
--- /dev/null
+++ b/test/Modules/Inputs/Conflicts/module.map
@@ -0,0 +1,10 @@
+module Conflicts {
+  explicit module A {
+    header "conflict_a.h"
+    conflict B, "we just don't like B"
+  }
+
+  module B {
+    header "conflict_b.h"
+  }
+}
diff --git a/test/Modules/conflicts.m b/test/Modules/conflicts.m
new file mode 100644
index 0000000..2388e6f
--- /dev/null
+++ b/test/Modules/conflicts.m
@@ -0,0 +1,7 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -Wauto-import -fmodules-cache-path=%t -fmodules -I %S/Inputs/Conflicts %s -verify
+
+@import Conflicts;
+
+@import Conflicts.A; // expected-warning{{module 'Conflicts.A' conflicts with already-imported module 'Conflicts.B': we just don't like B}}
+
diff --git a/unittests/Basic/SourceManagerTest.cpp b/unittests/Basic/SourceManagerTest.cpp
index 130ea0a..3f09cbb 100644
--- a/unittests/Basic/SourceManagerTest.cpp
+++ b/unittests/Basic/SourceManagerTest.cpp
@@ -61,7 +61,8 @@
 
   virtual void makeModuleVisible(Module *Mod,
                                  Module::NameVisibilityKind Visibility,
-                                 SourceLocation ImportLoc) { }
+                                 SourceLocation ImportLoc,
+                                 bool Complain) { }
 };
 
 TEST_F(SourceManagerTest, isBeforeInTranslationUnit) {
diff --git a/unittests/Lex/LexerTest.cpp b/unittests/Lex/LexerTest.cpp
index 505be75..c9b1840 100644
--- a/unittests/Lex/LexerTest.cpp
+++ b/unittests/Lex/LexerTest.cpp
@@ -62,7 +62,8 @@
 
   virtual void makeModuleVisible(Module *Mod,
                                  Module::NameVisibilityKind Visibility,
-                                 SourceLocation ImportLoc) { }
+                                 SourceLocation ImportLoc,
+                                 bool Complain) { }
 };
 
 TEST_F(LexerTest, LexAPI) {
diff --git a/unittests/Lex/PPCallbacksTest.cpp b/unittests/Lex/PPCallbacksTest.cpp
index 13104cb..36bd5f9 100644
--- a/unittests/Lex/PPCallbacksTest.cpp
+++ b/unittests/Lex/PPCallbacksTest.cpp
@@ -39,7 +39,8 @@
 
   virtual void makeModuleVisible(Module *Mod,
                                  Module::NameVisibilityKind Visibility,
-                                 SourceLocation ImportLoc) { }
+                                 SourceLocation ImportLoc,
+                                 bool Complain) { }
 };
 
 // Stub to collect data from InclusionDirective callbacks.
diff --git a/unittests/Lex/PPConditionalDirectiveRecordTest.cpp b/unittests/Lex/PPConditionalDirectiveRecordTest.cpp
index 856a8ff..082eced 100644
--- a/unittests/Lex/PPConditionalDirectiveRecordTest.cpp
+++ b/unittests/Lex/PPConditionalDirectiveRecordTest.cpp
@@ -62,7 +62,8 @@
 
   virtual void makeModuleVisible(Module *Mod,
                                  Module::NameVisibilityKind Visibility,
-                                 SourceLocation ImportLoc) { }
+                                 SourceLocation ImportLoc,
+                                 bool Complain) { }
 };
 
 TEST_F(PPConditionalDirectiveRecordTest, PPRecAPI) {