Index: lld/trunk/include/lld/Core/LinkingContext.h
===================================================================
--- lld/trunk/include/lld/Core/LinkingContext.h
+++ lld/trunk/include/lld/Core/LinkingContext.h
@@ -307,6 +307,15 @@
   // Derived classes may use it to change the list of input files.
   virtual void finalizeInputFiles() {}
 
+  /// Callback invoked for each file the Resolver decides we are going to load.
+  /// This can be used to update context state based on the file, and emit
+  /// errors for any differences between the context state and a loaded file.
+  /// For example, we can error if we try to load a file which is a different
+  /// arch from that being linked.
+  virtual std::error_code handleLoadedFile(File &file) {
+    return std::error_code();
+  }
+
   TaskGroup &getTaskGroup() { return _taskGroup; }
 
   /// @}
Index: lld/trunk/include/lld/Core/Resolver.h
===================================================================
--- lld/trunk/include/lld/Core/Resolver.h
+++ lld/trunk/include/lld/Core/Resolver.h
@@ -17,6 +17,7 @@
 #include "lld/Core/SymbolTable.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
+#include "llvm/Support/ErrorOr.h"
 #include <set>
 #include <unordered_map>
 #include <unordered_set>
@@ -43,13 +44,13 @@
 
   // Handle files, this adds atoms from the current file thats
   // being processed by the resolver
-  bool handleFile(File &);
+  ErrorOr<bool> handleFile(File &);
 
   // Handle an archive library file.
-  bool handleArchiveFile(File &);
+  ErrorOr<bool> handleArchiveFile(File &);
 
   // Handle a shared library file.
-  void handleSharedLibrary(File &);
+  std::error_code handleSharedLibrary(File &);
 
   /// @brief do work of merging and resolving and return list
   bool resolve();
@@ -57,7 +58,7 @@
   std::unique_ptr<SimpleFile> resultFile() { return std::move(_result); }
 
 private:
-  typedef std::function<void(StringRef, bool)> UndefCallback;
+  typedef std::function<ErrorOr<bool>(StringRef, bool)> UndefCallback;
 
   bool undefinesAdded(int begin, int end);
   File *getFile(int &index);
@@ -73,7 +74,8 @@
   bool checkUndefines();
   void removeCoalescedAwayAtoms();
   void checkDylibSymbolCollisions();
-  void forEachUndefines(File &file, bool searchForOverrides, UndefCallback callback);
+  ErrorOr<bool> forEachUndefines(File &file, bool searchForOverrides,
+                                 UndefCallback callback);
 
   void markLive(const Atom *atom);
   void addAtoms(const std::vector<const DefinedAtom *>&);
Index: lld/trunk/include/lld/ReaderWriter/MachOLinkingContext.h
===================================================================
--- lld/trunk/include/lld/ReaderWriter/MachOLinkingContext.h
+++ lld/trunk/include/lld/ReaderWriter/MachOLinkingContext.h
@@ -342,6 +342,8 @@
 
   void finalizeInputFiles() override;
 
+  std::error_code handleLoadedFile(File &file) override;
+
   bool customAtomOrderer(const DefinedAtom *left, const DefinedAtom *right,
                          bool &leftBeforeRight) const;
 
Index: lld/trunk/lib/Core/Resolver.cpp
===================================================================
--- lld/trunk/lib/Core/Resolver.cpp
+++ lld/trunk/lib/Core/Resolver.cpp
@@ -29,7 +29,9 @@
 
 namespace lld {
 
-bool Resolver::handleFile(File &file) {
+ErrorOr<bool> Resolver::handleFile(File &file) {
+  if (auto ec = _ctx.handleLoadedFile(file))
+    return ec;
   bool undefAdded = false;
   for (const DefinedAtom *atom : file.defined())
     doDefinedAtom(*atom);
@@ -46,9 +48,10 @@
   return undefAdded;
 }
 
-void Resolver::forEachUndefines(File &file, bool searchForOverrides,
-                                UndefCallback callback) {
+ErrorOr<bool> Resolver::forEachUndefines(File &file, bool searchForOverrides,
+                                         UndefCallback callback) {
   size_t i = _undefineIndex[&file];
+  bool undefAdded = false;
   do {
     for (; i < _undefines.size(); ++i) {
       StringRef undefName = _undefines[i];
@@ -60,7 +63,10 @@
         _undefines[i] = "";
         continue;
       }
-      callback(undefName, false);
+      auto undefAddedOrError = callback(undefName, false);
+      if (undefAddedOrError.getError())
+        return undefAddedOrError;
+      undefAdded |= undefAddedOrError.get();
     }
     if (!searchForOverrides)
       continue;
@@ -69,43 +75,57 @@
       // something that overrode this tentative, so always check.
       const Atom *curAtom = _symbolTable.findByName(tentDefName);
       assert(curAtom != nullptr);
-      if (const DefinedAtom *curDefAtom = dyn_cast<DefinedAtom>(curAtom))
-        if (curDefAtom->merge() == DefinedAtom::mergeAsTentative)
-          callback(tentDefName, true);
+      if (const DefinedAtom *curDefAtom = dyn_cast<DefinedAtom>(curAtom)) {
+        if (curDefAtom->merge() == DefinedAtom::mergeAsTentative) {
+          auto undefAddedOrError = callback(tentDefName, true);
+          if (undefAddedOrError.getError())
+            return undefAddedOrError;
+          undefAdded |= undefAddedOrError.get();
+        }
+      }
     }
   } while (i < _undefines.size());
   _undefineIndex[&file] = i;
+  return undefAdded;
 }
 
-bool Resolver::handleArchiveFile(File &file) {
+ErrorOr<bool> Resolver::handleArchiveFile(File &file) {
   ArchiveLibraryFile *archiveFile = cast<ArchiveLibraryFile>(&file);
   bool searchForOverrides =
       _ctx.searchArchivesToOverrideTentativeDefinitions();
-  bool undefAdded = false;
-  forEachUndefines(file, searchForOverrides,
-                   [&](StringRef undefName, bool dataSymbolOnly) {
+  return forEachUndefines(file, searchForOverrides,
+                          [&](StringRef undefName,
+                              bool dataSymbolOnly)->ErrorOr<bool> {
     if (File *member = archiveFile->find(undefName, dataSymbolOnly)) {
       member->setOrdinal(_ctx.getNextOrdinalAndIncrement());
       member->beforeLink();
       updatePreloadArchiveMap();
-      undefAdded = handleFile(*member) || undefAdded;
+      return handleFile(*member);
     }
+    return false;
   });
-  return undefAdded;
 }
 
-void Resolver::handleSharedLibrary(File &file) {
+std::error_code Resolver::handleSharedLibrary(File &file) {
   // Add all the atoms from the shared library
   SharedLibraryFile *sharedLibrary = cast<SharedLibraryFile>(&file);
-  handleFile(*sharedLibrary);
+  auto undefAddedOrError = handleFile(*sharedLibrary);
+  if (undefAddedOrError.getError())
+    return undefAddedOrError.getError();
   bool searchForOverrides =
       _ctx.searchSharedLibrariesToOverrideTentativeDefinitions();
-  forEachUndefines(file, searchForOverrides,
-                   [&](StringRef undefName, bool dataSymbolOnly) {
+  undefAddedOrError = forEachUndefines(file, searchForOverrides,
+                                       [&](StringRef undefName,
+                                           bool dataSymbolOnly)->ErrorOr<bool> {
     if (const SharedLibraryAtom *atom =
             sharedLibrary->exports(undefName, dataSymbolOnly))
       doSharedLibraryAtom(*atom);
+    return false;
   });
+
+  if (undefAddedOrError.getError())
+    return undefAddedOrError.getError();
+  return std::error_code();
 }
 
 bool Resolver::doUndefinedAtom(const UndefinedAtom &atom) {
@@ -318,7 +338,7 @@
     file->beforeLink();
     updatePreloadArchiveMap();
     switch (file->kind()) {
-    case File::kindObject:
+    case File::kindObject: {
       // The same file may be visited more than once if the file is
       // in --start-group and --end-group. Only library files should
       // be processed more than once.
@@ -327,17 +347,35 @@
       seen.insert(file);
       assert(!file->hasOrdinal());
       file->setOrdinal(_ctx.getNextOrdinalAndIncrement());
-      undefAdded = handleFile(*file);
+      auto undefAddedOrError = handleFile(*file);
+      if (undefAddedOrError.getError()) {
+        llvm::errs() << "Error in " + file->path()
+                     << ": " << undefAddedOrError.getError().message() << "\n";
+        return false;
+      }
+      undefAdded = undefAddedOrError.get();
       break;
-    case File::kindArchiveLibrary:
+    }
+    case File::kindArchiveLibrary: {
       if (!file->hasOrdinal())
         file->setOrdinal(_ctx.getNextOrdinalAndIncrement());
-      undefAdded = handleArchiveFile(*file);
+      auto undefAddedOrError = handleArchiveFile(*file);
+      if (undefAddedOrError.getError()) {
+        llvm::errs() << "Error in " + file->path()
+                     << ": " << undefAddedOrError.getError().message() << "\n";
+        return false;
+      }
+      undefAdded = undefAddedOrError.get();
       break;
+    }
     case File::kindSharedLibrary:
       if (!file->hasOrdinal())
         file->setOrdinal(_ctx.getNextOrdinalAndIncrement());
-      handleSharedLibrary(*file);
+      if (auto EC = handleSharedLibrary(*file)) {
+        llvm::errs() << "Error in " + file->path()
+                     << ": " << EC.message() << "\n";
+        return false;
+      }
       break;
     }
     _newUndefinesAdded[file] = undefAdded;
Index: lld/trunk/lib/ReaderWriter/MachO/MachOLinkingContext.cpp
===================================================================
--- lld/trunk/lib/ReaderWriter/MachO/MachOLinkingContext.cpp
+++ lld/trunk/lib/ReaderWriter/MachO/MachOLinkingContext.cpp
@@ -990,4 +990,8 @@
   elements.push_back(llvm::make_unique<GroupEnd>(numLibs));
 }
 
+std::error_code MachOLinkingContext::handleLoadedFile(File &file) {
+  return std::error_code();
+}
+
 } // end namespace lld