diff --git a/include/clang/Sema/ExternalSemaSource.h b/include/clang/Sema/ExternalSemaSource.h
index a626a09..fd32d7f 100644
--- a/include/clang/Sema/ExternalSemaSource.h
+++ b/include/clang/Sema/ExternalSemaSource.h
@@ -14,6 +14,7 @@
 #define LLVM_CLANG_SEMA_EXTERNAL_SEMA_SOURCE_H
 
 #include "clang/AST/ExternalASTSource.h"
+#include "clang/Sema/TypoCorrection.h"
 #include "clang/Sema/Weak.h"
 #include "llvm/ADT/MapVector.h"
 #include <utility>
@@ -187,6 +188,22 @@
   virtual void ReadLateParsedTemplates(
       llvm::DenseMap<const FunctionDecl *, LateParsedTemplate *> &LPTMap) {}
 
+  /// \copydoc Sema::CorrectTypo
+  /// \note LookupKind must correspond to a valid Sema::LookupNameKind
+  ///
+  /// ExternalSemaSource::CorrectTypo is always given the first chance to
+  /// correct a typo (really, to offer suggestions to repair a failed lookup).
+  /// It will even be called when SpellChecking is turned off or after a
+  /// fatal error has already been detected.
+  virtual TypoCorrection CorrectTypo(const DeclarationNameInfo &Typo,
+                                     int LookupKind, Scope *S, CXXScopeSpec *SS,
+                                     CorrectionCandidateCallback &CCC,
+                                     DeclContext *MemberContext,
+                                     bool EnteringContext,
+                                     const ObjCObjectPointerType *OPT) {
+    return TypoCorrection();
+  }
+
   // isa/cast/dyn_cast support
   static bool classof(const ExternalASTSource *Source) {
     return Source->SemaSource;
diff --git a/include/clang/Sema/MultiplexExternalSemaSource.h b/include/clang/Sema/MultiplexExternalSemaSource.h
index 0334007..bb7ba23 100644
--- a/include/clang/Sema/MultiplexExternalSemaSource.h
+++ b/include/clang/Sema/MultiplexExternalSemaSource.h
@@ -331,6 +331,15 @@
   virtual void ReadLateParsedTemplates(
       llvm::DenseMap<const FunctionDecl *, LateParsedTemplate *> &LPTMap);
 
+  /// \copydoc ExternalSemaSource::CorrectTypo
+  /// \note Returns the first nonempty correction.
+  virtual TypoCorrection CorrectTypo(const DeclarationNameInfo &Typo,
+                                     int LookupKind, Scope *S, CXXScopeSpec *SS,
+                                     CorrectionCandidateCallback &CCC,
+                                     DeclContext *MemberContext,
+                                     bool EnteringContext,
+                                     const ObjCObjectPointerType *OPT);
+
   // isa/cast/dyn_cast support
   static bool classof(const MultiplexExternalSemaSource*) { return true; }
   //static bool classof(const ExternalSemaSource*) { return true; }
diff --git a/lib/Sema/MultiplexExternalSemaSource.cpp b/lib/Sema/MultiplexExternalSemaSource.cpp
index de790e9..43bafd3 100644
--- a/lib/Sema/MultiplexExternalSemaSource.cpp
+++ b/lib/Sema/MultiplexExternalSemaSource.cpp
@@ -273,3 +273,19 @@
   for (size_t i = 0; i < Sources.size(); ++i)
     Sources[i]->ReadLateParsedTemplates(LPTMap);
 }
+
+TypoCorrection MultiplexExternalSemaSource::CorrectTypo(
+                                     const DeclarationNameInfo &Typo,
+                                     int LookupKind, Scope *S, CXXScopeSpec *SS,
+                                     CorrectionCandidateCallback &CCC,
+                                     DeclContext *MemberContext,
+                                     bool EnteringContext,
+                                     const ObjCObjectPointerType *OPT) {
+  for (size_t I = 0, E = Sources.size(); I < E; ++I) {
+    if (TypoCorrection C = Sources[I]->CorrectTypo(Typo, LookupKind, S, SS, CCC,
+                                                   MemberContext,
+                                                   EnteringContext, OPT))
+      return C;
+  }
+  return TypoCorrection();
+}
diff --git a/lib/Sema/SemaLookup.cpp b/lib/Sema/SemaLookup.cpp
index e95ffce..d0bb9f6 100644
--- a/lib/Sema/SemaLookup.cpp
+++ b/lib/Sema/SemaLookup.cpp
@@ -3872,6 +3872,14 @@
                                  DeclContext *MemberContext,
                                  bool EnteringContext,
                                  const ObjCObjectPointerType *OPT) {
+  // Always let the ExternalSource have the first chance at correction, even
+  // if we would otherwise have given up.
+  if (ExternalSource) {
+    if (TypoCorrection Correction = ExternalSource->CorrectTypo(
+        TypoName, LookupKind, S, SS, CCC, MemberContext, EnteringContext, OPT))
+      return Correction;
+  }
+
   if (Diags.hasFatalErrorOccurred() || !getLangOpts().SpellChecking)
     return TypoCorrection();
 
diff --git a/unittests/CMakeLists.txt b/unittests/CMakeLists.txt
index fed775e..479b36f 100644
--- a/unittests/CMakeLists.txt
+++ b/unittests/CMakeLists.txt
@@ -19,4 +19,5 @@
   add_subdirectory(AST)
   add_subdirectory(Tooling)
   add_subdirectory(Format)
+  add_subdirectory(Sema)
 endif()
diff --git a/unittests/Makefile b/unittests/Makefile
index e01a6ac..542863e 100644
--- a/unittests/Makefile
+++ b/unittests/Makefile
@@ -23,7 +23,7 @@
 endif
 
 ifeq ($(ENABLE_CLANG_REWRITER),1)
-PARALLEL_DIRS += ASTMatchers AST Tooling
+PARALLEL_DIRS += ASTMatchers AST Tooling Sema
 endif
 
 ifeq ($(ENABLE_CLANG_STATIC_ANALYZER),1)