Index: lib/AST/CMakeLists.txt
===================================================================
--- lib/AST/CMakeLists.txt
+++ lib/AST/CMakeLists.txt
@@ -31,6 +31,7 @@
   ExprConstant.cpp
   ExprCXX.cpp
   ExprObjC.cpp
+  ExternalASTMerger.cpp
   ExternalASTSource.cpp
   InheritViz.cpp
   ItaniumCXXABI.cpp
Index: test/Import/forward-declared-struct/Inputs/S1.c
===================================================================
--- test/Import/forward-declared-struct/Inputs/S1.c
+++ test/Import/forward-declared-struct/Inputs/S1.c
@@ -0,0 +1 @@
+struct S;
Index: test/Import/forward-declared-struct/Inputs/S2.c
===================================================================
--- test/Import/forward-declared-struct/Inputs/S2.c
+++ test/Import/forward-declared-struct/Inputs/S2.c
@@ -0,0 +1,3 @@
+struct S {
+  int a;
+};
Index: test/Import/forward-declared-struct/test.c
===================================================================
--- test/Import/forward-declared-struct/test.c
+++ test/Import/forward-declared-struct/test.c
@@ -0,0 +1,5 @@
+// RUN: clang-import-test -import %S/Inputs/S1.c --import %S/Inputs/S2.c -expression %s
+void expr() {
+  struct S MyS;
+  MyS.a = 3;
+}
Index: test/Import/member-in-struct/Inputs/S.c
===================================================================
--- test/Import/member-in-struct/Inputs/S.c
+++ test/Import/member-in-struct/Inputs/S.c
@@ -0,0 +1,3 @@
+struct S {
+  int a;
+};
Index: test/Import/member-in-struct/test.c
===================================================================
--- test/Import/member-in-struct/test.c
+++ test/Import/member-in-struct/test.c
@@ -0,0 +1,5 @@
+// RUN: clang-import-test -import %S/Inputs/S.c -expression %s
+void expr() {
+  struct S MyS;
+  MyS.a = 3;
+}
Index: test/Import/multiple-forward-declarations/Inputs/S1.c
===================================================================
--- test/Import/multiple-forward-declarations/Inputs/S1.c
+++ test/Import/multiple-forward-declarations/Inputs/S1.c
@@ -0,0 +1 @@
+struct S;
Index: test/Import/multiple-forward-declarations/Inputs/S2.c
===================================================================
--- test/Import/multiple-forward-declarations/Inputs/S2.c
+++ test/Import/multiple-forward-declarations/Inputs/S2.c
@@ -0,0 +1 @@
+struct S;
Index: test/Import/multiple-forward-declarations/test.c
===================================================================
--- test/Import/multiple-forward-declarations/test.c
+++ test/Import/multiple-forward-declarations/test.c
@@ -0,0 +1,4 @@
+// RUN: clang-import-test -import %S/Inputs/S1.c --import %S/Inputs/S2.c -expression %s
+void expr() {
+  struct S *MySPtr;
+}
Index: test/Import/overloaded-function/Inputs/F1.c
===================================================================
--- test/Import/overloaded-function/Inputs/F1.c
+++ test/Import/overloaded-function/Inputs/F1.c
@@ -0,0 +1 @@
+void f(int arg) { }
Index: test/Import/overloaded-function/Inputs/F2.c
===================================================================
--- test/Import/overloaded-function/Inputs/F2.c
+++ test/Import/overloaded-function/Inputs/F2.c
@@ -0,0 +1,4 @@
+struct S { int a; };
+
+void f(const char *arg) { }
+void f(S arg) { }
Index: test/Import/overloaded-function/test.c
===================================================================
--- test/Import/overloaded-function/test.c
+++ test/Import/overloaded-function/test.c
@@ -0,0 +1,7 @@
+// RUN: clang-import-test -import %S/Inputs/F1.c -import %S/Inputs/F2.c -expression %s
+void expr() {
+  f(2);
+  f("world");
+  S s;
+  f(s);
+}
Index: test/Import/struct-in-namespace/Inputs/N1.cpp
===================================================================
--- test/Import/struct-in-namespace/Inputs/N1.cpp
+++ test/Import/struct-in-namespace/Inputs/N1.cpp
@@ -0,0 +1,11 @@
+namespace N {
+  struct S {
+    int a;
+  };
+}
+
+namespace N {
+  struct T {
+    int b;
+  };
+}
Index: test/Import/struct-in-namespace/Inputs/N2.cpp
===================================================================
--- test/Import/struct-in-namespace/Inputs/N2.cpp
+++ test/Import/struct-in-namespace/Inputs/N2.cpp
@@ -0,0 +1,5 @@
+namespace N {
+  struct U {
+    int c;
+  };
+}
Index: test/Import/struct-in-namespace/Inputs/N3.cpp
===================================================================
--- test/Import/struct-in-namespace/Inputs/N3.cpp
+++ test/Import/struct-in-namespace/Inputs/N3.cpp
@@ -0,0 +1,5 @@
+namespace M {
+  struct V {
+    int d;
+  };
+}
Index: test/Import/struct-in-namespace/test.cpp
===================================================================
--- test/Import/struct-in-namespace/test.cpp
+++ test/Import/struct-in-namespace/test.cpp
@@ -0,0 +1,7 @@
+// RUN: clang-import-test -import %S/Inputs/N1.cpp -import %S/Inputs/N2.cpp -import %S/Inputs/N3.cpp -expression %s
+void expr() {
+  N::S s;
+  N::T t;
+  N::U u;
+  int d = s.a + t.b + u.c;
+}
Index: test/Import/template-specialization/Inputs/T.cpp
===================================================================
--- test/Import/template-specialization/Inputs/T.cpp
+++ test/Import/template-specialization/Inputs/T.cpp
@@ -0,0 +1,14 @@
+template <typename T> struct A {
+};
+
+template <> struct A<int> {
+  struct B {
+    int f;
+  };
+};
+
+template <> struct A<bool> {
+  struct B {
+    int g;
+  };
+};
Index: test/Import/template-specialization/test.cpp
===================================================================
--- test/Import/template-specialization/test.cpp
+++ test/Import/template-specialization/test.cpp
@@ -0,0 +1,7 @@
+// RUN: clang-import-test -import %S/Inputs/T.cpp -expression %s
+// XFAIL: *
+void expr() {
+  A<int>::B b1;
+  A<bool>::B b2;
+  b1.f + b2.g;
+}
Index: tools/clang-import-test/clang-import-test.cpp
===================================================================
--- tools/clang-import-test/clang-import-test.cpp
+++ tools/clang-import-test/clang-import-test.cpp
@@ -9,6 +9,8 @@
 
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTImporter.h"
+#include "clang/AST/DeclObjC.h"
+#include "clang/AST/ExternalASTMerger.h"
 #include "clang/Basic/Builtins.h"
 #include "clang/Basic/IdentifierTable.h"
 #include "clang/Basic/SourceLocation.h"
@@ -189,61 +191,18 @@
 } // end namespace
 
 namespace {
-class TestExternalASTSource : public ExternalASTSource {
-private:
-  llvm::ArrayRef<std::unique_ptr<CompilerInstance>> ImportCIs;
-  std::map<CompilerInstance *, std::unique_ptr<ASTImporter>> ForwardImporters;
-  std::map<CompilerInstance *, std::unique_ptr<ASTImporter>> ReverseImporters;
-
-public:
-  TestExternalASTSource(
-      CompilerInstance &ExpressionCI,
-      llvm::ArrayRef<std::unique_ptr<CompilerInstance>> ImportCIs)
-      : ImportCIs(ImportCIs) {
-    for (const std::unique_ptr<CompilerInstance> &ImportCI : ImportCIs) {
-      ForwardImporters[ImportCI.get()] = llvm::make_unique<ASTImporter>(
-          ExpressionCI.getASTContext(), ExpressionCI.getFileManager(),
-          ImportCI->getASTContext(), ImportCI->getFileManager(),
-          /*MinimalImport=*/true);
-      ReverseImporters[ImportCI.get()] = llvm::make_unique<ASTImporter>(
-          ImportCI->getASTContext(), ImportCI->getFileManager(),
-          ExpressionCI.getASTContext(), ExpressionCI.getFileManager(),
-          /*MinimalImport=*/true);
-    }
-  }
-
-  bool FindExternalVisibleDeclsByName(const DeclContext *DC,
-                                      DeclarationName Name) override {
-    llvm::SmallVector<NamedDecl *, 1> Decls;
-
-    if (isa<TranslationUnitDecl>(DC)) {
-      for (const std::unique_ptr<CompilerInstance> &I : ImportCIs) {
-        DeclarationName FromName = ReverseImporters[I.get()]->Import(Name);
-        DeclContextLookupResult Result =
-            I->getASTContext().getTranslationUnitDecl()->lookup(FromName);
-        for (NamedDecl *FromD : Result) {
-          NamedDecl *D =
-              llvm::cast<NamedDecl>(ForwardImporters[I.get()]->Import(FromD));
-          Decls.push_back(D);
-        }
-      }
-    }
-    if (Decls.empty()) {
-      return false;
-    } else {
-      SetExternalVisibleDeclsForName(DC, Name, Decls);
-      return true;
-    }
-  }
-};
-
+ 
 void AddExternalSource(
     CompilerInstance &CI,
     llvm::ArrayRef<std::unique_ptr<CompilerInstance>> Imports) {
-  ASTContext &AST = CI.getASTContext();
-  auto ES = llvm::make_unique<TestExternalASTSource>(CI, Imports);
-  AST.setExternalSource(ES.release());
-  AST.getTranslationUnitDecl()->setHasExternalVisibleStorage();
+  ExternalASTMerger::ImporterEndpoint Target({CI.getASTContext(), CI.getFileManager()});
+  llvm::SmallVector<ExternalASTMerger::ImporterEndpoint, 3> Sources;
+  for (const std::unique_ptr<CompilerInstance> &CI : Imports) {
+    Sources.push_back({CI->getASTContext(), CI->getFileManager()});
+  }
+  auto ES = llvm::make_unique<ExternalASTMerger>(Target, Sources);
+  CI.getASTContext().setExternalSource(ES.release());
+  CI.getASTContext().getTranslationUnitDecl()->setHasExternalVisibleStorage();
 }
 
 llvm::Error ParseSource(const std::string &Path, CompilerInstance &CI,
@@ -292,6 +251,7 @@
     return std::move(CI);
   }
 }
+
 } // end namespace
 
 int main(int argc, const char **argv) {