Index: cfe/trunk/include/clang/AST/ExternalASTMerger.h
===================================================================
--- cfe/trunk/include/clang/AST/ExternalASTMerger.h
+++ cfe/trunk/include/clang/AST/ExternalASTMerger.h
@@ -14,6 +14,7 @@
 #define LLVM_CLANG_AST_EXTERNALASTMERGER_H
 
 #include "clang/AST/ASTImporter.h"
+#include "clang/AST/ASTImporterSharedState.h"
 #include "clang/AST/ExternalASTSource.h"
 #include "llvm/Support/raw_ostream.h"
 
@@ -88,6 +89,11 @@
 private:
   /// The target for this ExtenralASTMerger.
   ImporterTarget Target;
+  /// ExternalASTMerger has multiple ASTImporters that import into the same
+  /// TU. This is the shared state for all ASTImporters of this
+  /// ExternalASTMerger.
+  /// See also the CrossTranslationUnitContext that has a similar setup.
+  std::shared_ptr<ASTImporterSharedState> SharedState;
 
 public:
   ExternalASTMerger(const ImporterTarget &Target,
Index: cfe/trunk/lib/AST/ExternalASTMerger.cpp
===================================================================
--- cfe/trunk/lib/AST/ExternalASTMerger.cpp
+++ cfe/trunk/lib/AST/ExternalASTMerger.cpp
@@ -107,11 +107,13 @@
   LazyASTImporter(ExternalASTMerger &_Parent, ASTContext &ToContext,
                   FileManager &ToFileManager, ASTContext &FromContext,
                   FileManager &FromFileManager,
-                  const ExternalASTMerger::OriginMap &_FromOrigins)
+                  const ExternalASTMerger::OriginMap &_FromOrigins,
+                  std::shared_ptr<ASTImporterSharedState> SharedState)
       : ASTImporter(ToContext, ToFileManager, FromContext, FromFileManager,
-                    /*MinimalImport=*/true),
+                    /*MinimalImport=*/true, SharedState),
         Parent(_Parent), Reverse(FromContext, FromFileManager, ToContext,
-                                 ToFileManager, /*MinimalImport=*/true), FromOrigins(_FromOrigins) {}
+                                 ToFileManager, /*MinimalImport=*/true),
+        FromOrigins(_FromOrigins) {}
 
   /// Whenever a DeclContext is imported, ensure that ExternalASTSource's origin
   /// map is kept up to date.  Also set the appropriate flags.
@@ -314,6 +316,8 @@
 
 ExternalASTMerger::ExternalASTMerger(const ImporterTarget &Target,
                                      llvm::ArrayRef<ImporterSource> Sources) : LogStream(&llvm::nulls()), Target(Target) {
+  SharedState = std::make_shared<ASTImporterSharedState>(
+      *Target.AST.getTranslationUnitDecl());
   AddSources(Sources);
 }
 
@@ -321,7 +325,7 @@
   for (const ImporterSource &S : Sources) {
     assert(&S.AST != &Target.AST);
     Importers.push_back(std::make_unique<LazyASTImporter>(
-        *this, Target.AST, Target.FM, S.AST, S.FM, S.OM));
+        *this, Target.AST, Target.FM, S.AST, S.FM, S.OM, SharedState));
   }
 }
 
Index: lldb/trunk/packages/Python/lldbsuite/test/functionalities/modern-type-lookup/libcxx/Makefile
===================================================================
--- lldb/trunk/packages/Python/lldbsuite/test/functionalities/modern-type-lookup/libcxx/Makefile
+++ lldb/trunk/packages/Python/lldbsuite/test/functionalities/modern-type-lookup/libcxx/Makefile
@@ -0,0 +1,3 @@
+CXX_SOURCES := main.cpp
+USE_LIBCPP := 1
+include Makefile.rules
Index: lldb/trunk/packages/Python/lldbsuite/test/functionalities/modern-type-lookup/libcxx/TestLibCxxModernTypeLookup.py
===================================================================
--- lldb/trunk/packages/Python/lldbsuite/test/functionalities/modern-type-lookup/libcxx/TestLibCxxModernTypeLookup.py
+++ lldb/trunk/packages/Python/lldbsuite/test/functionalities/modern-type-lookup/libcxx/TestLibCxxModernTypeLookup.py
@@ -0,0 +1,20 @@
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+class LibcxxModernTypeLookup(TestBase):
+
+    mydir = TestBase.compute_mydir(__file__)
+
+    @add_test_categories(["libc++"])
+    def test(self):
+        self.build()
+
+        # Activate modern-type-lookup.
+        self.runCmd("settings set target.experimental.use-modern-type-lookup true")
+
+        lldbutil.run_to_source_breakpoint(self,
+            "// Set break point at this line.", lldb.SBFileSpec("main.cpp"))
+
+        # Test a few simple expressions that should still work with modern-type-lookup.
+        self.expect("expr pair", substrs=["(std::", "pair<int, long", "= (first = 1, second = 2)"])
Index: lldb/trunk/packages/Python/lldbsuite/test/functionalities/modern-type-lookup/libcxx/main.cpp
===================================================================
--- lldb/trunk/packages/Python/lldbsuite/test/functionalities/modern-type-lookup/libcxx/main.cpp
+++ lldb/trunk/packages/Python/lldbsuite/test/functionalities/modern-type-lookup/libcxx/main.cpp
@@ -0,0 +1,6 @@
+#include <utility>
+
+int main(int argc, char **argv) {
+  std::pair<int, long> pair = std::make_pair<int, float>(1, 2L);
+  return pair.first; // Set break point at this line.
+}