Index: include/clang/Basic/DiagnosticCrossTUKinds.td
===================================================================
--- include/clang/Basic/DiagnosticCrossTUKinds.td
+++ include/clang/Basic/DiagnosticCrossTUKinds.td
@@ -15,4 +15,7 @@
 
 def err_multiple_def_index : Error<
   "multiple definitions are found for the same key in index ">;
+
+def err_ctu_incompat_triple : Error<
+  "imported AST from '%0' had been generated for a different target, current: %1, imported: %2">;
 }
Index: include/clang/CrossTU/CrossTranslationUnit.h
===================================================================
--- include/clang/CrossTU/CrossTranslationUnit.h
+++ include/clang/CrossTU/CrossTranslationUnit.h
@@ -41,7 +41,9 @@
   missing_definition,
   failed_import,
   failed_to_get_external_ast,
-  failed_to_generate_usr
+  failed_to_generate_usr,
+  triple_mismatch,
+  lang_mismatch
 };
 
 class IndexError : public llvm::ErrorInfo<IndexError> {
Index: lib/CrossTU/CrossTranslationUnit.cpp
===================================================================
--- lib/CrossTU/CrossTranslationUnit.cpp
+++ lib/CrossTU/CrossTranslationUnit.cpp
@@ -28,6 +28,36 @@
 #include <fstream>
 #include <sstream>
 
+namespace llvm {
+// Same as Triple's equality operator, but we check a field only if that is
+// known in both instances.
+bool hasEqualKnownFields(const Triple &Lhs, const Triple &Rhs) {
+  return ((Lhs.getArch() != Triple::UnknownArch &&
+           Rhs.getArch() != Triple::UnknownArch)
+              ? Lhs.getArch() == Rhs.getArch()
+              : true) &&
+         ((Lhs.getSubArch() != Triple::NoSubArch &&
+           Rhs.getSubArch() != Triple::NoSubArch)
+              ? Lhs.getSubArch() == Rhs.getSubArch()
+              : true) &&
+         ((Lhs.getVendor() != Triple::UnknownVendor &&
+           Rhs.getVendor() != Triple::UnknownVendor)
+              ? Lhs.getVendor() == Rhs.getVendor()
+              : true) &&
+         ((Lhs.getOS() != Triple::UnknownOS && Rhs.getOS() != Triple::UnknownOS)
+              ? Lhs.getOS() == Rhs.getOS()
+              : true) &&
+         ((Lhs.getEnvironment() != Triple::UnknownEnvironment &&
+           Rhs.getEnvironment() != Triple::UnknownEnvironment)
+              ? Lhs.getEnvironment() == Rhs.getEnvironment()
+              : true) &&
+         ((Lhs.getObjectFormat() != Triple::UnknownObjectFormat &&
+           Rhs.getObjectFormat() != Triple::UnknownObjectFormat)
+              ? Lhs.getObjectFormat() == Rhs.getObjectFormat()
+              : true);
+}
+}
+
 namespace clang {
 namespace cross_tu {
 
@@ -55,6 +85,10 @@
       return "Failed to load external AST source.";
     case index_error_code::failed_to_generate_usr:
       return "Failed to generate USR.";
+    case index_error_code::triple_mismatch:
+      return "Triple mismatch";
+    case index_error_code::lang_mismatch:
+      return "Language mismatch";
     }
     llvm_unreachable("Unrecognized index_error_code.");
   }
@@ -166,6 +200,30 @@
   assert(&Unit->getFileManager() ==
          &Unit->getASTContext().getSourceManager().getFileManager());
 
+  const auto& TripleTo = Context.getTargetInfo().getTriple();
+  const auto& TripleFrom = Unit->getASTContext().getTargetInfo().getTriple();
+  // The imported AST had been generated for a different target.
+  // Some parts of the triple in the loaded ASTContext can be unknown while the
+  // very same parts in the target ASTContext are known. Thus we check for the
+  // known parts only.
+  if (!hasEqualKnownFields(TripleTo, TripleFrom)) {
+    // TODO pass the SourceLocation of the CallExpression for more precise
+    // diagnostics
+    Context.getDiagnostics().Report(diag::err_ctu_incompat_triple)
+        << Unit->getMainFileName() << TripleTo.str() << TripleFrom.str();
+    // TODO Add statistics here
+    return llvm::make_error<IndexError>(index_error_code::triple_mismatch);
+  }
+
+  const auto& LangTo = Context.getLangOpts();
+  const auto& LangFrom = Unit->getASTContext().getLangOpts();
+  // FIXME: Currenty we do not support CTU across C++ and C and across
+  // different dialects of C++.
+  if (LangTo.CPlusPlus != LangFrom.CPlusPlus) {
+    // TODO Add statistics here.
+    return llvm::make_error<IndexError>(index_error_code::lang_mismatch);
+  }
+
   TranslationUnitDecl *TU = Unit->getASTContext().getTranslationUnitDecl();
   if (const FunctionDecl *ResultDecl =
           findFunctionInDeclContext(TU, LookupFnName))