diff --git a/clang/lib/AST/ExternalASTMerger.cpp b/clang/lib/AST/ExternalASTMerger.cpp
index 1dc472a..b746eda 100644
--- a/clang/lib/AST/ExternalASTMerger.cpp
+++ b/clang/lib/AST/ExternalASTMerger.cpp
@@ -41,6 +41,7 @@
   Decl *Imported(Decl *From, Decl *To) override {
     if (auto ToTag = dyn_cast<TagDecl>(To)) {
       ToTag->setHasExternalLexicalStorage();
+      ToTag->setMustBuildLookupTable();
     } else if (auto ToNamespace = dyn_cast<NamespaceDecl>(To)) {
       ToNamespace->setHasExternalVisibleStorage();
     }