Index: lib/AST/ASTImporter.cpp
===================================================================
--- lib/AST/ASTImporter.cpp
+++ lib/AST/ASTImporter.cpp
@@ -2305,9 +2305,16 @@
       if (!FoundDecl->isInIdentifierNamespace(IDNS))
         continue;
       if (auto *FoundTypedef = dyn_cast<TypedefNameDecl>(FoundDecl)) {
-        if (Importer.IsStructurallyEquivalent(
-                D->getUnderlyingType(), FoundTypedef->getUnderlyingType()))
+        QualType FromUT = D->getUnderlyingType();
+        QualType FoundUT = FoundTypedef->getUnderlyingType();
+        if (Importer.IsStructurallyEquivalent(FromUT, FoundUT)) {
+          // If the "From" context has a complete underlying type but we
+          // already have a complete underlying type then return with that.
+          if (!FromUT->isIncompleteType() && !FoundUT->isIncompleteType())
             return Importer.MapImported(D, FoundTypedef);
+        }
+        // FIXME Handle redecl chain.
+        break;
       }
 
       ConflictingDecls.push_back(FoundDecl);
Index: unittests/AST/ASTImporterTest.cpp
===================================================================
--- unittests/AST/ASTImporterTest.cpp
+++ unittests/AST/ASTImporterTest.cpp
@@ -3726,6 +3726,38 @@
   EXPECT_EQ(To1->getPreviousDecl(), To0);
 }
 
+TEST_P(ASTImporterTestBase, ImportingTypedefShouldImportTheCompleteType) {
+  // We already have an incomplete underlying type in the "To" context.
+  auto Code =
+      R"(
+      template <typename T>
+      struct S {
+        void foo();
+      };
+      using U = S<int>;
+      )";
+  Decl *ToTU = getToTuDecl(Code, Lang_CXX11);
+  auto *ToD = FirstDeclMatcher<TypedefNameDecl>().match(ToTU,
+      typedefNameDecl(hasName("U")));
+  ASSERT_TRUE(ToD->getUnderlyingType()->isIncompleteType());
+
+  // The "From" context has the same typedef, but the underlying type is
+  // complete this time.
+  Decl *FromTU = getTuDecl(std::string(Code) +
+      R"(
+      void foo(U* u) {
+        u->foo();
+      }
+      )", Lang_CXX11);
+  auto *FromD = FirstDeclMatcher<TypedefNameDecl>().match(FromTU,
+      typedefNameDecl(hasName("U")));
+  ASSERT_FALSE(FromD->getUnderlyingType()->isIncompleteType());
+
+  // The imported type should be complete.
+  auto *ImportedD = cast<TypedefNameDecl>(Import(FromD, Lang_CXX11));
+  EXPECT_FALSE(ImportedD->getUnderlyingType()->isIncompleteType());
+}
+
 INSTANTIATE_TEST_CASE_P(ParameterizedTests, DeclContextTest,
                         ::testing::Values(ArgVector()), );