diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -8176,15 +8176,22 @@
   // need it to have a definition.
   if (auto *ToRecord = dyn_cast<RecordDecl>(ToDC)) {
     auto *FromRecord = cast<RecordDecl>(FromDC);
-    if (ToRecord->isCompleteDefinition()) {
-      // Do nothing.
-    } else if (FromRecord->isCompleteDefinition()) {
+    if (ToRecord->isCompleteDefinition())
+      return ToDC;
+
+    // If FromRecord is not defined we need to force it to be.
+    // Simply calling CompleteDecl(...) for a RecordDecl will break some cases
+    // it will start the definition but we never finish it.
+    // If there are base classes they won't be imported and we will
+    // be missing anything that we inherit from those bases.
+    if (FromRecord->getASTContext().getExternalSource() &&
+        !FromRecord->isCompleteDefinition())
+      FromRecord->getASTContext().getExternalSource()->CompleteType(FromRecord);
+
+    if (FromRecord->isCompleteDefinition())
       if (Error Err = ASTNodeImporter(*this).ImportDefinition(
           FromRecord, ToRecord, ASTNodeImporter::IDK_Basic))
         return std::move(Err);
-    } else {
-      CompleteDecl(ToRecord);
-    }
   } else if (auto *ToEnum = dyn_cast<EnumDecl>(ToDC)) {
     auto *FromEnum = cast<EnumDecl>(FromDC);
     if (ToEnum->isCompleteDefinition()) {
diff --git a/clang/unittests/AST/ASTImporterTest.cpp b/clang/unittests/AST/ASTImporterTest.cpp
--- a/clang/unittests/AST/ASTImporterTest.cpp
+++ b/clang/unittests/AST/ASTImporterTest.cpp
@@ -5942,6 +5942,70 @@
   return ::testing::ValuesIn(Copy);
 }
 
+struct ImportWithExternalSource : ASTImporterOptionSpecificTestBase {
+  ImportWithExternalSource() {
+    Creator = [](ASTContext &ToContext, FileManager &ToFileManager,
+                 ASTContext &FromContext, FileManager &FromFileManager,
+                 bool MinimalImport,
+                 const std::shared_ptr<ASTImporterSharedState> &SharedState) {
+      return new ASTImporter(ToContext, ToFileManager, FromContext,
+                             FromFileManager, MinimalImport,
+                             // We use the regular lookup.
+                             /*SharedState=*/nullptr);
+    };
+  }
+};
+
+/// An ExternalASTSource that keeps track of the tags is completed.
+struct SourceWithCompletedTagList : clang::ExternalASTSource {
+  std::vector<clang::TagDecl *> &CompletedTags;
+  SourceWithCompletedTagList(std::vector<clang::TagDecl *> &CompletedTags)
+      : CompletedTags(CompletedTags) {}
+  void CompleteType(TagDecl *Tag) override {
+    auto *Record = cast<CXXRecordDecl>(Tag);
+    Record->startDefinition();
+    Record->completeDefinition();
+    CompletedTags.push_back(Tag);
+  }
+  void
+  FindExternalLexicalDecls(const DeclContext *DC,
+                           llvm::function_ref<bool(Decl::Kind)> IsKindWeWant,
+                           SmallVectorImpl<Decl *> &Result) override {}
+};
+
+TEST_P(ImportWithExternalSource, CompleteRecordBeforeImporting) {
+  // Create an empty TU.
+  TranslationUnitDecl *FromTU = getTuDecl("", Lang_CXX, "input.cpp");
+
+  // Create and add the test ExternalASTSource.
+  std::vector<clang::TagDecl *> CompletedTags;
+  IntrusiveRefCntPtr<ExternalASTSource> source =
+      new SourceWithCompletedTagList(CompletedTags);
+  clang::ASTContext &Context = FromTU->getASTContext();
+  Context.setExternalSource(std::move(source));
+
+  // Create a dummy class by hand with external lexical storage.
+  IdentifierInfo &Ident = Context.Idents.get("test_class");
+  auto *Record = CXXRecordDecl::Create(
+      Context, TTK_Class, FromTU, SourceLocation(), SourceLocation(), &Ident);
+  Record->setHasExternalLexicalStorage();
+  FromTU->addDecl(Record);
+
+  // Do a minimal import of the created class.
+  EXPECT_EQ(0U, CompletedTags.size());
+  Import(Record, Lang_CXX);
+  EXPECT_EQ(0U, CompletedTags.size());
+
+  // Import the definition of the created class.
+  llvm::Error Err = findFromTU(Record)->Importer->ImportDefinition(Record);
+  EXPECT_FALSE((bool)Err);
+  consumeError(std::move(Err));
+
+  // Make sure the class was completed once.
+  EXPECT_EQ(1U, CompletedTags.size());
+  EXPECT_EQ(Record, CompletedTags.front());
+}
+
 INSTANTIATE_TEST_CASE_P(ParameterizedTests, ASTImporterLookupTableTest,
                         DefaultTestValuesForRunOptions, );
 
@@ -6007,5 +6071,8 @@
 INSTANTIATE_TEST_CASE_P(ParameterizedTests, ImportSourceLocations,
                         DefaultTestValuesForRunOptions, );
 
+INSTANTIATE_TEST_CASE_P(ParameterizedTests, ImportWithExternalSource,
+                        DefaultTestValuesForRunOptions, );
+
 } // end namespace ast_matchers
 } // end namespace clang
diff --git a/lldb/test/API/commands/expression/import_base_class_when_class_has_derived_member/TestImportBaseClassWhenClassHasDerivedMember.py b/lldb/test/API/commands/expression/import_base_class_when_class_has_derived_member/TestImportBaseClassWhenClassHasDerivedMember.py
new file mode 100644
--- /dev/null
+++ b/lldb/test/API/commands/expression/import_base_class_when_class_has_derived_member/TestImportBaseClassWhenClassHasDerivedMember.py
@@ -0,0 +1,4 @@
+from lldbsuite.test import lldbinline
+from lldbsuite.test import decorators
+
+lldbinline.MakeInlineTest(__file__, globals())
diff --git a/lldb/test/API/commands/expression/import_base_class_when_class_has_derived_member/main.cpp b/lldb/test/API/commands/expression/import_base_class_when_class_has_derived_member/main.cpp
new file mode 100644
--- /dev/null
+++ b/lldb/test/API/commands/expression/import_base_class_when_class_has_derived_member/main.cpp
@@ -0,0 +1,35 @@
+struct B {
+  int dump() const;
+};
+
+int B::dump() const { return 42; }
+
+// Derived class D obtains dump() method from base class B
+struct D : public B {
+  // Introduce a TypedefNameDecl
+  using Iterator = D *;
+};
+
+struct C {
+  // This will cause use to invoke VisitTypedefNameDecl(...) when Importing
+  // the DeclContext of C.
+  // We will invoke ImportContext(...) which should force the From Decl to
+  // be defined if it not already defined. We will then Import the definition
+  // to the To Decl.
+  // This will force processing of the base class of D which allows us to see
+  // base class methods such as dump().
+  D::Iterator iter;
+
+  bool f(D *DD) {
+    return true; //%self.expect_expr("DD->dump()", result_type="int", result_value="42")
+  }
+};
+
+int main() {
+  C c;
+  D d;
+
+  c.f(&d);
+
+  return 0;
+}