Index: llvm/trunk/include/llvm/Support/CommandLine.h
===================================================================
--- llvm/trunk/include/llvm/Support/CommandLine.h
+++ llvm/trunk/include/llvm/Support/CommandLine.h
@@ -349,6 +349,8 @@
 
   virtual void printOptionValue(size_t GlobalWidth, bool Force) const = 0;
 
+  virtual void setDefault() = 0;
+
   static void printHelpStr(StringRef HelpStr, size_t Indent,
                            size_t FirstLineIndentedBy);
 
@@ -1318,6 +1320,20 @@
     }
   }
 
+  template <class T, class = typename std::enable_if<
+            std::is_assignable<T&, T>::value>::type>
+  void setDefaultImpl() {
+    const OptionValue<DataType> &V = this->getDefault();
+    if (V.hasValue())
+      this->setValue(V.getValue());
+  }
+
+  template <class T, class = typename std::enable_if<
+            !std::is_assignable<T&, T>::value>::type>
+  void setDefaultImpl(...) {}
+
+  void setDefault() override { setDefaultImpl<DataType>(); }
+
   void done() {
     addArgument();
     Parser.initialize();
@@ -1493,6 +1509,8 @@
   void printOptionValue(size_t /*GlobalWidth*/, bool /*Force*/) const override {
   }
 
+  void setDefault() override {}
+
   void done() {
     addArgument();
     Parser.initialize();
@@ -1634,6 +1652,8 @@
   void printOptionValue(size_t /*GlobalWidth*/, bool /*Force*/) const override {
   }
 
+  void setDefault() override {}
+
   void done() {
     addArgument();
     Parser.initialize();
@@ -1684,6 +1704,8 @@
   void printOptionValue(size_t /*GlobalWidth*/, bool /*Force*/) const override {
   }
 
+  void setDefault() override { AliasFor->setDefault(); }
+
   ValueExpected getValueExpectedFlagDefault() const override {
     return AliasFor->getValueExpectedFlag();
   }
Index: llvm/trunk/unittests/Support/CommandLineTest.cpp
===================================================================
--- llvm/trunk/unittests/Support/CommandLineTest.cpp
+++ llvm/trunk/unittests/Support/CommandLineTest.cpp
@@ -613,4 +613,39 @@
   llvm::sys::fs::remove(TestDir);
 }
 
+TEST(CommandLineTest, SetDefautValue) {
+  cl::ResetCommandLineParser();
+
+  StackOption<std::string> Opt1("opt1", cl::init("true"));
+  StackOption<bool> Opt2("opt2", cl::init(true));
+  cl::alias Alias("alias", llvm::cl::aliasopt(Opt2));
+  StackOption<int> Opt3("opt3", cl::init(3));
+
+  const char *args[] = {"prog", "-opt1=false", "-opt2", "-opt3"};
+
+  EXPECT_TRUE(
+    cl::ParseCommandLineOptions(2, args, StringRef(), &llvm::nulls()));
+
+  EXPECT_TRUE(Opt1 == "false");
+  EXPECT_TRUE(Opt2);
+  EXPECT_TRUE(Opt3 == 3);
+
+  Opt2 = false;
+  Opt3 = 1;
+
+  cl::ResetAllOptionOccurrences();
+
+  for (auto &OM : cl::getRegisteredOptions(*cl::TopLevelSubCommand)) {
+    cl::Option *O = OM.second;
+    if (O->ArgStr == "opt2") {
+      continue;
+    }
+    O->setDefault();
+  }
+
+  EXPECT_TRUE(Opt1 == "true");
+  EXPECT_TRUE(Opt2);
+  EXPECT_TRUE(Opt3 == 3);
+}
+
 }  // anonymous namespace