Index: include/llvm/Support/CommandLine.h
===================================================================
--- include/llvm/Support/CommandLine.h
+++ 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 +1320,22 @@
     }
   }
 
+  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 +1511,9 @@
   void printOptionValue(size_t /*GlobalWidth*/, bool /*Force*/) const override {
   }
 
+  void setDefault() override {
+  }
+
   void done() {
     addArgument();
     Parser.initialize();
@@ -1634,6 +1655,9 @@
   void printOptionValue(size_t /*GlobalWidth*/, bool /*Force*/) const override {
   }
 
+  void setDefault() override {
+  }
+
   void done() {
     addArgument();
     Parser.initialize();
@@ -1684,6 +1708,10 @@
   void printOptionValue(size_t /*GlobalWidth*/, bool /*Force*/) const override {
   }
 
+  void setDefault() override {
+    AliasFor->setDefault();
+  }
+
   ValueExpected getValueExpectedFlagDefault() const override {
     return AliasFor->getValueExpectedFlag();
   }