Index: include/llvm/Support/CommandLine.h
===================================================================
--- include/llvm/Support/CommandLine.h
+++ include/llvm/Support/CommandLine.h
@@ -845,9 +845,20 @@
 template <class DataType> class basic_parser : public basic_parser_impl {
 public:
   using parser_data_type = DataType;
+  
+  using Val = DataType;
+  using ValParam = typename std::conditional<std::is_scalar<DataType>::value,
+                                             Val, const Val&>::type;
   using OptVal = OptionValue<DataType>;
+  using OptValParam = typename std::conditional<std::is_scalar<DataType>::value,
+                                                OptVal, const OptVal&>::type;
 
   basic_parser(Option &O) : basic_parser_impl(O) {}
+  
+  virtual void printOptionDiff(const Option &O, ValParam V, OptValParam Default,
+                               size_t GlobalWidth) const {
+    printOptionNoValue(O, GlobalWidth);
+  }
 
 protected:
   ~basic_parser() = default;
@@ -873,7 +884,7 @@
   StringRef getValueName() const override { return StringRef(); }
 
   void printOptionDiff(const Option &O, bool V, OptVal Default,
-                       size_t GlobalWidth) const;
+                       size_t GlobalWidth) const override;
 
   // An out-of-line virtual method to provide a 'home' for this class.
   void anchor() override;
@@ -899,7 +910,7 @@
   StringRef getValueName() const override { return StringRef(); }
 
   void printOptionDiff(const Option &O, boolOrDefault V, OptVal Default,
-                       size_t GlobalWidth) const;
+                       size_t GlobalWidth) const override;
 
   // An out-of-line virtual method to provide a 'home' for this class.
   void anchor() override;
@@ -921,7 +932,7 @@
   StringRef getValueName() const override { return "int"; }
 
   void printOptionDiff(const Option &O, int V, OptVal Default,
-                       size_t GlobalWidth) const;
+                       size_t GlobalWidth) const override;
 
   // An out-of-line virtual method to provide a 'home' for this class.
   void anchor() override;
@@ -943,7 +954,7 @@
   StringRef getValueName() const override { return "uint"; }
 
   void printOptionDiff(const Option &O, unsigned V, OptVal Default,
-                       size_t GlobalWidth) const;
+                       size_t GlobalWidth) const override;
 
   // An out-of-line virtual method to provide a 'home' for this class.
   void anchor() override;
@@ -968,7 +979,7 @@
   StringRef getValueName() const override { return "uint"; }
 
   void printOptionDiff(const Option &O, unsigned long long V, OptVal Default,
-                       size_t GlobalWidth) const;
+                       size_t GlobalWidth) const override;
 
   // An out-of-line virtual method to provide a 'home' for this class.
   void anchor() override;
@@ -990,7 +1001,7 @@
   StringRef getValueName() const override { return "number"; }
 
   void printOptionDiff(const Option &O, double V, OptVal Default,
-                       size_t GlobalWidth) const;
+                       size_t GlobalWidth) const override;
 
   // An out-of-line virtual method to provide a 'home' for this class.
   void anchor() override;
@@ -1012,7 +1023,7 @@
   StringRef getValueName() const override { return "number"; }
 
   void printOptionDiff(const Option &O, float V, OptVal Default,
-                       size_t GlobalWidth) const;
+                       size_t GlobalWidth) const override;
 
   // An out-of-line virtual method to provide a 'home' for this class.
   void anchor() override;
@@ -1036,8 +1047,9 @@
   // getValueName - Overload in subclass to provide a better default value.
   StringRef getValueName() const override { return "string"; }
 
-  void printOptionDiff(const Option &O, StringRef V, const OptVal &Default,
-                       size_t GlobalWidth) const;
+  void printOptionDiff(const Option &O, const std::string &V,
+                       const OptVal &Default,
+                       size_t GlobalWidth) const override;
 
   // An out-of-line virtual method to provide a 'home' for this class.
   void anchor() override;
@@ -1062,7 +1074,7 @@
   StringRef getValueName() const override { return "char"; }
 
   void printOptionDiff(const Option &O, char V, OptVal Default,
-                       size_t GlobalWidth) const;
+                       size_t GlobalWidth) const override;
 
   // An out-of-line virtual method to provide a 'home' for this class.
   void anchor() override;
@@ -1087,7 +1099,7 @@
 // This is instantiated for basic parsers when the parsed value has a different
 // type than the option value. e.g. HelpPrinter.
 template <class ParserDT, class ValDT> struct OptionDiffPrinter {
-  void print(const Option &O, const parser<ParserDT> &P, const ValDT & /*V*/,
+  void print(const Option &O, const basic_parser<ParserDT> &P, const ValDT & /*V*/,
              const OptionValue<ValDT> & /*Default*/, size_t GlobalWidth) {
     P.printOptionNoValue(O, GlobalWidth);
   }
@@ -1096,7 +1108,7 @@
 // This is instantiated for basic parsers when the parsed value has the same
 // type as the option value.
 template <class DT> struct OptionDiffPrinter<DT, DT> {
-  void print(const Option &O, const parser<DT> &P, const DT &V,
+  void print(const Option &O, const basic_parser<DT> &P, const DT &V,
              const OptionValue<DT> &Default, size_t GlobalWidth) {
     P.printOptionDiff(O, V, Default, GlobalWidth);
   }
@@ -1109,10 +1121,8 @@
     const Option &O,
     const basic_parser<typename ParserClass::parser_data_type> &P,
     const ValDT &V, const OptionValue<ValDT> &Default, size_t GlobalWidth) {
-
   OptionDiffPrinter<typename ParserClass::parser_data_type, ValDT> printer;
-  printer.print(O, static_cast<const ParserClass &>(P), V, Default,
-                GlobalWidth);
+  printer.print(O, P, V, Default, GlobalWidth);
 }
 
 //===----------------------------------------------------------------------===//
Index: lib/Support/CommandLine.cpp
===================================================================
--- lib/Support/CommandLine.cpp
+++ lib/Support/CommandLine.cpp
@@ -1649,7 +1649,7 @@
 PRINT_OPT_DIFF(float)
 PRINT_OPT_DIFF(char)
 
-void parser<std::string>::printOptionDiff(const Option &O, StringRef V,
+void parser<std::string>::printOptionDiff(const Option &O, const std::string &V,
                                           const OptionValue<std::string> &D,
                                           size_t GlobalWidth) const {
   printOptionName(O, GlobalWidth);
Index: unittests/Support/CommandLineTest.cpp
===================================================================
--- unittests/Support/CommandLineTest.cpp
+++ unittests/Support/CommandLineTest.cpp
@@ -50,9 +50,10 @@
   const char *const name;
 };
 
-template <typename T>
-class StackOption : public cl::opt<T> {
-  typedef cl::opt<T> Base;
+template <typename T, bool ExternalStorage = false,
+          class ParserClass = cl::parser<T>>
+class StackOption : public cl::opt<T, ExternalStorage, ParserClass> {
+  typedef cl::opt<T, ExternalStorage, ParserClass> Base;
 public:
   // One option...
   template<class M0t>
@@ -648,4 +649,41 @@
   EXPECT_TRUE(Opt3 == 3);
 }
 
+/// Custom dummy parser for int options.
+struct CustomIntParser final : cl::basic_parser<int> {
+  static const int MAGIC_NR = 42;
+  static int printOptionDiffCount;
+  CustomIntParser(cl::Option &O) : cl::basic_parser<int>(O) {}
+  bool parse(cl::Option &O, StringRef ArgName, StringRef Arg, int &Value) {
+    Value = MAGIC_NR;
+    return false;
+  }
+  void printOptionDiff(const cl::Option &O, int V, OptVal Default,
+                       size_t GlobalWidth) const {
+    //printOptionName(O, GlobalWidth);
+    //llvm::outs() << "= *always " << MAGIC_NR << "*\n";
+    printOptionDiffCount++;
+  }
+};
+int CustomIntParser::printOptionDiffCount = 0;
+
+TEST(CommandLineTest, PrintOptionValueWithCustomParser) {
+  cl::ResetCommandLineParser();
+  
+  StackOption<int, false, CustomIntParser> Opt1("opt1", cl::init(0));
+  
+  const char *args[] = {"prog", "-opt1=0"};
+  EXPECT_TRUE(
+    cl::ParseCommandLineOptions(2, args, StringRef(), &llvm::nulls()));
+  
+  EXPECT_TRUE(Opt1 == CustomIntParser::MAGIC_NR);
+  
+  for (auto &OM : cl::getRegisteredOptions(*cl::TopLevelSubCommand)) {
+    cl::Option *O = OM.second;
+    O->printOptionValue(8, true);
+  }
+  
+  EXPECT_TRUE(CustomIntParser::printOptionDiffCount > 0);
+}
+
 }  // anonymous namespace