Index: llvm/include/llvm/Support/CommandLine.h
===================================================================
--- llvm/include/llvm/Support/CommandLine.h
+++ llvm/include/llvm/Support/CommandLine.h
@@ -1406,7 +1406,7 @@
 
   // Make sure we initialize the value with the default constructor for the
   // type.
-  opt_storage() : Value(DataType()), Default(DataType()) {}
+  opt_storage() : Value(DataType()), Default() {}
 
   template <class T> void setValue(const T &V, bool initial = false) {
     Value = V;
Index: llvm/unittests/Support/CommandLineTest.cpp
===================================================================
--- llvm/unittests/Support/CommandLineTest.cpp
+++ llvm/unittests/Support/CommandLineTest.cpp
@@ -1789,11 +1789,11 @@
 TEST(CommandLineTest, Callback) {
   cl::ResetCommandLineParser();
 
-  StackOption<bool> OptA("a", cl::desc("option a"));
-  StackOption<bool> OptB(
+  StackOption<bool> OptA("a", cl::desc("option a"), cl::init(false));
+  StackOption<bool> OptB(cl::init(false),
       "b", cl::desc("option b -- This option turns on option a"),
       cl::callback([&](const bool &) { OptA = true; }));
-  StackOption<bool> OptC(
+  StackOption<bool> OptC(cl::init(false),
       "c", cl::desc("option c -- This option turns on options a and b"),
       cl::callback([&](const bool &) { OptB = true; }));
   StackOption<std::string, cl::list<std::string>> List(
@@ -1904,7 +1904,7 @@
   cl::ResetCommandLineParser();
 
   // -option [sink] input [args]
-  StackOption<bool> Option("option");
+  StackOption<bool> Option("option", cl::init(false));
   StackOption<std::string, cl::list<std::string>> Sink(cl::Sink);
   StackOption<std::string> Input(cl::Positional);
   StackOption<std::string, cl::list<std::string>> ExtraArgs(cl::ConsumeAfter);
@@ -1930,4 +1930,60 @@
   EXPECT_EQ(0, (int)ExtraArgs.size());
 }
 
+TEST(CommandLineTest, DefaultValue) {
+  cl::ResetCommandLineParser();
+
+  StackOption<bool> BoolOption("bool-option");
+  StackOption<std::string> StrOption("str-option");
+  StackOption<bool> BoolInitOption("bool-init-option", cl::init(true));
+  StackOption<std::string> StrInitOption("str-init-option",
+                                         cl::init("str-default-value"));
+
+  const char *Args[] = {"prog"}; // no options
+
+  std::string Errs;
+  raw_string_ostream OS(Errs);
+  EXPECT_TRUE(cl::ParseCommandLineOptions(1, Args, StringRef(), &OS));
+  EXPECT_TRUE(OS.str().empty());
+
+  EXPECT_TRUE(!BoolOption);
+  EXPECT_FALSE(BoolOption.Default.hasValue());
+  EXPECT_EQ(0, BoolOption.getNumOccurrences());
+
+  EXPECT_EQ("", StrOption);
+  EXPECT_FALSE(StrOption.Default.hasValue());
+  EXPECT_EQ(0, StrOption.getNumOccurrences());
+
+  EXPECT_TRUE(BoolInitOption);
+  EXPECT_TRUE(BoolInitOption.Default.hasValue());
+  EXPECT_EQ(0, BoolInitOption.getNumOccurrences());
+
+  EXPECT_EQ("str-default-value", StrInitOption);
+  EXPECT_TRUE(StrInitOption.Default.hasValue());
+  EXPECT_EQ(0, StrInitOption.getNumOccurrences());
+
+  const char *Args2[] = {"prog", "-bool-option", "-str-option=str-value",
+                         "-bool-init-option=0",
+                         "-str-init-option=str-init-value"};
+
+  EXPECT_TRUE(cl::ParseCommandLineOptions(5, Args2, StringRef(), &OS));
+  EXPECT_TRUE(OS.str().empty());
+
+  EXPECT_TRUE(BoolOption);
+  EXPECT_FALSE(BoolOption.Default.hasValue());
+  EXPECT_EQ(1, BoolOption.getNumOccurrences());
+
+  EXPECT_EQ("str-value", StrOption);
+  EXPECT_FALSE(StrOption.Default.hasValue());
+  EXPECT_EQ(1, StrOption.getNumOccurrences());
+
+  EXPECT_FALSE(BoolInitOption);
+  EXPECT_TRUE(BoolInitOption.Default.hasValue());
+  EXPECT_EQ(1, BoolInitOption.getNumOccurrences());
+
+  EXPECT_EQ("str-init-value", StrInitOption);
+  EXPECT_TRUE(StrInitOption.Default.hasValue());
+  EXPECT_EQ(1, StrInitOption.getNumOccurrences());
+}
+
 } // anonymous namespace