diff --git a/llvm/include/llvm/Support/CommandLine.h b/llvm/include/llvm/Support/CommandLine.h
--- a/llvm/include/llvm/Support/CommandLine.h
+++ b/llvm/include/llvm/Support/CommandLine.h
@@ -1694,7 +1694,7 @@
   unsigned *Location = nullptr; // Where to store the bits...
 
   template <class T> static unsigned Bit(const T &V) {
-    unsigned BitPos = reinterpret_cast<unsigned>(V);
+    unsigned BitPos = static_cast<unsigned>(V);
     assert(BitPos < sizeof(unsigned) * CHAR_BIT &&
            "enum exceeds width of bit vector!");
     return 1 << BitPos;
@@ -1719,6 +1719,11 @@
 
   unsigned getBits() { return *Location; }
 
+  void clear() {
+    if (Location)
+      *Location = 0;
+  }
+
   template <class T> bool isSet(const T &V) {
     return (*Location & Bit(V)) != 0;
   }
@@ -1728,10 +1733,10 @@
 // This makes us exactly compatible with the bits in all cases that it is used.
 //
 template <class DataType> class bits_storage<DataType, bool> {
-  unsigned Bits; // Where to store the bits...
+  unsigned Bits{0}; // Where to store the bits...
 
   template <class T> static unsigned Bit(const T &V) {
-    unsigned BitPos = (unsigned)V;
+    unsigned BitPos = static_cast<unsigned>(V);
     assert(BitPos < sizeof(unsigned) * CHAR_BIT &&
            "enum exceeds width of bit vector!");
     return 1 << BitPos;
@@ -1742,6 +1747,8 @@
 
   unsigned getBits() { return Bits; }
 
+  void clear() { Bits = 0; }
+
   template <class T> bool isSet(const T &V) { return (Bits & Bit(V)) != 0; }
 };
 
@@ -1788,7 +1795,7 @@
   void printOptionValue(size_t /*GlobalWidth*/, bool /*Force*/) const override {
   }
 
-  void setDefault() override {}
+  void setDefault() override { bits_storage<DataType, Storage>::clear(); }
 
   void done() {
     addArgument();
diff --git a/llvm/unittests/Support/CommandLineTest.cpp b/llvm/unittests/Support/CommandLineTest.cpp
--- a/llvm/unittests/Support/CommandLineTest.cpp
+++ b/llvm/unittests/Support/CommandLineTest.cpp
@@ -1931,20 +1931,27 @@
 TEST(CommandLineTest, ResetAllOptionOccurrences) {
   cl::ResetCommandLineParser();
 
-  // -option [sink] input [args]
+  // -option -enableA -enableC [sink] input [args]
   StackOption<bool> Option("option");
+  enum Vals { ValA, ValB, ValC };
+  StackOption<Vals, cl::bits<Vals>> Bits(
+      cl::values(clEnumValN(ValA, "enableA", "Enable A"),
+                 clEnumValN(ValB, "enableB", "Enable B"),
+                 clEnumValN(ValC, "enableC", "Enable C")));
   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);
 
-  const char *Args[] = {"prog", "-option", "-unknown", "input", "-arg"};
+  const char *Args[] = {"prog",     "-option", "-enableA", "-enableC",
+                        "-unknown", "input",   "-arg"};
 
   std::string Errs;
   raw_string_ostream OS(Errs);
-  EXPECT_TRUE(cl::ParseCommandLineOptions(5, Args, StringRef(), &OS));
+  EXPECT_TRUE(cl::ParseCommandLineOptions(7, Args, StringRef(), &OS));
   EXPECT_TRUE(OS.str().empty());
 
   EXPECT_TRUE(Option);
+  EXPECT_EQ((1u << ValA) | (1u << ValC), Bits.getBits());
   EXPECT_EQ(1u, Sink.size());
   EXPECT_EQ("-unknown", Sink[0]);
   EXPECT_EQ("input", Input);
@@ -1953,6 +1960,7 @@
 
   cl::ResetAllOptionOccurrences();
   EXPECT_FALSE(Option);
+  EXPECT_EQ(0u, Bits.getBits());
   EXPECT_EQ(0u, Sink.size());
   EXPECT_EQ(0, Input.getNumOccurrences());
   EXPECT_EQ(0u, ExtraArgs.size());