Index: include/llvm/Support/circular_raw_ostream.h
===================================================================
--- include/llvm/Support/circular_raw_ostream.h
+++ include/llvm/Support/circular_raw_ostream.h
@@ -108,7 +108,7 @@
     ///
     circular_raw_ostream(raw_ostream &Stream, const char *Header,
                          size_t BuffSize = 0, bool Owns = REFERENCE_ONLY)
-        : raw_ostream(/*unbuffered*/ true), TheStream(nullptr),
+        : raw_ostream(BufferKind::Unbuffered), TheStream(nullptr),
           OwnsStream(Owns), BufferSize(BuffSize), BufferArray(nullptr),
           Filled(false), Banner(Header) {
       if (BufferSize != 0)
Index: include/llvm/Support/raw_ostream.h
===================================================================
--- include/llvm/Support/raw_ostream.h
+++ include/llvm/Support/raw_ostream.h
@@ -60,11 +60,12 @@
   /// this buffer.
   char *OutBufStart, *OutBufEnd, *OutBufCur;
 
-  enum BufferKind {
-    Unbuffered = 0,
-    InternalBuffer,
-    ExternalBuffer
-  } BufferMode;
+public:
+  /// Specify the current state of the stream.
+  enum class BufferKind { Unbuffered, InternalBuffer, ExternalBuffer };
+
+private:
+  BufferKind BufferMode;
 
 public:
   // color order matches ANSI escape sequence, don't change
@@ -80,8 +81,8 @@
     SAVEDCOLOR
   };
 
-  explicit raw_ostream(bool unbuffered = false)
-      : BufferMode(unbuffered ? Unbuffered : InternalBuffer) {
+  explicit raw_ostream(BufferKind BufferMode = BufferKind::InternalBuffer)
+      : BufferMode(BufferMode) {
     // Start out ready to flush.
     OutBufStart = OutBufEnd = OutBufCur = nullptr;
   }
@@ -102,13 +103,13 @@
   /// Set the stream to be buffered, using the specified buffer size.
   void SetBufferSize(size_t Size) {
     flush();
-    SetBufferAndMode(new char[Size], Size, InternalBuffer);
+    SetBufferAndMode(new char[Size], Size, BufferKind::InternalBuffer);
   }
 
   size_t GetBufferSize() const {
     // If we're supposed to be buffered but haven't actually gotten around
     // to allocating the buffer yet, return the value that would be used.
-    if (BufferMode != Unbuffered && OutBufStart == nullptr)
+    if (BufferMode != BufferKind::Unbuffered && OutBufStart == nullptr)
       return preferred_buffer_size();
 
     // Otherwise just return the size of the allocated buffer.
@@ -120,7 +121,7 @@
   /// when the stream is being set to unbuffered.
   void SetUnbuffered() {
     flush();
-    SetBufferAndMode(nullptr, 0, Unbuffered);
+    SetBufferAndMode(nullptr, 0, BufferKind::Unbuffered);
   }
 
   size_t GetNumBytesInBuffer() const {
@@ -287,7 +288,7 @@
   /// use only by subclasses which can arrange for the output to go directly
   /// into the desired output buffer, instead of being copied on each flush.
   void SetBuffer(char *BufferStart, size_t Size) {
-    SetBufferAndMode(BufferStart, Size, ExternalBuffer);
+    SetBufferAndMode(BufferStart, Size, BufferKind::ExternalBuffer);
   }
 
   /// Return an efficient buffer size for the underlying output mechanism.
@@ -320,7 +321,7 @@
   virtual void pwrite_impl(const char *Ptr, size_t Size, uint64_t Offset) = 0;
 
 public:
-  explicit raw_pwrite_stream(bool Unbuffered = false)
+  explicit raw_pwrite_stream(BufferKind Unbuffered = BufferKind::InternalBuffer)
       : raw_ostream(Unbuffered) {}
   void pwrite(const char *Ptr, size_t Size, uint64_t Offset) {
 #ifndef NDBEBUG
@@ -387,7 +388,8 @@
 
   /// FD is the file descriptor that this writes to.  If ShouldClose is true,
   /// this closes the file when the stream is destroyed.
-  raw_fd_ostream(int fd, bool shouldClose, bool unbuffered=false);
+  raw_fd_ostream(int fd, bool shouldClose,
+                 BufferKind Unbuffered = BufferKind::InternalBuffer);
 
   ~raw_fd_ostream() override;
 
@@ -530,7 +532,7 @@
   uint64_t current_pos() const override;
 
 public:
-  explicit raw_null_ostream() {}
+  explicit raw_null_ostream() : raw_pwrite_stream(BufferKind::Unbuffered) {}
   ~raw_null_ostream() override;
 };
 
Index: lib/Bitcode/Writer/BitWriter.cpp
===================================================================
--- lib/Bitcode/Writer/BitWriter.cpp
+++ lib/Bitcode/Writer/BitWriter.cpp
@@ -30,7 +30,10 @@
 
 int LLVMWriteBitcodeToFD(LLVMModuleRef M, int FD, int ShouldClose,
                          int Unbuffered) {
-  raw_fd_ostream OS(FD, ShouldClose, Unbuffered);
+  raw_ostream::BufferKind Mode = Unbuffered
+                                     ? raw_ostream::BufferKind::Unbuffered
+                                     : raw_ostream::BufferKind::InternalBuffer;
+  raw_fd_ostream OS(FD, ShouldClose, Mode);
 
   WriteBitcodeToFile(unwrap(M), OS);
   return 0;
Index: lib/Support/raw_ostream.cpp
===================================================================
--- lib/Support/raw_ostream.cpp
+++ lib/Support/raw_ostream.cpp
@@ -65,7 +65,7 @@
   assert(OutBufCur == OutBufStart &&
          "raw_ostream destructor called with non-empty buffer!");
 
-  if (BufferMode == InternalBuffer)
+  if (BufferMode == BufferKind::InternalBuffer)
     delete [] OutBufStart;
 }
 
@@ -88,14 +88,14 @@
 
 void raw_ostream::SetBufferAndMode(char *BufferStart, size_t Size,
                                    BufferKind Mode) {
-  assert(((Mode == Unbuffered && !BufferStart && Size == 0) ||
-          (Mode != Unbuffered && BufferStart && Size != 0)) &&
+  assert(((Mode == BufferKind::Unbuffered && !BufferStart && Size == 0) ||
+          (Mode != BufferKind::Unbuffered && BufferStart && Size != 0)) &&
          "stream must be unbuffered or have at least one byte");
   // Make sure the current buffer is free of content (we can't flush here; the
   // child buffer management logic will be in write_impl).
   assert(GetNumBytesInBuffer() == 0 && "Current buffer is non-empty!");
 
-  if (BufferMode == InternalBuffer)
+  if (BufferMode == BufferKind::InternalBuffer)
     delete [] OutBufStart;
   OutBufStart = BufferStart;
   OutBufEnd = OutBufStart+Size;
@@ -277,7 +277,7 @@
   // Group exceptional cases into a single branch.
   if (LLVM_UNLIKELY(OutBufCur >= OutBufEnd)) {
     if (LLVM_UNLIKELY(!OutBufStart)) {
-      if (BufferMode == Unbuffered) {
+      if (BufferMode == BufferKind::Unbuffered) {
         write_impl(reinterpret_cast<char*>(&C), 1);
         return *this;
       }
@@ -297,7 +297,7 @@
   // Group exceptional cases into a single branch.
   if (LLVM_UNLIKELY(size_t(OutBufEnd - OutBufCur) < Size)) {
     if (LLVM_UNLIKELY(!OutBufStart)) {
-      if (BufferMode == Unbuffered) {
+      if (BufferMode == BufferKind::Unbuffered) {
         write_impl(Ptr, Size);
         return *this;
       }
@@ -515,9 +515,9 @@
 
 /// FD is the file descriptor that this writes to.  If ShouldClose is true, this
 /// closes the file when the stream is destroyed.
-raw_fd_ostream::raw_fd_ostream(int fd, bool shouldClose, bool unbuffered)
-    : raw_pwrite_stream(unbuffered), FD(fd), ShouldClose(shouldClose),
-      Error(false), UseAtomicWrites(false) {
+raw_fd_ostream::raw_fd_ostream(int fd, bool shouldClose, BufferKind Mode)
+    : raw_pwrite_stream(Mode), FD(fd), ShouldClose(shouldClose), Error(false),
+      UseAtomicWrites(false) {
   if (FD < 0 ) {
     ShouldClose = false;
     return;
@@ -728,7 +728,8 @@
 /// Use it like: errs() << "foo" << "bar";
 raw_ostream &llvm::errs() {
   // Set standard error to be unbuffered by default.
-  static raw_fd_ostream S(STDERR_FILENO, false, true);
+  static raw_fd_ostream S(STDERR_FILENO, false,
+                          raw_ostream::BufferKind::Unbuffered);
   return S;
 }
 
Index: unittests/Support/MemoryBufferTest.cpp
===================================================================
--- unittests/Support/MemoryBufferTest.cpp
+++ unittests/Support/MemoryBufferTest.cpp
@@ -71,7 +71,7 @@
   SmallString<64> TestPath;
   sys::fs::createTemporaryFile("MemoryBufferTest_NullTerminator4K", "temp",
                                TestFD, TestPath);
-  raw_fd_ostream OF(TestFD, true, /*unbuffered=*/true);
+  raw_fd_ostream OF(TestFD, true, raw_ostream::BufferKind::Unbuffered);
   for (unsigned i = 0; i < 4096 / 16; ++i) {
     OF << "0123456789abcdef";
   }
@@ -136,7 +136,9 @@
   // OF is responsible for closing the file; If the file is not
   // reopened, it will be unbuffered so that the results are
   // immediately visible through the fd.
-  raw_fd_ostream OF(TestFD, true, !Reopen);
+  raw_fd_ostream OF(TestFD, true, Reopen
+                                      ? raw_ostream::BufferKind::InternalBuffer
+                                      : raw_ostream::BufferKind::Unbuffered);
   for (int i = 0; i < 60000; ++i) {
     OF << "0123456789";
   }
@@ -175,7 +177,7 @@
   int FD;
   SmallString<64> TestPath;
   sys::fs::createTemporaryFile("MemoryBufferTest_Slice", "temp", FD, TestPath);
-  raw_fd_ostream OF(FD, true, /*unbuffered=*/true);
+  raw_fd_ostream OF(FD, true, raw_ostream::BufferKind::Unbuffered);
   for (unsigned i = 0; i < 0x2000 / 8; ++i) {
     OF << "12345678";
   }
Index: unittests/Support/SpecialCaseListTest.cpp
===================================================================
--- unittests/Support/SpecialCaseListTest.cpp
+++ unittests/Support/SpecialCaseListTest.cpp
@@ -36,7 +36,7 @@
     int FD;
     SmallString<64> Path;
     sys::fs::createTemporaryFile("SpecialCaseListTest", "temp", FD, Path);
-    raw_fd_ostream OF(FD, true, true);
+    raw_fd_ostream OF(FD, true, raw_ostream::BufferKind::Unbuffered);
     OF << Contents;
     OF.close();
     return Path.str();