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(StreamMode::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
@@ -31,6 +31,9 @@
 }
 }
 
+/// Enable or disable buffering when creating a raw_ostream-derived class.
+enum class StreamMode : bool { Buffered, Unbuffered };
+
 /// This class implements an extremely fast bulk output stream that can *only*
 /// output to a stream.  It does not support seeking, reopening, rewinding, line
 /// buffered disciplines etc. It is a simple buffer that outputs
@@ -60,11 +63,8 @@
   /// this buffer.
   char *OutBufStart, *OutBufEnd, *OutBufCur;
 
-  enum BufferKind {
-    Unbuffered = 0,
-    InternalBuffer,
-    ExternalBuffer
-  } BufferMode;
+  /// Specify the current state of the stream.
+  enum BufferKind { Unbuffered, InternalBuffer, ExternalBuffer } BufferMode;
 
 public:
   // color order matches ANSI escape sequence, don't change
@@ -80,8 +80,8 @@
     SAVEDCOLOR
   };
 
-  explicit raw_ostream(bool unbuffered = false)
-      : BufferMode(unbuffered ? Unbuffered : InternalBuffer) {
+  explicit raw_ostream(StreamMode Mode = StreamMode::Buffered)
+      : BufferMode(Mode == StreamMode::Buffered ? InternalBuffer : Unbuffered) {
     // Start out ready to flush.
     OutBufStart = OutBufEnd = OutBufCur = nullptr;
   }
@@ -320,8 +320,8 @@
   virtual void pwrite_impl(const char *Ptr, size_t Size, uint64_t Offset) = 0;
 
 public:
-  explicit raw_pwrite_stream(bool Unbuffered = false)
-      : raw_ostream(Unbuffered) {}
+  explicit raw_pwrite_stream(StreamMode Mode = StreamMode::Buffered)
+      : raw_ostream(Mode) {}
   void pwrite(const char *Ptr, size_t Size, uint64_t Offset) {
 #ifndef NDBEBUG
     uint64_t Pos = tell();
@@ -387,7 +387,8 @@
 
   /// 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,
+                 StreamMode Mode = StreamMode::Buffered);
 
   ~raw_fd_ostream() override;
 
Index: lib/Bitcode/Writer/BitWriter.cpp
===================================================================
--- lib/Bitcode/Writer/BitWriter.cpp
+++ lib/Bitcode/Writer/BitWriter.cpp
@@ -30,7 +30,8 @@
 
 int LLVMWriteBitcodeToFD(LLVMModuleRef M, int FD, int ShouldClose,
                          int Unbuffered) {
-  raw_fd_ostream OS(FD, ShouldClose, Unbuffered);
+  StreamMode Mode = Unbuffered ? StreamMode::Unbuffered : StreamMode::Buffered;
+  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
@@ -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, StreamMode Mode)
+    : raw_pwrite_stream(Mode), FD(fd), ShouldClose(shouldClose), Error(false),
+      UseAtomicWrites(false) {
   if (FD < 0 ) {
     ShouldClose = false;
     return;
@@ -728,7 +728,7 @@
 /// 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, StreamMode::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, StreamMode::Unbuffered);
   for (unsigned i = 0; i < 4096 / 16; ++i) {
     OF << "0123456789abcdef";
   }
@@ -136,7 +136,8 @@
   // 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 ? StreamMode::Buffered : StreamMode::Unbuffered);
   for (int i = 0; i < 60000; ++i) {
     OF << "0123456789";
   }
@@ -175,7 +176,7 @@
   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, StreamMode::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, StreamMode::Unbuffered);
     OF << Contents;
     OF.close();
     return Path.str();