diff --git a/llvm/include/llvm/Support/raw_ostream.h b/llvm/include/llvm/Support/raw_ostream.h
--- a/llvm/include/llvm/Support/raw_ostream.h
+++ b/llvm/include/llvm/Support/raw_ostream.h
@@ -622,6 +622,9 @@
 
 /// A raw_ostream that writes to an std::string.  This is a simple adaptor
 /// class. This class does not encounter output errors.
+/// raw_string_ostream operates without a buffer, delegating all memory
+/// management to the std::string. Thus the std::string is always up-to-date,
+/// may be used directly and there is no need to call flush().
 class raw_string_ostream : public raw_ostream {
   std::string &OS;
 
@@ -636,14 +639,10 @@
   explicit raw_string_ostream(std::string &O) : OS(O) {
     SetUnbuffered();
   }
-  ~raw_string_ostream() override;
+  ~raw_string_ostream() override = default;
 
-  /// Flushes the stream contents to the target string and returns  the string's
-  /// reference.
-  std::string& str() {
-    flush();
-    return OS;
-  }
+  /// Returns the string's reference.
+  std::string& str() { return OS; }
 
   void reserveExtraSpace(uint64_t ExtraSize) override {
     OS.reserve(tell() + ExtraSize);
diff --git a/llvm/lib/Support/raw_ostream.cpp b/llvm/lib/Support/raw_ostream.cpp
--- a/llvm/lib/Support/raw_ostream.cpp
+++ b/llvm/lib/Support/raw_ostream.cpp
@@ -937,10 +937,6 @@
 //  raw_string_ostream
 //===----------------------------------------------------------------------===//
 
-raw_string_ostream::~raw_string_ostream() {
-  flush();
-}
-
 void raw_string_ostream::write_impl(const char *Ptr, size_t Size) {
   OS.append(Ptr, Size);
 }
diff --git a/llvm/unittests/Support/raw_ostream_test.cpp b/llvm/unittests/Support/raw_ostream_test.cpp
--- a/llvm/unittests/Support/raw_ostream_test.cpp
+++ b/llvm/unittests/Support/raw_ostream_test.cpp
@@ -25,6 +25,7 @@
     llvm::raw_string_ostream OS(res);
     OS.SetBuffered();
     OS << Value;
+    OS.flush();
   }
   return res;
 }
@@ -141,6 +142,7 @@
   OS << "hello";
   OS << 1;
   OS << 'w' << 'o' << 'r' << 'l' << 'd';
+  OS.flush();
   EXPECT_EQ("hello1world", OS.str());
 }
 
@@ -457,6 +459,9 @@
   TiedTo << "y";
   TiedStream << "0";
   EXPECT_EQ("acegostuv", TiedToBuffer);
+
+  TiedTo.flush();
+  TiedStream.flush();
 }
 
 TEST(raw_ostreamTest, reserve_stream) {