Implement LWG#1203 for raw_ostream.

Implement LWG#1203 (https://cplusplus.github.io/LWG/issue1203) for raw_ostream
like libc++ does for std::basic_ostream<...>.

Add a operator<< overload that takes an rvalue reference of a typed derived from
raw_ostream, streams the value to it and returns the stream of the same type as
the argument.

This allows free operator<< to work with rvalue reference raw_ostreams:
raw_ostream& operator<<(raw_ostream&, const SomeType& Value);
raw_os_ostream(std::cout) << SomeType();

It also allows using the derived type like:
auto Foo = (raw_string_ostream(buffer) << "foo").str();

Author: Christian Sigg <csigg@google.com>
Differential Revision: https://reviews.llvm.org/D70686

include/llvm/Support/raw_ostream.h

@@ -21,6 +21,7 @@
 #include <cstring>
 #include <string>
 #include <system_error>
+#include <type_traits>
 
 namespace llvm {
 
@@ -354,6 +355,17 @@ private:
   virtual void anchor();
 };
 
+/// Call the appropriate insertion operator, given an rvalue reference to a
+/// raw_ostream object and return a stream of the same type as the argument.
+template <typename OStream, typename T>
+typename std::enable_if<!std::is_reference<OStream>::value &&
+                            std::is_base_of<raw_ostream, OStream>::value,
+                        OStream &&>::type
+operator<<(OStream &&OS, const T &Value) {
+  OS << Value;
+  return std::move(OS);
+}
+
 /// An abstract base class for streams implementations that also support a
 /// pwrite operation. This is useful for code that can mostly stream out data,
 /// but needs to patch in a header that needs to know the output size.

unittests/Support/raw_ostream_test.cpp

@@ -18,8 +18,7 @@ namespace {
 
 template<typename T> std::string printToString(const T &Value) {
   std::string res;
-  llvm::raw_string_ostream(res) << Value;
-  return res;    
+  return (llvm::raw_string_ostream(res) << Value).str();
 }
 
 /// printToString - Print the given value to a stream which only has \arg
@@ -47,6 +46,10 @@ template<typename T> std::string printToStringUnbuffered(const T &Value) {
   return res;
 }
 
+struct X {};
+
+raw_ostream &operator<<(raw_ostream &OS, const X &) { return OS << 'X'; }
+
 TEST(raw_ostreamTest, Types_Buffered) {
   // Char
   EXPECT_EQ("c", printToString('c'));
@@ -76,6 +79,9 @@ TEST(raw_ostreamTest, Types_Buffered) {
   // Min and max.
   EXPECT_EQ("18446744073709551615", printToString(UINT64_MAX));
   EXPECT_EQ("-9223372036854775808", printToString(INT64_MIN));
+
+  // X, checking free operator<<().
+  EXPECT_EQ("X", printToString(X{}));
 }
 
 TEST(raw_ostreamTest, Types_Unbuffered) {  
@@ -107,6 +113,9 @@ TEST(raw_ostreamTest, Types_Unbuffered) {
   // Min and max.
   EXPECT_EQ("18446744073709551615", printToStringUnbuffered(UINT64_MAX));
   EXPECT_EQ("-9223372036854775808", printToStringUnbuffered(INT64_MIN));
+
+  // X, checking free operator<<().
+  EXPECT_EQ("X", printToString(X{}));
 }
 
 TEST(raw_ostreamTest, BufferEdge) {