Fix StringPiece ostream overloads to properly not rely on <ostream>

Summary:
std::ostream may be incomplete because Range.h doesn't use <ostream>,
only <iosfwd>. Changing the operator<< overloads to be templated on the char
type defers typechecking until callsites, which will avoid the potential problem.

Reviewed By: yfeldblum, ericniebler

Differential Revision: D5494648

fbshipit-source-id: e59b6fdfba6c08ec70ebb1e10c14a43307a1119f

folly/Range.h

@@ -970,15 +970,19 @@ typedef Range<char*> MutableStringPiece;
 typedef Range<const unsigned char*> ByteRange;
 typedef Range<unsigned char*> MutableByteRange;
 
-inline std::ostream& operator<<(std::ostream& os,
-                                const StringPiece piece) {
-  os.write(piece.start(), std::streamsize(piece.size()));
+template <class C>
+std::basic_ostream<C>& operator<<(
+    std::basic_ostream<C>& os,
+    Range<C const*> piece) {
+  using StreamSize = decltype(os.width());
+  os.write(piece.start(), static_cast<StreamSize>(piece.size()));
   return os;
 }
 
-inline std::ostream& operator<<(std::ostream& os,
-                                const MutableStringPiece piece) {
-  os.write(piece.start(), std::streamsize(piece.size()));
+template <class C>
+std::basic_ostream<C>& operator<<(std::basic_ostream<C>& os, Range<C*> piece) {
+  using StreamSize = decltype(os.width());
+  os.write(piece.start(), static_cast<StreamSize>(piece.size()));
   return os;
 }