Make parseTo work with arbitrary string types

Summary: `to<T>` already works in simple cases when `IsSomeString<T>`; by generalizing `parseTo` any string-like `T` gets full functionality without having to copy-paste the overload.

Reviewed By: yfeldblum

Differential Revision: D26831450

fbshipit-source-id: c09629bb7173dde1e0986654128b8e1d7c48b9ba

folly/Conv.h

@@ -1462,20 +1462,36 @@ inline Expected<StringPiece, ConversionCode> parseTo(
   return StringPiece{in.end(), in.end()};
 }
 
-FOLLY_NODISCARD
-inline Expected<StringPiece, ConversionCode> parseTo(
-    StringPiece in, std::string& out) {
+namespace detail {
+
+template <class Str>
+FOLLY_ERASE Expected<StringPiece, ConversionCode> parseToStr(
+    StringPiece in, Str& out) {
   out.clear();
   out.append(in.data(), in.size()); // TODO try/catch?
   return StringPiece{in.end(), in.end()};
 }
 
+} // namespace detail
+
+FOLLY_NODISCARD
+inline Expected<StringPiece, ConversionCode> parseTo(
+    StringPiece in, std::string& out) {
+  return detail::parseToStr(in, out);
+}
+
 FOLLY_NODISCARD
 inline Expected<StringPiece, ConversionCode> parseTo(
     StringPiece in, fbstring& out) {
-  out.clear();
-  out.append(in.data(), in.size()); // TODO try/catch?
-  return StringPiece{in.end(), in.end()};
+  return detail::parseToStr(in, out);
+}
+
+template <class Str>
+FOLLY_NODISCARD inline typename std::enable_if<
+    IsSomeString<Str>::value,
+    Expected<StringPiece, ConversionCode>>::type
+parseTo(StringPiece in, Str& out) {
+  return detail::parseToStr(in, out);
 }
 
 namespace detail {