From: Eric Niebler <eniebler@fb.com>
Date: Mon, 15 Aug 2016 21:03:58 +0000 (-0700)
Subject: Use folly::Expected to implement folly::tryTo, a non-throwing variant of folly::to
X-Git-Tag: v2016.08.22.00~37
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=89cc32931877aa684e1a1f06776f564a49d39eab;p=folly.git

Use folly::Expected to implement folly::tryTo, a non-throwing variant of folly::to

Summary:
This change adds a non-throwing interface for folly::to<T>: tryTo<T>, which
returns an Expected<T, ConversionCode>.

Here is how the non-throwing interface compares to the regular interface in
terms of performance. On the successful path, there's generally not much
difference between using the throwing and non-throwing interfaces. For the
error path, tryTo<> is about three orders of magnitude faster than to<>.

Reviewed By: mhx

Differential Revision: D3557832

fbshipit-source-id: 25b29834b6fda474a7cdd43a7dea69ff4ee183c8
---

diff --git a/folly/Conv.cpp b/folly/Conv.cpp
index e11d8693..0ab6dcf5 100644
--- a/folly/Conv.cpp
+++ b/folly/Conv.cpp
@@ -207,8 +207,10 @@ struct ErrorString {
   bool quote;
 };
 
-// Keep this in sync with ConversionError::Code in Conv.h
-constexpr const std::array<ErrorString, ConversionError::NUM_ERROR_CODES>
+// Keep this in sync with ConversionCode in Conv.h
+constexpr const std::array<
+    ErrorString,
+    static_cast<std::size_t>(ConversionCode::NUM_ERROR_CODES)>
     kErrorStrings{{
         {"Success", true},
         {"Empty input string", true},
@@ -255,34 +257,11 @@ inline bool bool_str_cmp(const char** b, size_t len, const char* value) {
 
 } // anonymous namespace
 
-ConversionError makeConversionError(
-    ConversionError::Code code,
-    const char* input,
-    size_t inputLen) {
-  assert(code >= 0 && code < kErrorStrings.size());
-  const ErrorString& err = kErrorStrings[code];
-  if (code == ConversionError::EMPTY_INPUT_STRING && inputLen == 0) {
-    return ConversionError(err.string, code);
-  }
-  std::string tmp(err.string);
-  tmp.append(": ");
-  if (err.quote) {
-    tmp.append(1, '"');
-  }
-  if (input && inputLen > 0) {
-    tmp.append(input, inputLen);
-  }
-  if (err.quote) {
-    tmp.append(1, '"');
-  }
-  return ConversionError(tmp, code);
-}
-
-ConversionResult<bool> str_to_bool(StringPiece* src) {
+Expected<bool, ConversionCode> str_to_bool(StringPiece* src) noexcept {
   auto b = src->begin(), e = src->end();
   for (;; ++b) {
     if (b >= e) {
-      return ConversionResult<bool>(ConversionError::EMPTY_INPUT_STRING);
+      return makeUnexpected(ConversionCode::EMPTY_INPUT_STRING);
     }
     if (!std::isspace(*b)) {
       break;
@@ -297,7 +276,7 @@ ConversionResult<bool> str_to_bool(StringPiece* src) {
       result = false;
       for (; b < e && isdigit(*b); ++b) {
         if (result || (*b != '0' && *b != '1')) {
-          return ConversionResult<bool>(ConversionError::BOOL_OVERFLOW);
+          return makeUnexpected(ConversionCode::BOOL_OVERFLOW);
         }
         result = (*b == '1');
       }
@@ -338,16 +317,16 @@ ConversionResult<bool> str_to_bool(StringPiece* src) {
       } else if (bool_str_cmp(&b, len, "off")) {
         result = false;
       } else {
-        return ConversionResult<bool>(ConversionError::BOOL_INVALID_VALUE);
+        return makeUnexpected(ConversionCode::BOOL_INVALID_VALUE);
       }
       break;
     default:
-      return ConversionResult<bool>(ConversionError::BOOL_INVALID_VALUE);
+      return makeUnexpected(ConversionCode::BOOL_INVALID_VALUE);
   }
 
   src->assign(b, e);
 
-  return ConversionResult<bool>(result);
+  return result;
 }
 
 /**
@@ -355,7 +334,7 @@ ConversionResult<bool> str_to_bool(StringPiece* src) {
  * StringPiece parameter to munch the already-parsed characters.
  */
 template <class Tgt>
-ConversionResult<Tgt> str_to_floating(StringPiece* src) {
+Expected<Tgt, ConversionCode> str_to_floating(StringPiece* src) noexcept {
   using namespace double_conversion;
   static StringToDoubleConverter
     conv(StringToDoubleConverter::ALLOW_TRAILING_JUNK
@@ -366,7 +345,7 @@ ConversionResult<Tgt> str_to_floating(StringPiece* src) {
          nullptr, nullptr);
 
   if (src->empty()) {
-    return ConversionResult<Tgt>(ConversionError::EMPTY_INPUT_STRING);
+    return makeUnexpected(ConversionCode::EMPTY_INPUT_STRING);
   }
 
   int length;
@@ -383,10 +362,10 @@ ConversionResult<Tgt> str_to_floating(StringPiece* src) {
     // that was processed, so we need to check if that character was
     // whitespace or not.
     if (length == 0 || (result == 0.0 && std::isspace((*src)[length - 1]))) {
-      return ConversionResult<Tgt>(ConversionError::EMPTY_INPUT_STRING);
+      return makeUnexpected(ConversionCode::EMPTY_INPUT_STRING);
     }
     src->advance(length);
-    return ConversionResult<Tgt>(result);
+    return result;
   }
 
   auto* e = src->end();
@@ -435,7 +414,7 @@ ConversionResult<Tgt> str_to_floating(StringPiece* src) {
 
   if (result == 0.0) {
     // All bets are off
-    return ConversionResult<Tgt>(ConversionError::STRING_TO_FLOAT_ERROR);
+    return makeUnexpected(ConversionCode::STRING_TO_FLOAT_ERROR);
   }
 
   if (negative) {
@@ -444,11 +423,13 @@ ConversionResult<Tgt> str_to_floating(StringPiece* src) {
 
   src->assign(b, e);
 
-  return ConversionResult<Tgt>(result);
+  return result;
 }
 
-template ConversionResult<float> str_to_floating<float>(StringPiece* src);
-template ConversionResult<double> str_to_floating<double>(StringPiece* src);
+template Expected<float, ConversionCode> str_to_floating<float>(
+    StringPiece* src) noexcept;
+template Expected<double, ConversionCode> str_to_floating<double>(
+    StringPiece* src) noexcept;
 
 /**
  * This class takes care of additional processing needed for signed values,
@@ -460,39 +441,39 @@ class SignedValueHandler;
 template <typename T>
 class SignedValueHandler<T, true> {
  public:
-  ConversionError::Code init(const char*& b) {
+  ConversionCode init(const char*& b) {
     negative_ = false;
     if (!std::isdigit(*b)) {
       if (*b == '-') {
         negative_ = true;
       } else if (UNLIKELY(*b != '+')) {
-        return ConversionError::INVALID_LEADING_CHAR;
+        return ConversionCode::INVALID_LEADING_CHAR;
       }
       ++b;
     }
-    return ConversionError::SUCCESS;
+    return ConversionCode::SUCCESS;
   }
 
-  ConversionError::Code overflow() {
-    return negative_ ? ConversionError::NEGATIVE_OVERFLOW
-                     : ConversionError::POSITIVE_OVERFLOW;
+  ConversionCode overflow() {
+    return negative_ ? ConversionCode::NEGATIVE_OVERFLOW
+                     : ConversionCode::POSITIVE_OVERFLOW;
   }
 
   template <typename U>
-  ConversionResult<T> finalize(U value) {
+  Expected<T, ConversionCode> finalize(U value) {
     T rv;
     if (negative_) {
       rv = -value;
       if (UNLIKELY(rv > 0)) {
-        return ConversionResult<T>(ConversionError::NEGATIVE_OVERFLOW);
+        return makeUnexpected(ConversionCode::NEGATIVE_OVERFLOW);
       }
     } else {
       rv = value;
       if (UNLIKELY(rv < 0)) {
-        return ConversionResult<T>(ConversionError::POSITIVE_OVERFLOW);
+        return makeUnexpected(ConversionCode::POSITIVE_OVERFLOW);
       }
     }
-    return ConversionResult<T>(rv);
+    return rv;
   }
 
  private:
@@ -503,16 +484,16 @@ class SignedValueHandler<T, true> {
 template <typename T>
 class SignedValueHandler<T, false> {
  public:
-  ConversionError::Code init(const char*&) {
-    return ConversionError::SUCCESS;
+  ConversionCode init(const char*&) {
+    return ConversionCode::SUCCESS;
   }
 
-  ConversionError::Code overflow() {
-    return ConversionError::POSITIVE_OVERFLOW;
+  ConversionCode overflow() {
+    return ConversionCode::POSITIVE_OVERFLOW;
   }
 
-  ConversionResult<T> finalize(T value) {
-    return ConversionResult<T>(value);
+  Expected<T, ConversionCode> finalize(T value) {
+    return value;
   }
 };
 
@@ -524,15 +505,17 @@ class SignedValueHandler<T, false> {
  * an appropriate error.
  */
 template <class Tgt>
-inline ConversionResult<Tgt> digits_to(const char* b, const char* const e) {
+inline Expected<Tgt, ConversionCode> digits_to(
+    const char* b,
+    const char* const e) noexcept {
   using UT = typename std::make_unsigned<Tgt>::type;
   assert(b <= e);
 
   SignedValueHandler<Tgt> sgn;
 
   auto err = sgn.init(b);
-  if (UNLIKELY(err != ConversionError::SUCCESS)) {
-    return ConversionResult<Tgt>(err);
+  if (UNLIKELY(err != ConversionCode::SUCCESS)) {
+    return makeUnexpected(err);
   }
 
   size_t size = e - b;
@@ -545,7 +528,7 @@ inline ConversionResult<Tgt> digits_to(const char* b, const char* const e) {
     if (b < e && *b == '0') {
       for (++b;; ++b) {
         if (b == e) {
-          return ConversionResult<Tgt>(Tgt(0)); // just zeros, e.g. "0000"
+          return Tgt(0); // just zeros, e.g. "0000"
         }
         if (*b != '0') {
           size = e - b;
@@ -556,7 +539,7 @@ inline ConversionResult<Tgt> digits_to(const char* b, const char* const e) {
     if (size > std::numeric_limits<UT>::digits10 &&
         (size != std::numeric_limits<UT>::digits10 + 1 ||
          strncmp(b, MaxString<UT>::value, size) > 0)) {
-      return ConversionResult<Tgt>(sgn.overflow());
+      return makeUnexpected(sgn.overflow());
     }
   }
 
@@ -611,7 +594,7 @@ inline ConversionResult<Tgt> digits_to(const char* b, const char* const e) {
   default:
     assert(b == e);
     if (size == 0) {
-      return ConversionResult<Tgt>(ConversionError::NO_DIGITS);
+      return makeUnexpected(ConversionCode::NO_DIGITS);
     }
     break;
   }
@@ -619,46 +602,52 @@ inline ConversionResult<Tgt> digits_to(const char* b, const char* const e) {
   return sgn.finalize(result);
 
 outOfRange:
-  return ConversionResult<Tgt>(ConversionError::NON_DIGIT_CHAR);
+  return makeUnexpected(ConversionCode::NON_DIGIT_CHAR);
 }
 
-template ConversionResult<char> digits_to<char>(const char*, const char*);
-template ConversionResult<signed char> digits_to<signed char>(
+template Expected<char, ConversionCode> digits_to<char>(
     const char*,
-    const char*);
-template ConversionResult<unsigned char> digits_to<unsigned char>(
+    const char*) noexcept;
+template Expected<signed char, ConversionCode> digits_to<signed char>(
     const char*,
-    const char*);
-
-template ConversionResult<short> digits_to<short>(const char*, const char*);
-template ConversionResult<unsigned short> digits_to<unsigned short>(
+    const char*) noexcept;
+template Expected<unsigned char, ConversionCode> digits_to<unsigned char>(
     const char*,
-    const char*);
+    const char*) noexcept;
 
-template ConversionResult<int> digits_to<int>(const char*, const char*);
-template ConversionResult<unsigned int> digits_to<unsigned int>(
+template Expected<short, ConversionCode> digits_to<short>(
     const char*,
-    const char*);
+    const char*) noexcept;
+template Expected<unsigned short, ConversionCode> digits_to<unsigned short>(
+    const char*,
+    const char*) noexcept;
 
-template ConversionResult<long> digits_to<long>(const char*, const char*);
-template ConversionResult<unsigned long> digits_to<unsigned long>(
+template Expected<int, ConversionCode> digits_to<int>(
     const char*,
-    const char*);
+    const char*) noexcept;
+template Expected<unsigned int, ConversionCode> digits_to<unsigned int>(
+    const char*,
+    const char*) noexcept;
 
-template ConversionResult<long long> digits_to<long long>(
+template Expected<long, ConversionCode> digits_to<long>(
     const char*,
     const char*);
-template ConversionResult<unsigned long long> digits_to<unsigned long long>(
+template Expected<unsigned long, ConversionCode> digits_to<unsigned long>(
     const char*,
-    const char*);
+    const char*) noexcept;
 
-#if FOLLY_HAVE_INT128_T
-template ConversionResult<__int128> digits_to<__int128>(
+template Expected<long long, ConversionCode> digits_to<long long>(
     const char*,
-    const char*);
-template ConversionResult<unsigned __int128> digits_to<unsigned __int128>(
+    const char*) noexcept;
+template Expected<unsigned long long, ConversionCode>
+digits_to<unsigned long long>(const char*, const char*) noexcept;
+
+#if FOLLY_HAVE_INT128_T
+template Expected<__int128, ConversionCode> digits_to<__int128>(
     const char*,
-    const char*);
+    const char*) noexcept;
+template Expected<unsigned __int128, ConversionCode>
+digits_to<unsigned __int128>(const char*, const char*) noexcept;
 #endif
 
 /**
@@ -666,14 +655,14 @@ template ConversionResult<unsigned __int128> digits_to<unsigned __int128>(
  * StringPiece parameter to munch the already-parsed characters.
  */
 template <class Tgt>
-ConversionResult<Tgt> str_to_integral(StringPiece* src) {
+Expected<Tgt, ConversionCode> str_to_integral(StringPiece* src) noexcept {
   using UT = typename std::make_unsigned<Tgt>::type;
 
   auto b = src->data(), past = src->data() + src->size();
 
   for (;; ++b) {
     if (UNLIKELY(b >= past)) {
-      return ConversionResult<Tgt>(ConversionError::EMPTY_INPUT_STRING);
+      return makeUnexpected(ConversionCode::EMPTY_INPUT_STRING);
     }
     if (!std::isspace(*b)) {
       break;
@@ -683,63 +672,93 @@ ConversionResult<Tgt> str_to_integral(StringPiece* src) {
   SignedValueHandler<Tgt> sgn;
   auto err = sgn.init(b);
 
-  if (UNLIKELY(err != ConversionError::SUCCESS)) {
-    return ConversionResult<Tgt>(err);
+  if (UNLIKELY(err != ConversionCode::SUCCESS)) {
+    return makeUnexpected(err);
   }
   if (std::is_signed<Tgt>::value && UNLIKELY(b >= past)) {
-    return ConversionResult<Tgt>(ConversionError::NO_DIGITS);
+    return makeUnexpected(ConversionCode::NO_DIGITS);
   }
   if (UNLIKELY(!isdigit(*b))) {
-    return ConversionResult<Tgt>(ConversionError::NON_DIGIT_CHAR);
+    return makeUnexpected(ConversionCode::NON_DIGIT_CHAR);
   }
 
   auto m = findFirstNonDigit(b + 1, past);
 
   auto tmp = digits_to<UT>(b, m);
 
-  if (UNLIKELY(!tmp.success())) {
-    return ConversionResult<Tgt>(
-        tmp.error == ConversionError::POSITIVE_OVERFLOW ? sgn.overflow()
-                                                        : tmp.error);
+  if (UNLIKELY(!tmp.hasValue())) {
+    return makeUnexpected(
+        tmp.error() == ConversionCode::POSITIVE_OVERFLOW ? sgn.overflow()
+                                                         : tmp.error());
   }
 
-  auto res = sgn.finalize(tmp.value);
+  auto res = sgn.finalize(tmp.value());
 
-  if (res.success()) {
+  if (res.hasValue()) {
     src->advance(m - src->data());
   }
 
   return res;
 }
 
-template ConversionResult<char> str_to_integral<char>(StringPiece* src);
-template ConversionResult<signed char> str_to_integral<signed char>(
-    StringPiece* src);
-template ConversionResult<unsigned char> str_to_integral<unsigned char>(
-    StringPiece* src);
-
-template ConversionResult<short> str_to_integral<short>(StringPiece* src);
-template ConversionResult<unsigned short> str_to_integral<unsigned short>(
-    StringPiece* src);
-
-template ConversionResult<int> str_to_integral<int>(StringPiece* src);
-template ConversionResult<unsigned int> str_to_integral<unsigned int>(
-    StringPiece* src);
-
-template ConversionResult<long> str_to_integral<long>(StringPiece* src);
-template ConversionResult<unsigned long> str_to_integral<unsigned long>(
-    StringPiece* src);
-
-template ConversionResult<long long> str_to_integral<long long>(
-    StringPiece* src);
-template ConversionResult<unsigned long long>
-str_to_integral<unsigned long long>(StringPiece* src);
+template Expected<char, ConversionCode> str_to_integral<char>(
+    StringPiece* src) noexcept;
+template Expected<signed char, ConversionCode> str_to_integral<signed char>(
+    StringPiece* src) noexcept;
+template Expected<unsigned char, ConversionCode> str_to_integral<unsigned char>(
+    StringPiece* src) noexcept;
+
+template Expected<short, ConversionCode> str_to_integral<short>(
+    StringPiece* src) noexcept;
+template Expected<unsigned short, ConversionCode>
+str_to_integral<unsigned short>(StringPiece* src) noexcept;
+
+template Expected<int, ConversionCode> str_to_integral<int>(
+    StringPiece* src) noexcept;
+template Expected<unsigned int, ConversionCode> str_to_integral<unsigned int>(
+    StringPiece* src) noexcept;
+
+template Expected<long, ConversionCode> str_to_integral<long>(
+    StringPiece* src) noexcept;
+template Expected<unsigned long, ConversionCode> str_to_integral<unsigned long>(
+    StringPiece* src) noexcept;
+
+template Expected<long long, ConversionCode> str_to_integral<long long>(
+    StringPiece* src) noexcept;
+template Expected<unsigned long long, ConversionCode>
+str_to_integral<unsigned long long>(StringPiece* src) noexcept;
 
 #if FOLLY_HAVE_INT128_T
-template ConversionResult<__int128> str_to_integral<__int128>(StringPiece* src);
-template ConversionResult<unsigned __int128> str_to_integral<unsigned __int128>(
-    StringPiece* src);
+template Expected<__int128, ConversionCode> str_to_integral<__int128>(
+    StringPiece* src) noexcept;
+template Expected<unsigned __int128, ConversionCode>
+str_to_integral<unsigned __int128>(StringPiece* src) noexcept;
 #endif
 
 } // namespace detail
+
+ConversionError makeConversionError(ConversionCode code, StringPiece input) {
+  using namespace detail;
+  static_assert(
+      std::is_unsigned<std::underlying_type<ConversionCode>::type>::value,
+      "ConversionCode should be unsigned");
+  assert((std::size_t)code < kErrorStrings.size());
+  const ErrorString& err = kErrorStrings[(std::size_t)code];
+  if (code == ConversionCode::EMPTY_INPUT_STRING && input.empty()) {
+    return {err.string, code};
+  }
+  std::string tmp(err.string);
+  tmp.append(": ");
+  if (err.quote) {
+    tmp.append(1, '"');
+  }
+  if (input.size() > 0) {
+    tmp.append(input.data(), input.size());
+  }
+  if (err.quote) {
+    tmp.append(1, '"');
+  }
+  return {tmp, code};
+}
+
 } // namespace folly
diff --git a/folly/Conv.h b/folly/Conv.h
index df34f2fa..9fc19696 100644
--- a/folly/Conv.h
+++ b/folly/Conv.h
@@ -39,75 +39,88 @@
 #include <double-conversion/double-conversion.h> // V8 JavaScript implementation
 
 #include <folly/Demangle.h>
+#include <folly/Expected.h>
 #include <folly/FBString.h>
 #include <folly/Likely.h>
 #include <folly/Range.h>
+#include <folly/Unit.h>
 #include <folly/portability/Math.h>
 
 namespace folly {
 
-class ConversionError : public std::range_error {
- public:
-  // Keep this in sync with kErrorStrings in Conv.cpp
-  enum Code {
-    SUCCESS,
-    EMPTY_INPUT_STRING,
-    NO_DIGITS,
-    BOOL_OVERFLOW,
-    BOOL_INVALID_VALUE,
-    NON_DIGIT_CHAR,
-    INVALID_LEADING_CHAR,
-    POSITIVE_OVERFLOW,
-    NEGATIVE_OVERFLOW,
-    STRING_TO_FLOAT_ERROR,
-    NON_WHITESPACE_AFTER_END,
-    ARITH_POSITIVE_OVERFLOW,
-    ARITH_NEGATIVE_OVERFLOW,
-    ARITH_LOSS_OF_PRECISION,
-    NUM_ERROR_CODES, // has to be the last entry
-  };
+// Keep this in sync with kErrorStrings in Conv.cpp
+enum class ConversionCode : unsigned char {
+  SUCCESS,
+  EMPTY_INPUT_STRING,
+  NO_DIGITS,
+  BOOL_OVERFLOW,
+  BOOL_INVALID_VALUE,
+  NON_DIGIT_CHAR,
+  INVALID_LEADING_CHAR,
+  POSITIVE_OVERFLOW,
+  NEGATIVE_OVERFLOW,
+  STRING_TO_FLOAT_ERROR,
+  NON_WHITESPACE_AFTER_END,
+  ARITH_POSITIVE_OVERFLOW,
+  ARITH_NEGATIVE_OVERFLOW,
+  ARITH_LOSS_OF_PRECISION,
+  NUM_ERROR_CODES, // has to be the last entry
+};
 
-  ConversionError(const std::string& str, Code code)
-      : std::range_error(str), code_(code) {}
+struct ConversionErrorBase : std::range_error {
+  using std::range_error::range_error;
+};
 
-  ConversionError(const char* str, Code code)
-      : std::range_error(str), code_(code) {}
+class ConversionError : public ConversionErrorBase {
+ public:
+  ConversionError(const std::string& str, ConversionCode code)
+      : ConversionErrorBase(str), code_(code) {}
 
-  Code errorCode() const { return code_; }
+  ConversionError(const char* str, ConversionCode code)
+      : ConversionErrorBase(str), code_(code) {}
+
+  ConversionCode errorCode() const {
+    return code_;
+  }
 
  private:
-  Code code_;
+  ConversionCode code_;
 };
 
-namespace detail {
-
-ConversionError makeConversionError(
-    ConversionError::Code code,
-    const char* input,
-    size_t inputLen);
-
-inline ConversionError makeConversionError(
-    ConversionError::Code code,
-    const std::string& str) {
-  return makeConversionError(code, str.data(), str.size());
-}
-
-inline ConversionError makeConversionError(
-    ConversionError::Code code,
-    StringPiece sp) {
-  return makeConversionError(code, sp.data(), sp.size());
-}
+/*******************************************************************************
+ * Custom Error Translation
+ *
+ * Your overloaded parseTo() function can return a custom error code on failure.
+ * ::folly::to() will call makeConversionError to translate that error code into
+ * an object to throw. makeConversionError is found by argument-dependent
+ * lookup. It should have this signature:
+ *
+ * namespace other_namespace {
+ * enum YourErrorCode { BAD_ERROR, WORSE_ERROR };
+ *
+ * struct YourConversionError : ConversionErrorBase {
+ *   YourConversionError(const char* what) : ConversionErrorBase(what) {}
+ * };
+ *
+ * YourConversionError
+ * makeConversionError(YourErrorCode code, ::folly::StringPiece sp) {
+ *   ...
+ *   return YourConversionError(messageString);
+ * }
+ ******************************************************************************/
+ConversionError makeConversionError(ConversionCode code, StringPiece sp);
 
+namespace detail {
 /**
  * Enforce that the suffix following a number is made up only of whitespace.
  */
-inline ConversionError::Code enforceWhitespaceErr(StringPiece sp) {
+inline ConversionCode enforceWhitespaceErr(StringPiece sp) {
   for (auto c : sp) {
-    if (!std::isspace(c)) {
-      return ConversionError::NON_WHITESPACE_AFTER_END;
+    if (UNLIKELY(!std::isspace(c))) {
+      return ConversionCode::NON_WHITESPACE_AFTER_END;
     }
   }
-  return ConversionError::SUCCESS;
+  return ConversionCode::SUCCESS;
 }
 
 /**
@@ -115,42 +128,57 @@ inline ConversionError::Code enforceWhitespaceErr(StringPiece sp) {
  */
 inline void enforceWhitespace(StringPiece sp) {
   auto err = enforceWhitespaceErr(sp);
-  if (err != ConversionError::SUCCESS) {
-    throw detail::makeConversionError(err, sp);
+  if (err != ConversionCode::SUCCESS) {
+    throw makeConversionError(err, sp);
   }
 }
+}
 
 /**
- * A simple std::pair-like wrapper to wrap both a value and an error
+ * The identity conversion function.
+ * tryTo<T>(T) returns itself for all types T.
  */
-template <typename T>
-struct ConversionResult {
-  explicit ConversionResult(T v) : value(v) {}
-  explicit ConversionResult(ConversionError::Code e) : error(e) {}
-
-  bool success() const {
-    return error == ConversionError::SUCCESS;
-  }
+template <class Tgt, class Src>
+typename std::enable_if<
+    std::is_same<Tgt, typename std::decay<Src>::type>::value,
+    Expected<Tgt, ConversionCode>>::type
+tryTo(Src&& value) {
+  return std::forward<Src>(value);
+}
 
-  T value;
-  ConversionError::Code error{ConversionError::SUCCESS};
-};
+template <class Tgt, class Src>
+typename std::enable_if<
+    std::is_same<Tgt, typename std::decay<Src>::type>::value,
+    Tgt>::type
+to(Src&& value) {
+  return std::forward<Src>(value);
 }
 
+/*******************************************************************************
+ * Arithmetic to boolean
+ ******************************************************************************/
+
 /**
- * The identity conversion function.
- * to<T>(T) returns itself for all types T.
+ * Unchecked conversion from arithmetic to boolean. This is different from the
+ * other arithmetic conversions because we use the C convention of treating any
+ * non-zero value as true, instead of range checking.
  */
 template <class Tgt, class Src>
-typename std::enable_if<std::is_same<Tgt, Src>::value, Tgt>::type
-to(const Src & value) {
-  return value;
+typename std::enable_if<
+    std::is_arithmetic<Src>::value && !std::is_same<Tgt, Src>::value &&
+        std::is_same<Tgt, bool>::value,
+    Expected<Tgt, ConversionCode>>::type
+tryTo(const Src& value) {
+  return value != Src();
 }
 
 template <class Tgt, class Src>
-typename std::enable_if<std::is_same<Tgt, Src>::value, Tgt>::type
-to(Src && value) {
-  return std::forward<Src>(value);
+typename std::enable_if<
+    std::is_arithmetic<Src>::value && !std::is_same<Tgt, Src>::value &&
+        std::is_same<Tgt, bool>::value,
+    Tgt>::type
+to(const Src& value) {
+  return value != Src();
 }
 
 /*******************************************************************************
@@ -159,34 +187,28 @@ to(Src && value) {
 
 namespace detail {
 
-template <class T>
-const T& getLastElement(const T & v) {
-  return v;
-}
+template <typename... Ts>
+struct LastElementImpl {
+  static void call(Ignored<Ts>...) {}
+};
 
-template <class T, class... Ts>
-typename std::tuple_element<
-  sizeof...(Ts),
-  std::tuple<T, Ts...> >::type const&
-  getLastElement(const T&, const Ts&... vs) {
-  return getLastElement(vs...);
+template <typename Head, typename... Ts>
+struct LastElementImpl<Head, Ts...> {
+  template <typename Last>
+  static Last call(Ignored<Ts>..., Last&& last) {
+    return std::forward<Last>(last);
+  }
+};
+
+template <typename... Ts>
+auto getLastElement(const Ts&... ts)
+    -> decltype(LastElementImpl<Ts...>::call(ts...)) {
+  return LastElementImpl<Ts...>::call(ts...);
 }
 
-// This class exists to specialize away std::tuple_element in the case where we
-// have 0 template arguments. Without this, Clang/libc++ will blow a
-// static_assert even if tuple_element is protected by an enable_if.
 template <class... Ts>
-struct last_element {
-  typedef typename std::enable_if<
-    sizeof...(Ts) >= 1,
-    typename std::tuple_element<
-      sizeof...(Ts) - 1, std::tuple<Ts...>
-    >::type>::type type;
-};
-
-template <>
-struct last_element<> {
-  typedef void type;
+struct LastElement : std::decay<decltype(
+                         LastElementImpl<Ts...>::call(std::declval<Ts>()...))> {
 };
 
 } // namespace detail
@@ -373,11 +395,10 @@ toAppend(Src value, Tgt * result) {
   }
 }
 
-template<class Src>
-typename std::enable_if<
-  std::is_convertible<Src, const char*>::value,
-  size_t>::type
-estimateSpaceNeeded(Src value) {
+template <class Src>
+typename std::enable_if<std::is_convertible<Src, const char*>::value, size_t>::
+    type
+    estimateSpaceNeeded(Src value) {
   const char *c = value;
   if (c) {
     return folly::StringPiece(value).size();
@@ -747,11 +768,10 @@ toAppendStrImpl(const T& v, Tgt result) {
 }
 
 template <class T, class... Ts>
-typename std::enable_if<sizeof...(Ts) >= 2
-  && IsSomeString<
-  typename std::remove_pointer<
-    typename detail::last_element<Ts...>::type
-  >::type>::value>::type
+typename std::enable_if<
+    sizeof...(Ts) >= 2 &&
+    IsSomeString<typename std::remove_pointer<
+        typename detail::LastElement<const Ts&...>::type>::type>::value>::type
 toAppendStrImpl(const T& v, const Ts&... vs) {
   toAppend(v, getLastElement(vs...));
   toAppendStrImpl(vs...);
@@ -765,11 +785,10 @@ toAppendDelimStrImpl(const Delimiter& /* delim */, const T& v, Tgt result) {
 }
 
 template <class Delimiter, class T, class... Ts>
-typename std::enable_if<sizeof...(Ts) >= 2
-  && IsSomeString<
-  typename std::remove_pointer<
-    typename detail::last_element<Ts...>::type
-  >::type>::value>::type
+typename std::enable_if<
+    sizeof...(Ts) >= 2 &&
+    IsSomeString<typename std::remove_pointer<
+        typename detail::LastElement<const Ts&...>::type>::type>::value>::type
 toAppendDelimStrImpl(const Delimiter& delim, const T& v, const Ts&... vs) {
   // we are really careful here, calling toAppend with just one element does
   // not try to estimate space needed (as we already did that). If we call
@@ -803,11 +822,10 @@ toAppendDelimStrImpl(const Delimiter& delim, const T& v, const Ts&... vs) {
  * }
  */
 template <class... Ts>
-typename std::enable_if<sizeof...(Ts) >= 3
-  && IsSomeString<
-  typename std::remove_pointer<
-    typename detail::last_element<Ts...>::type
-  >::type>::value>::type
+typename std::enable_if<
+    sizeof...(Ts) >= 3 &&
+    IsSomeString<typename std::remove_pointer<
+        typename detail::LastElement<const Ts&...>::type>::type>::value>::type
 toAppend(const Ts&... vs) {
   ::folly::detail::toAppendStrImpl(vs...);
 }
@@ -833,11 +851,8 @@ void toAppend(const pid_t a, Tgt* res) {
  * will probably save a few calls to malloc.
  */
 template <class... Ts>
-typename std::enable_if<
-  IsSomeString<
-  typename std::remove_pointer<
-    typename detail::last_element<Ts...>::type
-  >::type>::value>::type
+typename std::enable_if<IsSomeString<typename std::remove_pointer<
+    typename detail::LastElement<const Ts&...>::type>::type>::value>::type
 toAppendFit(const Ts&... vs) {
   ::folly::detail::reserveInTarget(vs...);
   toAppend(vs...);
@@ -874,11 +889,10 @@ typename std::enable_if<IsSomeString<Tgt>::value>::type toAppendDelim(
  * comments for toAppend for details about memory allocation.
  */
 template <class Delimiter, class... Ts>
-typename std::enable_if<sizeof...(Ts) >= 3
-  && IsSomeString<
-  typename std::remove_pointer<
-    typename detail::last_element<Ts...>::type
-  >::type>::value>::type
+typename std::enable_if<
+    sizeof...(Ts) >= 3 &&
+    IsSomeString<typename std::remove_pointer<
+        typename detail::LastElement<const Ts&...>::type>::type>::value>::type
 toAppendDelim(const Delimiter& delim, const Ts&... vs) {
   detail::toAppendDelimStrImpl(delim, vs...);
 }
@@ -887,11 +901,8 @@ toAppendDelim(const Delimiter& delim, const Ts&... vs) {
  * Detail in comment for toAppendFit
  */
 template <class Delimiter, class... Ts>
-typename std::enable_if<
-  IsSomeString<
-  typename std::remove_pointer<
-    typename detail::last_element<Ts...>::type
-  >::type>::value>::type
+typename std::enable_if<IsSomeString<typename std::remove_pointer<
+    typename detail::LastElement<const Ts&...>::type>::type>::value>::type
 toAppendDelimFit(const Delimiter& delim, const Ts&... vs) {
   detail::reserveInTargetDelim(delim, vs...);
   toAppendDelim(delim, vs...);
@@ -906,10 +917,11 @@ void toAppendDelimFit(const De&, const Ts&) {}
  */
 template <class Tgt, class... Ts>
 typename std::enable_if<
-  IsSomeString<Tgt>::value && (
-    sizeof...(Ts) != 1 ||
-    !std::is_same<Tgt, typename detail::last_element<Ts...>::type>::value),
-  Tgt>::type
+    IsSomeString<Tgt>::value &&
+        (sizeof...(Ts) != 1 ||
+         !std::is_same<Tgt, typename detail::LastElement<const Ts&...>::type>::
+             value),
+    Tgt>::type
 to(const Ts&... vs) {
   Tgt result;
   toAppendFit(vs..., &result);
@@ -933,10 +945,11 @@ toDelim(const Delim& /* delim */, const Src& value) {
  */
 template <class Tgt, class Delim, class... Ts>
 typename std::enable_if<
-  IsSomeString<Tgt>::value && (
-    sizeof...(Ts) != 1 ||
-    !std::is_same<Tgt, typename detail::last_element<Ts...>::type>::value),
-  Tgt>::type
+    IsSomeString<Tgt>::value &&
+        (sizeof...(Ts) != 1 ||
+         !std::is_same<Tgt, typename detail::LastElement<const Ts&...>::type>::
+             value),
+    Tgt>::type
 toDelim(const Delim& delim, const Ts&... vs) {
   Tgt result;
   toAppendDelimFit(delim, vs..., &result);
@@ -949,130 +962,121 @@ toDelim(const Delim& delim, const Ts&... vs) {
 
 namespace detail {
 
-ConversionResult<bool> str_to_bool(StringPiece* src);
+Expected<bool, ConversionCode> str_to_bool(StringPiece* src) noexcept;
 
 template <typename T>
-ConversionResult<T> str_to_floating(StringPiece* src);
+Expected<T, ConversionCode> str_to_floating(StringPiece* src) noexcept;
 
-extern template ConversionResult<float> str_to_floating<float>(
-    StringPiece* src);
-extern template ConversionResult<double> str_to_floating<double>(
-    StringPiece* src);
+extern template Expected<float, ConversionCode> str_to_floating<float>(
+    StringPiece* src) noexcept;
+extern template Expected<double, ConversionCode> str_to_floating<double>(
+    StringPiece* src) noexcept;
 
 template <class Tgt>
-ConversionResult<Tgt> digits_to(const char* b, const char* e);
+Expected<Tgt, ConversionCode> digits_to(const char* b, const char* e) noexcept;
 
-extern template ConversionResult<char> digits_to<char>(
-    const char*,
-    const char*);
-extern template ConversionResult<signed char> digits_to<signed char>(
+extern template Expected<char, ConversionCode> digits_to<char>(
     const char*,
-    const char*);
-extern template ConversionResult<unsigned char> digits_to<unsigned char>(
+    const char*) noexcept;
+extern template Expected<signed char, ConversionCode> digits_to<signed char>(
     const char*,
-    const char*);
+    const char*) noexcept;
+extern template Expected<unsigned char, ConversionCode>
+digits_to<unsigned char>(const char*, const char*) noexcept;
 
-extern template ConversionResult<short> digits_to<short>(
+extern template Expected<short, ConversionCode> digits_to<short>(
     const char*,
-    const char*);
-extern template ConversionResult<unsigned short> digits_to<unsigned short>(
-    const char*,
-    const char*);
+    const char*) noexcept;
+extern template Expected<unsigned short, ConversionCode>
+digits_to<unsigned short>(const char*, const char*) noexcept;
 
-extern template ConversionResult<int> digits_to<int>(const char*, const char*);
-extern template ConversionResult<unsigned int> digits_to<unsigned int>(
+extern template Expected<int, ConversionCode> digits_to<int>(
     const char*,
-    const char*);
-
-extern template ConversionResult<long> digits_to<long>(
+    const char*) noexcept;
+extern template Expected<unsigned int, ConversionCode> digits_to<unsigned int>(
     const char*,
-    const char*);
-extern template ConversionResult<unsigned long> digits_to<unsigned long>(
+    const char*) noexcept;
+
+extern template Expected<long, ConversionCode> digits_to<long>(
     const char*,
-    const char*);
+    const char*) noexcept;
+extern template Expected<unsigned long, ConversionCode>
+digits_to<unsigned long>(const char*, const char*) noexcept;
 
-extern template ConversionResult<long long> digits_to<long long>(
+extern template Expected<long long, ConversionCode> digits_to<long long>(
     const char*,
-    const char*);
-extern template ConversionResult<unsigned long long>
-digits_to<unsigned long long>(const char*, const char*);
+    const char*) noexcept;
+extern template Expected<unsigned long long, ConversionCode>
+digits_to<unsigned long long>(const char*, const char*) noexcept;
 
 #if FOLLY_HAVE_INT128_T
-extern template ConversionResult<__int128> digits_to<__int128>(
+extern template Expected<__int128, ConversionCode> digits_to<__int128>(
     const char*,
-    const char*);
-extern template ConversionResult<unsigned __int128>
-digits_to<unsigned __int128>(const char*, const char*);
+    const char*) noexcept;
+extern template Expected<unsigned __int128, ConversionCode>
+digits_to<unsigned __int128>(const char*, const char*) noexcept;
 #endif
 
 template <class T>
-ConversionResult<T> str_to_integral(StringPiece* src);
-
-extern template ConversionResult<char> str_to_integral<char>(StringPiece* src);
-extern template ConversionResult<signed char> str_to_integral<signed char>(
-    StringPiece* src);
-extern template ConversionResult<unsigned char> str_to_integral<unsigned char>(
-    StringPiece* src);
-
-extern template ConversionResult<short> str_to_integral<short>(
-    StringPiece* src);
-extern template ConversionResult<unsigned short>
-str_to_integral<unsigned short>(StringPiece* src);
-
-extern template ConversionResult<int> str_to_integral<int>(StringPiece* src);
-extern template ConversionResult<unsigned int> str_to_integral<unsigned int>(
-    StringPiece* src);
-
-extern template ConversionResult<long> str_to_integral<long>(StringPiece* src);
-extern template ConversionResult<unsigned long> str_to_integral<unsigned long>(
-    StringPiece* src);
-
-extern template ConversionResult<long long> str_to_integral<long long>(
-    StringPiece* src);
-extern template ConversionResult<unsigned long long>
-str_to_integral<unsigned long long>(StringPiece* src);
+Expected<T, ConversionCode> str_to_integral(StringPiece* src) noexcept;
+
+extern template Expected<char, ConversionCode> str_to_integral<char>(
+    StringPiece* src) noexcept;
+extern template Expected<signed char, ConversionCode>
+str_to_integral<signed char>(StringPiece* src) noexcept;
+extern template Expected<unsigned char, ConversionCode>
+str_to_integral<unsigned char>(StringPiece* src) noexcept;
+
+extern template Expected<short, ConversionCode> str_to_integral<short>(
+    StringPiece* src) noexcept;
+extern template Expected<unsigned short, ConversionCode>
+str_to_integral<unsigned short>(StringPiece* src) noexcept;
+
+extern template Expected<int, ConversionCode> str_to_integral<int>(
+    StringPiece* src) noexcept;
+extern template Expected<unsigned int, ConversionCode>
+str_to_integral<unsigned int>(StringPiece* src) noexcept;
+
+extern template Expected<long, ConversionCode> str_to_integral<long>(
+    StringPiece* src) noexcept;
+extern template Expected<unsigned long, ConversionCode>
+str_to_integral<unsigned long>(StringPiece* src) noexcept;
+
+extern template Expected<long long, ConversionCode> str_to_integral<long long>(
+    StringPiece* src) noexcept;
+extern template Expected<unsigned long long, ConversionCode>
+str_to_integral<unsigned long long>(StringPiece* src) noexcept;
 
 #if FOLLY_HAVE_INT128_T
-extern template ConversionResult<__int128> str_to_integral<__int128>(
-    StringPiece* src);
-extern template ConversionResult<unsigned __int128>
-str_to_integral<unsigned __int128>(StringPiece* src);
+extern template Expected<__int128, ConversionCode> str_to_integral<__int128>(
+    StringPiece* src) noexcept;
+extern template Expected<unsigned __int128, ConversionCode>
+str_to_integral<unsigned __int128>(StringPiece* src) noexcept;
 #endif
 
 template <typename T>
-typename std::enable_if<std::is_same<T, bool>::value, ConversionResult<T>>::type
-convertTo(StringPiece* src) {
+typename std::
+    enable_if<std::is_same<T, bool>::value, Expected<T, ConversionCode>>::type
+    convertTo(StringPiece* src) noexcept {
   return str_to_bool(src);
 }
 
 template <typename T>
 typename std::enable_if<
-    std::is_floating_point<T>::value, ConversionResult<T>>::type
-convertTo(StringPiece* src) {
+    std::is_floating_point<T>::value,
+    Expected<T, ConversionCode>>::type
+convertTo(StringPiece* src) noexcept {
   return str_to_floating<T>(src);
 }
 
 template <typename T>
 typename std::enable_if<
     std::is_integral<T>::value && !std::is_same<T, bool>::value,
-    ConversionResult<T>>::type
-convertTo(StringPiece* src) {
+    Expected<T, ConversionCode>>::type
+convertTo(StringPiece* src) noexcept {
   return str_to_integral<T>(src);
 }
 
-template <typename T>
-struct WrapperInfo { using type = T; };
-
-template <typename T, typename Gen>
-typename std::enable_if<
-    std::is_same<typename WrapperInfo<T>::type, T>::value, T>::type
-inline wrap(ConversionResult<typename WrapperInfo<T>::type> res, Gen&& gen) {
-  if (LIKELY(res.success())) {
-    return res.value;
-  }
-  throw detail::makeConversionError(res.error, gen());
-}
-
 } // namespace detail
 
 /**
@@ -1081,12 +1085,22 @@ inline wrap(ConversionResult<typename WrapperInfo<T>::type> res, Gen&& gen) {
  */
 template <typename Tgt>
 typename std::enable_if<
-    std::is_integral<typename detail::WrapperInfo<Tgt>::type>::value &&
-    !std::is_same<typename detail::WrapperInfo<Tgt>::type, bool>::value,
+    std::is_integral<Tgt>::value && !std::is_same<Tgt, bool>::value,
+    Expected<Tgt, ConversionCode>>::type
+tryTo(const char* b, const char* e) {
+  return detail::digits_to<Tgt>(b, e);
+}
+
+template <typename Tgt>
+typename std::enable_if<
+    std::is_integral<Tgt>::value && !std::is_same<Tgt, bool>::value,
     Tgt>::type
 to(const char* b, const char* e) {
-  auto res = detail::digits_to<typename detail::WrapperInfo<Tgt>::type>(b, e);
-  return detail::wrap<Tgt>(res, [&] { return StringPiece(b, e); });
+  return tryTo<Tgt>(b, e).thenOrThrow(
+      [](Tgt res) { return res; },
+      [=](ConversionCode code) {
+        return makeConversionError(code, StringPiece(b, e));
+      });
 }
 
 /*******************************************************************************
@@ -1094,47 +1108,21 @@ to(const char* b, const char* e) {
  ******************************************************************************/
 
 /**
- * Parsing strings to numeric types. These routines differ from
- * parseTo(str, numeric) routines in that they take a POINTER TO a StringPiece
- * and alter that StringPiece to reflect progress information.
+ * Parsing strings to numeric types.
  */
 template <typename Tgt>
-typename std::enable_if<
-    std::is_arithmetic<typename detail::WrapperInfo<Tgt>::type>::value>::type
-parseTo(StringPiece* src, Tgt& out) {
-  auto res = detail::convertTo<typename detail::WrapperInfo<Tgt>::type>(src);
-  out = detail::wrap<Tgt>(res, [&] { return *src; });
-}
-
-template <typename Tgt>
-typename std::enable_if<
-    std::is_arithmetic<typename detail::WrapperInfo<Tgt>::type>::value>::type
+FOLLY_WARN_UNUSED_RESULT inline typename std::enable_if<
+    std::is_arithmetic<Tgt>::value,
+    Expected<StringPiece, ConversionCode>>::type
 parseTo(StringPiece src, Tgt& out) {
-  auto res = detail::convertTo<typename detail::WrapperInfo<Tgt>::type>(&src);
-  if (LIKELY(res.success())) {
-    res.error = detail::enforceWhitespaceErr(src);
-  }
-  out = detail::wrap<Tgt>(res, [&] { return src; });
+  return detail::convertTo<Tgt>(&src).then(
+      [&](Tgt res) { return void(out = res), src; });
 }
 
 /*******************************************************************************
  * Integral / Floating Point to integral / Floating Point
  ******************************************************************************/
 
-/**
- * Unchecked conversion from arithmetic to boolean. This is different from the
- * other arithmetic conversions because we use the C convention of treating any
- * non-zero value as true, instead of range checking.
- */
-template <class Tgt, class Src>
-typename std::enable_if<
-    std::is_arithmetic<Src>::value && !std::is_same<Tgt, Src>::value &&
-        std::is_same<Tgt, bool>::value,
-    Tgt>::type
-to(const Src& value) {
-  return value != Src();
-}
-
 namespace detail {
 
 /**
@@ -1147,22 +1135,22 @@ typename std::enable_if<
     std::is_integral<Src>::value && !std::is_same<Tgt, Src>::value &&
         !std::is_same<Tgt, bool>::value &&
         std::is_integral<Tgt>::value,
-    ConversionResult<Tgt>>::type
-convertTo(const Src& value) {
+    Expected<Tgt, ConversionCode>>::type
+convertTo(const Src& value) noexcept {
   /* static */ if (
       std::numeric_limits<Tgt>::max() < std::numeric_limits<Src>::max()) {
     if (greater_than<Tgt, std::numeric_limits<Tgt>::max()>(value)) {
-      return ConversionResult<Tgt>(ConversionError::ARITH_POSITIVE_OVERFLOW);
+      return makeUnexpected(ConversionCode::ARITH_POSITIVE_OVERFLOW);
     }
   }
   /* static */ if (
       std::is_signed<Src>::value &&
       (!std::is_signed<Tgt>::value || sizeof(Src) > sizeof(Tgt))) {
     if (less_than<Tgt, std::numeric_limits<Tgt>::min()>(value)) {
-      return ConversionResult<Tgt>(ConversionError::ARITH_NEGATIVE_OVERFLOW);
+      return makeUnexpected(ConversionCode::ARITH_NEGATIVE_OVERFLOW);
     }
   }
-  return ConversionResult<Tgt>(static_cast<Tgt>(value));
+  return static_cast<Tgt>(value);
 }
 
 /**
@@ -1174,18 +1162,18 @@ template <class Tgt, class Src>
 typename std::enable_if<
     std::is_floating_point<Tgt>::value && std::is_floating_point<Src>::value &&
         !std::is_same<Tgt, Src>::value,
-    ConversionResult<Tgt>>::type
-convertTo(const Src& value) {
+    Expected<Tgt, ConversionCode>>::type
+convertTo(const Src& value) noexcept {
   /* static */ if (
       std::numeric_limits<Tgt>::max() < std::numeric_limits<Src>::max()) {
     if (value > std::numeric_limits<Tgt>::max()) {
-      return ConversionResult<Tgt>(ConversionError::ARITH_POSITIVE_OVERFLOW);
+      return makeUnexpected(ConversionCode::ARITH_POSITIVE_OVERFLOW);
     }
     if (value < std::numeric_limits<Tgt>::lowest()) {
-      return ConversionResult<Tgt>(ConversionError::ARITH_NEGATIVE_OVERFLOW);
+      return makeUnexpected(ConversionCode::ARITH_NEGATIVE_OVERFLOW);
     }
   }
-  return ConversionResult<Tgt>(boost::implicit_cast<Tgt>(value));
+  return boost::implicit_cast<Tgt>(value);
 }
 
 /**
@@ -1252,18 +1240,18 @@ template <typename Tgt, typename Src>
 typename std::enable_if<
     (std::is_integral<Src>::value && std::is_floating_point<Tgt>::value) ||
         (std::is_floating_point<Src>::value && std::is_integral<Tgt>::value),
-    ConversionResult<Tgt>>::type
-convertTo(const Src& value) {
+    Expected<Tgt, ConversionCode>>::type
+convertTo(const Src& value) noexcept {
   if (LIKELY(checkConversion<Tgt>(value))) {
     Tgt result = static_cast<Tgt>(value);
     if (LIKELY(checkConversion<Src>(result))) {
       Src witness = static_cast<Src>(result);
       if (LIKELY(value == witness)) {
-        return ConversionResult<Tgt>(result);
+        return result;
       }
     }
   }
-  return ConversionResult<Tgt>(ConversionError::ARITH_LOSS_OF_PRECISION);
+  return makeUnexpected(ConversionCode::ARITH_LOSS_OF_PRECISION);
 }
 
 template <typename Tgt, typename Src>
@@ -1286,13 +1274,20 @@ using IsArithToArith = std::integral_constant<
 
 template <typename Tgt, typename Src>
 typename std::enable_if<
-    detail::IsArithToArith<
-        typename detail::WrapperInfo<Tgt>::type, Src>::value, Tgt>::type
-to(const Src& value) {
-  auto res = detail::convertTo<typename detail::WrapperInfo<Tgt>::type>(value);
-  return detail::wrap<Tgt>(res, [&] {
-    return detail::errorValue<typename detail::WrapperInfo<Tgt>::type>(value);
-  });
+    detail::IsArithToArith<Tgt, Src>::value,
+    Expected<Tgt, ConversionCode>>::type
+tryTo(const Src& value) noexcept {
+  return detail::convertTo<Tgt>(value);
+}
+
+template <typename Tgt, typename Src>
+typename std::enable_if<detail::IsArithToArith<Tgt, Src>::value, Tgt>::type to(
+    const Src& value) {
+  return tryTo<Tgt>(value).thenOrThrow(
+      [](Tgt res) { return res; },
+      [&](ConversionCode e) {
+        return makeConversionError(e, detail::errorValue<Tgt>(value));
+      });
 }
 
 /*******************************************************************************
@@ -1303,49 +1298,157 @@ to(const Src& value) {
  * argument-dependent lookup:
  *
  * namespace other_namespace {
- * void parseTo(::folly::StringPiece, OtherType&);
+ * ::folly::Expected<::folly::StringPiece, SomeErrorCode>
+ *   parseTo(::folly::StringPiece, OtherType&) noexcept;
  * }
  ******************************************************************************/
 template <class T>
-typename std::enable_if<std::is_enum<T>::value>::type
-parseTo(StringPiece in, T& out) {
-  typename std::underlying_type<T>::type tmp;
-  parseTo(in, tmp);
-  out = static_cast<T>(tmp);
-}
-
-inline void parseTo(StringPiece in, StringPiece& out) {
+FOLLY_WARN_UNUSED_RESULT typename std::enable_if<
+    std::is_enum<T>::value,
+    Expected<StringPiece, ConversionCode>>::type
+parseTo(StringPiece in, T& out) noexcept {
+  typename std::underlying_type<T>::type tmp{};
+  auto restOrError = parseTo(in, tmp);
+  out = static_cast<T>(tmp); // Harmless if parseTo fails
+  return restOrError;
+}
+
+FOLLY_WARN_UNUSED_RESULT
+inline Expected<StringPiece, ConversionCode> parseTo(
+    StringPiece in,
+    StringPiece& out) noexcept {
   out = in;
+  return StringPiece{in.end(), in.end()};
 }
 
-inline void parseTo(StringPiece in, std::string& out) {
+FOLLY_WARN_UNUSED_RESULT
+inline Expected<StringPiece, ConversionCode> parseTo(
+    StringPiece in,
+    std::string& out) {
   out.clear();
-  out.append(in.data(), in.size());
+  out.append(in.data(), in.size()); // TODO try/catch?
+  return StringPiece{in.end(), in.end()};
 }
 
-inline void parseTo(StringPiece in, fbstring& out) {
+FOLLY_WARN_UNUSED_RESULT
+inline Expected<StringPiece, ConversionCode> parseTo(
+    StringPiece in,
+    fbstring& out) {
   out.clear();
-  out.append(in.data(), in.size());
+  out.append(in.data(), in.size()); // TODO try/catch?
+  return StringPiece{in.end(), in.end()};
 }
 
+namespace detail {
+template <typename Tgt>
+using ParseToResult = decltype(parseTo(StringPiece{}, std::declval<Tgt&>()));
+
+struct CheckTrailingSpace {
+  Expected<Unit, ConversionCode> operator()(StringPiece sp) const {
+    auto e = enforceWhitespaceErr(sp);
+    if (UNLIKELY(e != ConversionCode::SUCCESS))
+      return makeUnexpected(e);
+    return unit;
+  }
+};
+
+template <class Error>
+struct ReturnUnit {
+  template <class T>
+  constexpr Expected<Unit, Error> operator()(T&&) const {
+    return unit;
+  }
+};
+
+// Older versions of the parseTo customization point threw on error and
+// returned void. Handle that.
+template <class Tgt>
+inline typename std::enable_if<
+    std::is_void<ParseToResult<Tgt>>::value,
+    Expected<StringPiece, ConversionCode>>::type
+parseToWrap(StringPiece sp, Tgt& out) {
+  parseTo(sp, out);
+  return StringPiece(sp.end(), sp.end());
+}
+
+template <class Tgt>
+inline typename std::enable_if<
+    !std::is_void<ParseToResult<Tgt>>::value,
+    ParseToResult<Tgt>>::type
+parseToWrap(StringPiece sp, Tgt& out) {
+  return parseTo(sp, out);
+}
+
+template <typename Tgt>
+using ParseToError = ExpectedErrorType<decltype(
+    detail::parseToWrap(StringPiece{}, std::declval<Tgt&>()))>;
+
+} // namespace detail
+
 /**
  * String or StringPiece to target conversion. Accepts leading and trailing
  * whitespace, but no non-space trailing characters.
  */
 
 template <class Tgt>
-typename std::enable_if<!std::is_same<StringPiece, Tgt>::value, Tgt>::type
-to(StringPiece src) {
+inline typename std::enable_if<
+    !std::is_same<StringPiece, Tgt>::value,
+    Expected<Tgt, detail::ParseToError<Tgt>>>::type
+tryTo(StringPiece src) {
+  Tgt result{};
+  using Error = detail::ParseToError<Tgt>;
+  using Check = typename std::conditional<
+      std::is_arithmetic<Tgt>::value,
+      detail::CheckTrailingSpace,
+      detail::ReturnUnit<Error>>::type;
+  return parseTo(src, result).then(Check(), [&](Unit) {
+    return std::move(result);
+  });
+}
+
+template <class Tgt>
+inline
+    typename std::enable_if<!std::is_same<StringPiece, Tgt>::value, Tgt>::type
+    to(StringPiece src) {
+  Tgt result{};
+  using Error = detail::ParseToError<Tgt>;
+  using Check = typename std::conditional<
+      std::is_arithmetic<Tgt>::value,
+      detail::CheckTrailingSpace,
+      detail::ReturnUnit<Error>>::type;
+  auto tmp = detail::parseToWrap(src, result);
+  return tmp
+      .thenOrThrow(Check(), [&](Error e) { throw makeConversionError(e, src); })
+      .thenOrThrow(
+          [&](Unit) { return std::move(result); },
+          [&](Error e) { throw makeConversionError(e, tmp.value()); });
+}
+
+/**
+ * tryTo/to that take the strings by pointer so the caller gets information
+ * about how much of the string was consumed by the conversion. These do not
+ * check for trailing whitepsace.
+ */
+template <class Tgt>
+Expected<Tgt, detail::ParseToError<Tgt>> tryTo(StringPiece* src) {
   Tgt result;
-  parseTo(src, result);
-  return result;
+  return parseTo(*src, result).then([&, src](StringPiece sp) -> Tgt {
+    *src = sp;
+    return std::move(result);
+  });
 }
 
 template <class Tgt>
 Tgt to(StringPiece* src) {
   Tgt result;
-  parseTo(src, result);
-  return result;
+  using Error = detail::ParseToError<Tgt>;
+  return parseTo(*src, result)
+      .thenOrThrow(
+          [&, src](StringPiece sp) -> Tgt {
+            *src = sp;
+            return std::move(result);
+          },
+          [=](Error e) { return makeConversionError(e, *src); });
 }
 
 /*******************************************************************************
@@ -1354,8 +1457,27 @@ Tgt to(StringPiece* src) {
 
 template <class Tgt, class Src>
 typename std::enable_if<
-  std::is_enum<Src>::value && !std::is_same<Src, Tgt>::value, Tgt>::type
-to(const Src & value) {
+    std::is_enum<Src>::value && !std::is_same<Src, Tgt>::value,
+    Expected<Tgt, ConversionCode>>::type
+tryTo(const Src& value) {
+  using I = typename std::underlying_type<Src>::type;
+  return tryTo<Tgt>(static_cast<I>(value));
+}
+
+template <class Tgt, class Src>
+typename std::enable_if<
+    std::is_enum<Tgt>::value && !std::is_same<Src, Tgt>::value,
+    Tgt>::type
+tryTo(const Src& value) {
+  using I = typename std::underlying_type<Tgt>::type;
+  return tryTo<I>(value).then([](I i) { return static_cast<Tgt>(i); });
+}
+
+template <class Tgt, class Src>
+typename std::enable_if<
+    std::is_enum<Src>::value && !std::is_same<Src, Tgt>::value,
+    Tgt>::type
+to(const Src& value) {
   return to<Tgt>(static_cast<typename std::underlying_type<Src>::type>(value));
 }
 
diff --git a/folly/Portability.h b/folly/Portability.h
index 56dc7c95..e6d06a4d 100644
--- a/folly/Portability.h
+++ b/folly/Portability.h
@@ -92,6 +92,15 @@ constexpr bool kHasUnalignedAccess = false;
 # define FOLLY_ALWAYS_INLINE inline
 #endif
 
+// warn unused result
+#if defined(_MSC_VER) && (_MSC_VER >= 1700)
+#define FOLLY_WARN_UNUSED_RESULT _Check_return_
+#elif defined(__clang__) || defined(__GNUC__)
+#define FOLLY_WARN_UNUSED_RESULT __attribute__((__warn_unused_result__))
+#else
+#define FOLLY_WARN_UNUSED_RESULT
+#endif
+
 // target
 #ifdef _MSC_VER
 # define FOLLY_TARGET_ATTRIBUTE(target)
diff --git a/folly/String-inl.h b/folly/String-inl.h
index 1d613e9c..2bf0104a 100644
--- a/folly/String-inl.h
+++ b/folly/String-inl.h
@@ -328,7 +328,7 @@ bool splitFixed(const Delim& delimiter, StringPiece input, OutputType& output) {
   if (exact && UNLIKELY(std::string::npos != input.find(delimiter))) {
     return false;
   }
-  parseTo(input, output);
+  output = folly::to<OutputType>(input);
   return true;
 }
 
@@ -346,7 +346,7 @@ bool splitFixed(
   StringPiece tail(input.begin() + cut + detail::delimSize(delimiter),
                    input.end());
   if (LIKELY(splitFixed<exact>(delimiter, tail, outTail...))) {
-    parseTo(head, outHead);
+    outHead = folly::to<OutputType>(head);
     return true;
   }
   return false;
diff --git a/folly/String.h b/folly/String.h
index fc552f11..5d631340 100644
--- a/folly/String.h
+++ b/folly/String.h
@@ -491,7 +491,8 @@ struct IsConvertible {
 template <class T>
 struct IsConvertible<
     T,
-    decltype(parseTo(std::declval<folly::StringPiece>(), std::declval<T&>()))> {
+    decltype(static_cast<void>(
+        parseTo(std::declval<folly::StringPiece>(), std::declval<T&>())))> {
   enum { value = true };
 };
 
diff --git a/folly/docs/Conv.md b/folly/docs/Conv.md
index 8d285e9d..7156c116 100644
--- a/folly/docs/Conv.md
+++ b/folly/docs/Conv.md
@@ -215,3 +215,31 @@ is returned, which can be tested for as follows:
       // string could not be parsed
     }
 ```
+
+#### Non-throwing interfaces
+
+`tryTo<T>` is the non-throwing variant of `to<T>`. It returns
+an `Expected<T, ConversionCode>`. You can think of `Expected`
+as like an `Optional<T>`, but if the conversion failed, `Expected`
+stores an error code instead of a `T`.
+
+`tryTo<T>` has similar performance as `to<T>` when the
+conversion is successful. On the error path, you can expect
+`tryTo<T>` to be roughly three orders of magnitude faster than
+the throwing `to<T>` and to completely avoid any lock contention
+arising from stack unwinding.
+
+Here is how to use non-throwing conversions:
+
+``` Cpp
+    auto t1 = tryTo<int>(str);
+    if (t1.hasValue()) {
+      use(t1.value());
+    }
+```
+
+`Expected` has a composability feature to make the above pattern simpler.
+
+``` Cpp
+    tryTo<int>(str).then([](int i) { use(i); });
+```
diff --git a/folly/test/ConvBenchmark.cpp b/folly/test/ConvBenchmark.cpp
index 0464822b..ec975f3e 100644
--- a/folly/test/ConvBenchmark.cpp
+++ b/folly/test/ConvBenchmark.cpp
@@ -911,6 +911,16 @@ inline void stringToTypeClassic(const char* str, uint32_t n) {
   }
 }
 
+template <typename T>
+inline void stringToTypeOptional(const char* str, uint32_t n) {
+  for (uint32_t i = 0; i < n; ++i) {
+    auto val = tryTo<T>(str);
+    if (val.hasValue()) {
+      doNotOptimizeAway(val.value());
+    }
+  }
+}
+
 template <typename T>
 inline void ptrPairToIntClassic(StringPiece sp, uint32_t n) {
   for (uint32_t i = 0; i < n; ++i) {
@@ -924,6 +934,16 @@ inline void ptrPairToIntClassic(StringPiece sp, uint32_t n) {
   }
 }
 
+template <typename T>
+inline void ptrPairToIntOptional(StringPiece sp, uint32_t n) {
+  for (uint32_t i = 0; i < n; ++i) {
+    auto val = tryTo<T>(sp.begin(), sp.end());
+    if (val.hasValue()) {
+      doNotOptimizeAway(val.value());
+    }
+  }
+}
+
 constexpr uint32_t kArithNumIter = 10000;
 
 template <typename T, typename U>
@@ -944,6 +964,24 @@ inline size_t arithToArithClassic(const U* in, uint32_t numItems) {
   return kArithNumIter * numItems;
 }
 
+template <typename T, typename U>
+inline size_t arithToArithOptional(const U* in, uint32_t numItems) {
+  for (uint32_t i = 0; i < kArithNumIter; ++i) {
+    for (uint32_t j = 0; j < numItems; ++j) {
+      auto val = tryTo<T>(*in);
+      doNotOptimizeAway(val.hasValue());
+      if (val.hasValue()) {
+        auto v2 = val.value();
+        doNotOptimizeAway(v2);
+      }
+      doNotOptimizeAway(j);
+    }
+    doNotOptimizeAway(i);
+  }
+
+  return kArithNumIter * numItems;
+}
+
 } // namespace
 
 namespace folly {
@@ -980,6 +1018,12 @@ std::array<double, 4> double2IntBad{{1e100, 1.25, 2.5, 100.00001}};
   }                                                      \
   BENCHMARK(stringTo##name##ClassicError, n) {           \
     stringToTypeClassic<type>(fail, n);                  \
+  }                                                      \
+  BENCHMARK(stringTo##name##Optional, n) {               \
+    stringToTypeOptional<type>(pass, n);                 \
+  }                                                      \
+  BENCHMARK(stringTo##name##OptionalError, n) {          \
+    stringToTypeOptional<type>(fail, n);                 \
   }
 
 #define PTR_PAIR_TO_INT_BENCHMARK(type, name, pass, fail) \
@@ -988,14 +1032,26 @@ std::array<double, 4> double2IntBad{{1e100, 1.25, 2.5, 100.00001}};
   }                                                       \
   BENCHMARK(ptrPairTo##name##ClassicError, n) {           \
     ptrPairToIntClassic<type>(fail, n);                   \
+  }                                                       \
+  BENCHMARK(ptrPairTo##name##Optional, n) {               \
+    ptrPairToIntOptional<type>(pass, n);                  \
+  }                                                       \
+  BENCHMARK(ptrPairTo##name##OptionalError, n) {          \
+    ptrPairToIntOptional<type>(fail, n);                  \
   }
 
-#define ARITH_TO_ARITH_BENCHMARK(type, name, pass, fail)        \
-  BENCHMARK_MULTI(name##Classic) {                              \
-    return arithToArithClassic<type>(pass.data(), pass.size()); \
-  }                                                             \
-  BENCHMARK_MULTI(name##ClassicError) {                         \
-    return arithToArithClassic<type>(fail.data(), fail.size()); \
+#define ARITH_TO_ARITH_BENCHMARK(type, name, pass, fail)         \
+  BENCHMARK_MULTI(name##Classic) {                               \
+    return arithToArithClassic<type>(pass.data(), pass.size());  \
+  }                                                              \
+  BENCHMARK_MULTI(name##ClassicError) {                          \
+    return arithToArithClassic<type>(fail.data(), fail.size());  \
+  }                                                              \
+  BENCHMARK_MULTI(name##Optional) {                              \
+    return arithToArithOptional<type>(pass.data(), pass.size()); \
+  }                                                              \
+  BENCHMARK_MULTI(name##OptionalError) {                         \
+    return arithToArithOptional<type>(fail.data(), fail.size()); \
   }
 
 #define INT_TO_ARITH_BENCHMARK(type, name, pass, fail) \
diff --git a/folly/test/ConvTest.cpp b/folly/test/ConvTest.cpp
index 0ac0ed5a..2ff1ab08 100644
--- a/folly/test/ConvTest.cpp
+++ b/folly/test/ConvTest.cpp
@@ -18,6 +18,7 @@
 #include <folly/Conv.h>
 #include <folly/Foreach.h>
 #include <gtest/gtest.h>
+#include <algorithm>
 #include <limits>
 #include <sstream>
 #include <stdexcept>
@@ -820,6 +821,14 @@ TEST(Conv, StringToBool) {
   EXPECT_EQ(buf5, sp5.begin());
 }
 
+TEST(Conv, Transform) {
+  const std::vector<int64_t> in{1, 2, 3};
+  std::vector<std::string> out(in.size());
+  std::transform(in.begin(), in.end(), out.begin(), to<std::string, int64_t>);
+  const std::vector<std::string> ref{"1", "2", "3"};
+  EXPECT_EQ(ref, out);
+}
+
 TEST(Conv, FloatToInt) {
   EXPECT_EQ(to<int>(42.0f), 42);
   EXPECT_EQ(to<int8_t>(-128.0f), int8_t(-128));
@@ -874,7 +883,7 @@ template <typename F>
 void testConvError(
     F&& expr,
     const char* exprStr,
-    ConversionError::Code code,
+    ConversionCode code,
     const char* value,
     bool quotedValue,
     int line) {
@@ -907,7 +916,7 @@ void testConvError(
   testConvError(                                           \
       [&] { return expr; },                                \
       #expr,                                               \
-      ConversionError::code,                               \
+      ConversionCode::code,                                \
       value,                                               \
       quoted,                                              \
       __LINE__)
@@ -1026,6 +1035,91 @@ TEST(Conv, ConversionErrorFloatToInt) {
   EXPECT_CONV_ERROR_ARITH(int8_t, 65.5, ARITH_LOSS_OF_PRECISION);
 }
 
+TEST(Conv, TryStringToBool) {
+  auto rv1 = folly::tryTo<bool>("xxxx");
+  EXPECT_FALSE(rv1.hasValue());
+  auto rv2 = folly::tryTo<bool>("false");
+  EXPECT_TRUE(rv2.hasValue());
+  EXPECT_FALSE(rv2.value());
+  auto rv3 = folly::tryTo<bool>("yes");
+  EXPECT_TRUE(rv3.hasValue());
+  EXPECT_TRUE(rv3.value());
+}
+
+TEST(Conv, TryStringToInt) {
+  auto rv1 = folly::tryTo<int>("1000000000000000000000000000000");
+  EXPECT_FALSE(rv1.hasValue());
+  auto rv2 = folly::tryTo<int>("4711");
+  EXPECT_TRUE(rv2.hasValue());
+  EXPECT_EQ(rv2.value(), 4711);
+}
+
+TEST(Conv, TryStringToFloat) {
+  auto rv1 = folly::tryTo<float>("");
+  EXPECT_FALSE(rv1.hasValue());
+  auto rv2 = folly::tryTo<float>("3.14");
+  EXPECT_TRUE(rv2.hasValue());
+  EXPECT_NEAR(rv2.value(), 3.14, 1e-5);
+}
+
+TEST(Conv, TryStringToDouble) {
+  auto rv1 = folly::tryTo<double>("");
+  EXPECT_FALSE(rv1.hasValue());
+  auto rv2 = folly::tryTo<double>("3.14");
+  EXPECT_TRUE(rv2.hasValue());
+  EXPECT_NEAR(rv2.value(), 3.14, 1e-10);
+}
+
+TEST(Conv, TryIntToInt) {
+  auto rv1 = folly::tryTo<uint8_t>(256);
+  EXPECT_FALSE(rv1.hasValue());
+  auto rv2 = folly::tryTo<uint8_t>(255);
+  EXPECT_TRUE(rv2.hasValue());
+  EXPECT_EQ(rv2.value(), 255);
+}
+
+TEST(Conv, TryFloatToFloat) {
+  auto rv1 = folly::tryTo<float>(1e100);
+  EXPECT_FALSE(rv1.hasValue());
+  auto rv2 = folly::tryTo<double>(25.5f);
+  EXPECT_TRUE(rv2.hasValue());
+  EXPECT_NEAR(rv2.value(), 25.5, 1e-10);
+}
+
+TEST(Conv, TryFloatToInt) {
+  auto rv1 = folly::tryTo<int>(100.001);
+  EXPECT_FALSE(rv1.hasValue());
+  auto rv2 = folly::tryTo<int>(100.0);
+  EXPECT_TRUE(rv2.hasValue());
+  EXPECT_EQ(rv2.value(), 100);
+}
+
+TEST(Conv, TryIntToFloat) {
+  auto rv1 = folly::tryTo<float>(std::numeric_limits<uint64_t>::max());
+  EXPECT_FALSE(rv1.hasValue());
+  auto rv2 = folly::tryTo<float>(1000ULL);
+  EXPECT_TRUE(rv2.hasValue());
+  EXPECT_EQ(rv2.value(), 1000.0f);
+}
+
+TEST(Conv, TryPtrPairToInt) {
+  StringPiece sp1("1000000000000000000000000000000");
+  auto rv1 = folly::tryTo<int>(sp1.begin(), sp1.end());
+  EXPECT_FALSE(rv1.hasValue());
+  StringPiece sp2("4711");
+  auto rv2 = folly::tryTo<int>(sp2.begin(), sp2.end());
+  EXPECT_TRUE(rv2.hasValue());
+  EXPECT_EQ(rv2.value(), 4711);
+  StringPiece sp3("-4711");
+  auto rv3 = folly::tryTo<int>(sp3.begin(), sp3.end());
+  EXPECT_TRUE(rv3.hasValue());
+  EXPECT_EQ(rv3.value(), -4711);
+  StringPiece sp4("4711");
+  auto rv4 = folly::tryTo<uint16_t>(sp4.begin(), sp4.end());
+  EXPECT_TRUE(rv4.hasValue());
+  EXPECT_EQ(rv4.value(), 4711);
+}
+
 TEST(Conv, NewUint64ToString) {
   char buf[21];
 
@@ -1091,10 +1185,12 @@ struct Dimensions {
   }
 };
 
-void parseTo(folly::StringPiece in, Dimensions& out) {
-  out.w = folly::to<int>(&in);
-  in.removePrefix("x");
-  out.h = folly::to<int>(&in);
+Expected<StringPiece, ConversionCode> parseTo(
+    folly::StringPiece in,
+    Dimensions& out) {
+  return parseTo(in, out.w)
+      .then([](StringPiece sp) { return sp.removePrefix("x"), sp; })
+      .then([&](StringPiece sp) { return parseTo(sp, out.h); });
 }
 
 template <class String>
@@ -1117,3 +1213,10 @@ TEST(Conv, custom_kkproviders) {
   EXPECT_GT(str.capacity(), 2000);
   EXPECT_LT(str.capacity(), 2500);
 }
+
+TEST(Conv, TryToThenWithVoid) {
+  auto x = tryTo<int>("42").then([](int) {});
+  EXPECT_TRUE(x.hasValue());
+  Unit u = x.value();
+  (void)u;
+}
diff --git a/folly/test/StringTest.cpp b/folly/test/StringTest.cpp
index 76cf6ac4..c01cca5d 100644
--- a/folly/test/StringTest.cpp
+++ b/folly/test/StringTest.cpp
@@ -921,14 +921,27 @@ enum class Color {
   Blue,
 };
 
-void parseTo(folly::StringPiece in, Color& out) {
+enum class ColorErrorCode { INVALID_COLOR };
+
+struct ColorError : std::runtime_error {
+  using std::runtime_error::runtime_error;
+};
+
+ColorError makeConversionError(ColorErrorCode, StringPiece sp) {
+  return ColorError("Invalid my::Color representation : " + sp.str());
+}
+
+Expected<StringPiece, ColorErrorCode> parseTo(
+    StringPiece in,
+    Color& out) noexcept {
   if (in == "R") {
     out = Color::Red;
   } else if (in == "B") {
     out = Color::Blue;
   } else {
-    throw runtime_error("");
+    return makeUnexpected(ColorErrorCode::INVALID_COLOR);
   }
+  return StringPiece(in.end(), in.end());
 }
 }
 
@@ -938,6 +951,8 @@ TEST(Split, fixed_convert_custom) {
   EXPECT_TRUE(folly::split(',', "R,B", c1, c2));
   EXPECT_EQ(c1, my::Color::Red);
   EXPECT_EQ(c2, my::Color::Blue);
+
+  EXPECT_THROW(folly::split(',', "B,G", c1, c2), my::ColorError);
 }
 
 TEST(String, join) {
diff --git a/folly/test/TraitsTest.cpp b/folly/test/TraitsTest.cpp
index 23541b8e..402d1d2c 100644
--- a/folly/test/TraitsTest.cpp
+++ b/folly/test/TraitsTest.cpp
@@ -52,7 +52,7 @@ TEST(Traits, scalars) {
 
 TEST(Traits, containers) {
   EXPECT_TRUE  (IsRelocatable<vector<F1>>::value);
-  EXPECT_TRUE ((IsRelocatable<pair<F1, F1>>::value));
+  EXPECT_TRUE((IsRelocatable<pair<F1, F1>>::value));
   EXPECT_TRUE ((IsRelocatable<pair<T1, T2>>::value));
 }