to_signed and to_unsigned

Summary:
[Folly] `to_signed` and `to_unsigned`, for casting a value to the signed or unsigned version of the value's type.

And use them in time conversion functions.

Reviewed By: markw65

Differential Revision: D7020215

fbshipit-source-id: 47a6aeeea487cd7d59d23e7ded40cc66dcc57c52

folly/Utility.h

@@ -17,6 +17,7 @@
 #pragma once
 
 #include <cstdint>
+#include <limits>
 #include <type_traits>
 #include <utility>
 
@@ -364,4 +365,20 @@ using MoveOnly = moveonly_::MoveOnly;
 template <bool B>
 using Bool = std::integral_constant<bool, B>;
 
+template <typename T>
+constexpr auto to_signed(T const& t) -> typename std::make_signed<T>::type {
+  using S = typename std::make_signed<T>::type;
+  // note: static_cast<S>(t) would be more straightforward, but it would also be
+  // implementation-defined behavior and that is typically to be avoided; the
+  // following code optimized into the same thing, though
+  constexpr auto const s = std::numeric_limits<S>::max();
+  return s < t ? -static_cast<S>(~t) + S{-1} : static_cast<S>(t);
+}
+
+template <typename T>
+constexpr auto to_unsigned(T const& t) -> typename std::make_unsigned<T>::type {
+  using U = typename std::make_unsigned<T>::type;
+  return static_cast<U>(t);
+}
+
 } // namespace folly

folly/chrono/Conv.h

@@ -283,8 +283,7 @@ struct CheckOverflowToDuration {
       constexpr auto maxCount = std::numeric_limits<typename Tgt::rep>::max();
       constexpr auto maxSeconds = maxCount / Tgt::period::den;
 
-      auto unsignedSeconds =
-          static_cast<typename std::make_unsigned<Seconds>::type>(seconds);
+      auto unsignedSeconds = to_unsigned(seconds);
       if (LIKELY(unsignedSeconds < maxSeconds)) {
         return ConversionCode::SUCCESS;
       }
@@ -297,8 +296,7 @@ struct CheckOverflowToDuration {
         if (subseconds <= 0) {
           return ConversionCode::SUCCESS;
         }
-        if (static_cast<typename std::make_unsigned<Subseconds>::type>(
-                subseconds) <= maxSubseconds) {
+        if (to_unsigned(subseconds) <= maxSubseconds) {
           return ConversionCode::SUCCESS;
         }
       }
@@ -307,8 +305,7 @@ struct CheckOverflowToDuration {
       return ConversionCode::NEGATIVE_OVERFLOW;
     } else {
       constexpr auto minCount =
-          static_cast<typename std::make_signed<typename Tgt::rep>::type>(
-              std::numeric_limits<typename Tgt::rep>::lowest());
+          to_signed(std::numeric_limits<typename Tgt::rep>::lowest());
       constexpr auto minSeconds = (minCount / Tgt::period::den);
       if (LIKELY(seconds >= minSeconds)) {
         return ConversionCode::SUCCESS;

folly/test/UtilityTest.cpp

@@ -119,3 +119,29 @@ TEST_F(UtilityTest, MoveOnly) {
       std::is_nothrow_move_constructible<FooBar>::value,
       "Should have noexcept move constructor");
 }
+
+TEST_F(UtilityTest, to_signed) {
+  {
+    constexpr auto actual = folly::to_signed(int32_t(-12));
+    EXPECT_TRUE(std::is_signed<decltype(actual)>::value);
+    EXPECT_EQ(-12, actual);
+  }
+  {
+    constexpr auto actual = folly::to_signed(uint32_t(-12));
+    EXPECT_TRUE(std::is_signed<decltype(actual)>::value);
+    EXPECT_EQ(-12, actual);
+  }
+}
+
+TEST_F(UtilityTest, to_unsigned) {
+  {
+    constexpr auto actual = folly::to_unsigned(int32_t(-12));
+    EXPECT_TRUE(!std::is_signed<decltype(actual)>::value);
+    EXPECT_EQ(-12, actual);
+  }
+  {
+    constexpr auto actual = folly::to_unsigned(uint32_t(-12));
+    EXPECT_TRUE(!std::is_signed<decltype(actual)>::value);
+    EXPECT_EQ(-12, actual);
+  }
+}