Relax bit_cast requirements

Summary: The standard only requires `To` to be trivially copyable, not trivial.

Reviewed By: yfeldblum

Differential Revision: D17916612

fbshipit-source-id: 80ae35672ea4664fe4204c8181ac6402065c4e03

folly/lang/Bits.h

@@ -77,13 +77,13 @@ template <
     typename To,
     typename From,
     std::enable_if_t<
-        sizeof(From) == sizeof(To) && std::is_trivial<To>::value &&
+        sizeof(From) == sizeof(To) && is_trivially_copyable<To>::value &&
             is_trivially_copyable<From>::value,
         int> = 0>
 To bit_cast(const From& src) noexcept {
-  To to;
-  std::memcpy(&to, &src, sizeof(From));
-  return to;
+  aligned_storage_for_t<To> storage;
+  std::memcpy(&storage, &src, sizeof(From));
+  return reinterpret_cast<To&>(storage);
 }
 
 #endif

folly/lang/test/BitsTest.cpp

@@ -255,16 +255,25 @@ TEST(Bits, BitCastBasic) {
   auto one = std::make_unique<int>();
   auto two = folly::bit_cast<std::uintptr_t>(one.get());
   EXPECT_EQ(folly::bit_cast<int*>(two), one.get());
-}
 
-} // namespace folly
+  struct FancyInt {
+    FancyInt() {
+      ADD_FAILURE() << "Default constructor should not be called by bit_cast";
+    }
 
-TEST(Bits, BitCastCompatibilityTest) {
-  using namespace folly;
-  using namespace std;
+    int value;
+  };
 
+  int x = 5;
+  auto bi = folly::bit_cast<FancyInt>(x);
+  EXPECT_EQ(x, bi.value);
+}
+
+TEST(Bits, BitCastCompatibilityTest) {
   auto one = folly::Random::rand64();
-  auto pointer = bit_cast<std::uintptr_t>(one);
-  auto two = bit_cast<std::uint64_t>(pointer);
+  auto pointer = folly::bit_cast<std::uintptr_t>(one);
+  auto two = folly::bit_cast<std::uint64_t>(pointer);
   EXPECT_EQ(one, two);
 }
+
+} // namespace folly