Revert "[ADT] restrict bit_cast to trivially-constructible To"

Of course some bots don't have std::is_trivially_constructible...

llvm-svn: 342711

llvm/include/llvm/ADT/bit.h

@@ -20,12 +20,8 @@
 
 namespace llvm {
 
-// This implementation of bit_cast is different from the C++17 one in two ways:
-//  - It isn't constexpr because that requires compiler support.
-//  - It requires trivially-constructible To, to avoid UB in the implementation.
 template <typename To, typename From
           , typename = typename std::enable_if<sizeof(To) == sizeof(From)>::type
-          , typename = typename std::is_trivially_constructible<To>::type
 #if (__has_feature(is_trivially_copyable) && defined(_LIBCPP_VERSION)) || \
     (defined(__GNUC__) && __GNUC__ >= 5)
           , typename = typename std::enable_if<std::is_trivially_copyable<To>::value>::type
@@ -42,9 +38,17 @@ template <typename To, typename From
 #endif
 >
 inline To bit_cast(const From &from) noexcept {
-  To to;
-  std::memcpy(&to, &from, sizeof(To));
-  return to;
+  alignas(To) unsigned char storage[sizeof(To)];
+  std::memcpy(&storage, &from, sizeof(To));
+#if defined(__GNUC__)
+  // Before GCC 7.2, GCC thought that this violated strict aliasing.
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-aliasing"
+#endif
+  return reinterpret_cast<To &>(storage);
+#if defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
 }
 
 } // namespace llvm