[ADT] bit_cast: check for is_trivially_copyable more portably

[jfb]

Summary

It turns out that isPodLike isn't a good workaround for is_trivially_copyable for bit_cast's purpose. In D51872 Louis points out that tuple and pair really aren't a good fit, and for bit_cast I want to capture array. This patch instead checks is_trivially_copyable directly in bit_cast for all but GCC 4.x. In GCC 4.x developers only check for sizeof match, which means any mistake they make will succeed locally and fail on the bots. Realistically that's few developers and they'll be left behind once we upgrade past C++11.

This will allow using bit_cast with std::array.

[ldionne]

I think that works because memcpy this way is valid IFF To and From are trivially copyable. LGTM.

[jfb]

Extra fix in r341866 because __is_trivially_copyable is a function-like builtin, not a template.

[jfb]

And r341867 because I committed before saving the file edit...

llvm/trunk/include/llvm/ADT/bit.h

 //===-- llvm/ADT/bit.h - C++20 <bit> ----------------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements the C++20 <bit> header.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_ADT_BIT_H
 #define LLVM_ADT_BIT_H
 
-#include "llvm/Support/type_traits.h"
+#include "llvm/Support/Compiler.h"
 #include <cstring>
+#include <type_traits>
 
 namespace llvm {
 
-template <typename To, typename From,
-          typename = typename std::enable_if<sizeof(To) == sizeof(From)>::type,
-          typename = typename std::enable_if<isPodLike<To>::value>::type,
-          typename = typename std::enable_if<isPodLike<From>::value>::type>
+template <typename To, typename From
+          , typename = typename std::enable_if<sizeof(To) == sizeof(From)>::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
+          , typename = typename std::enable_if<std::is_trivially_copyable<From>::value>::type
+#elif __has_feature(is_trivially_copyable)
+          , typename = typename std::enable_if<__is_trivially_copyable<To>::value>::type
+          , typename = typename std::enable_if<__is_trivially_copyable<From>::value>::type
+#else
+  // This case is GCC 4.x. clang with libc++ or libstdc++ never get here. Unlike
+  // llvm/Support/type_traits.h's isPodLike we don't want to provide a
+  // good-enough answer here: developers in that configuration will hit
+  // compilation failures on the bots instead of locally. That's acceptable
+  // because it's very few developers, and only until we move past C++11.
+#endif
+>
 inline To bit_cast(const From &from) noexcept {
   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