[libcxx-commits] [libcxx] [libc++][modularization] Get rid of <cstddef> dependency in __datasizeof (PR #107394)

[A. Jiang via libcxx-commits]

================
@@ -37,15 +37,15 @@ struct _FirstPaddingByte {
   char __first_padding_byte_;
 };
 
-// _FirstPaddingByte<> is sometimes non-standard layout. Using `offsetof` is UB in that case, but GCC and Clang allow
-// the use as an extension.
+// _FirstPaddingByte<> is sometimes non-standard layout.
+// Using `__builtin_offsetof` is UB in that case, but GCC and Clang allow the use as an extension.
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frederick-vs-ja wrote:

I guess we shouldn't say UB here as [LWG2709](https://cplusplus.github.io/LWG/issue2709) changed the use from UB to a conditionally-supported feature. It seems fine to still say that it's an extension.

However, isn't `__builtin_offsetof` (not `offsetof`) **always** an extension?

```suggestion
// Using `__builtin_offsetof` is conditionally-supported, but GCC and Clang allow the use as an extension.
```

https://github.com/llvm/llvm-project/pull/107394