[libcxx-commits] [libcxx] [libc++] basic_ios<wchar_t> cannot store fill character WCHAR_MAX (PR #89305)

[Sean Perry via libcxx-commits]

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@@ -521,6 +522,31 @@ inline _LIBCPP_HIDE_FROM_ABI void ios_base::exceptions(iostate __iostate) {
   clear(__rdstate_);
 }
 
+template <class _Traits>
+// Attribute 'packed' is used to keep the layout compatible with the previous
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perry-ca wrote:

Can you expand this comment to explain this doesn't give 100% compat.  It will eliminate the tail padding and keep the member after this one in the right offset.  However, being packed the alignment of this struct will be 1 instead of the alignment of int_type.  If this struct was added as a member where the offset wasn't already a multiple of the alignment of int_type then the offset would not be preserved.

I don't think you can take the int_type/bool pair and put them in a struct that will preserve the size and alignment of the original pair.  You can either get the alignment correct or the size correct and not both.

https://github.com/llvm/llvm-project/pull/89305