[libc++] WIP: use no_unique_address for base in join_view

[jloser]

Summary

Let's see what CI says. I think this will work fine in clang13 onward which is
all we care about soon.

[• Quuxplusone]

libcxx/include/__ranges/join_view.h

71

Was a bug ever filed? If not, please do; if so, please give a link in the commit message.

FWIW, in the absence of any other information, I'd trust the old code comment that says "don't do this" over the CI results. ;) But I'm very curious to understand what the old code comment was talking about, because how on earth could the attribute be OK on __cache_ but ICE the compiler on __base_??

[jloser]

libcxx/include/__ranges/join_view.h

71

I don't understand how/why this could crash clang. I haven't seen a relevant bug report filed (nor am I convinced it is an actual bug). The closest one I could find is https://github.com/llvm/llvm-project/issues/50766.

@zoecarver @ldionne - any ideas here?

[mstorsjo]

The conflicting D119430 has landed now; sorry for the disturbance!

[• Quuxplusone]

Please update to use _LIBCPP_NO_UNIQUE_ADDRESS, but then, in the absence of any further information, I think we should just land this and see what breaks. My bet is still on "nothing." :)

libcxx/include/__ranges/join_view.h

private:
     using _InnerRange = range_reference_t<_View>;
 
     template<bool> struct __iterator;
     template<bool> struct __sentinel;
 
     static constexpr bool _UseCache = !is_reference_v<_InnerRange>;
     using _Cache = _If<_UseCache, __non_propagating_cache<remove_cvref_t<_InnerRange>>, __empty_cache>;
     _LIBCPP_NO_UNIQUE_ADDRESS _Cache __cache_;
-    _View __base_ = _View(); // TODO: [[no_unique_address]] makes clang crash! File a bug :)
+    _LIBCPP_NO_UNIQUE_ADDRESS _View __base_ = _View();
 
   public:
     _LIBCPP_HIDE_FROM_ABI
     join_view() requires default_initializable<_View> = default;
 
     _LIBCPP_HIDE_FROM_ABI
     constexpr explicit join_view(_View __base)
       : __base_(_VSTD::move(__base)) {}