[gcc r11-4074] libstdc++: Use reserved name for C++20 attribute

[gcc r11-4074] libstdc++: Use reserved name for C++20 attribute

[Jonathan Wakely]

https://gcc.gnu.org/g:fc77484c4a4feb308c64371233cb65b280333953

commit r11-4074-gfc77484c4a4feb308c64371233cb65b280333953
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Oct 19 17:57:14 2020 +0100

    libstdc++: Use reserved name for C++20 attribute
    
    Although the compiler supports the [[no_unique_address]] attribute, it's
    not a reserved name prior to C++20, so we can't use it in std::tuple.
    Use [[__no_unique_address__]] instead.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/tuple (_Head_base<Idx, Head, true>): Use reserved
            form of __no_unique_address__ attribute because
            no_unique_address is not reserved prior to C++20.

Diff:
---
 libstdc++-v3/include/std/tuple | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index 11ad1991108..adfd6213a14 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -122,7 +122,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static constexpr const _Head&
       _M_head(const _Head_base& __b) noexcept { return __b._M_head_impl; }
 
-      [[no_unique_address]] _Head _M_head_impl;
+      [[__no_unique_address__]] _Head _M_head_impl;
     };
 #else
   template<size_t _Idx, typename _Head>