[PATCH] PR libstdc++/86963 Implement LWG 2729 constraints on tuple assignment

[Jonathan Wakely]

On 17/08/18 19:01 +0100, Jonathan Wakely wrote:
>On 17/08/18 18:52 +0100, Jonathan Wakely wrote:
>>+  // The tag parameter ensures that in nested tuples each __tuple_base
>>+  // is a different type and can use the empty base-class optimisation.
>>+  template<typename _Tag>
>>+    class __tuple_base
>
>Specifically, this would fail if __tuple_base was not a class
>template:
>
>static_assert(sizeof(tuple<tuple<int>>) == sizeof(int), "");
>
>And also:
>
>struct empty {};
>static_assert(sizeof(tuple<tuple<empty>, tuple<empty>>) == 2, "");


In fact, it's just occurred to me that we don't really need the
__tuple_base class template at all. We can make _Tuple_impl
non-assignable (adding _M_assign members for the required
functionality). Then we don't need an extra base class.