Attempt to make std::pair piecewise constructor noexcept

[Jonathan Wakely]

On 21/12/18 18:21 +0100, François Dumont wrote:
>On 12/21/18 3:00 PM, Ville Voutilainen wrote:
>>On Fri, 21 Dec 2018 at 15:47, Jonathan Wakely <jwakely@redhat.com> wrote:
>>>On 16/12/18 18:48 +0100, François Dumont wrote:
>>>>@@ -372,7 +361,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>>>         second(std::forward<_U2>(__p.second)) { }
>>>>
>>>>       template<typename... _Args1, typename... _Args2>
>>>>-        pair(piecewise_construct_t, tuple<_Args1...>, tuple<_Args2...>);
>>>>+      pair(piecewise_construct_t,
>>>>+           tuple<_Args1...> __first, tuple<_Args2...> __second)
>>>>+      noexcept( noexcept(
>>>>+        pair(declval<tuple<_Args1...>&>(), declval<tuple<_Args2...>&>(),
>>>>+             declval<typename _Build_index_tuple<sizeof...(_Args1)>::__type>(),
>>>>+             declval<typename _Build_index_tuple<sizeof...(_Args2)>::__type>())) )
>>>>+      : pair(__first, __second,
>>>>+             typename _Build_index_tuple<sizeof...(_Args1)>::__type(),
>>>>+             typename _Build_index_tuple<sizeof...(_Args2)>::__type())
>>>>+      { }
>>>Surely just:
>>>
>>>       noexcept(
>>>         is_nothrow_constructible<_T1, _Args1...>
>>>         &&
>>>         is_nothrow_constructible<_T2, _Args2...>
>>>       )
>>>
>>>?
>>Correct. Piecewise construction will end up doing that anyway, and we
>>know it will.
>>
>Indeed, it is simpler and avoids moving a lot of code in different 
>headers. It just make hypothesis on implementation details which I 
>usually try to avoid.

It's not an implementation detail, it's what the constructor is
required to do by the standard. The whole point of that constructor is
to construct the first member with the elements of the first tuple,
and construct the second member with the elements of the second tuple.
So the constructor arguments are the tuple elements, and their types
are _Args1... and _Args2....

>Good point for anyone willing to push such a patch. But not me as I 
>have no interest to do so anymore.

OK.