[libstdc++ PATCH] Implement __is_nothrow_swappable and use it

[Jonathan Wakely]

On 01/05/15 12:47 +0300, Ville Voutilainen wrote:
>This patch partially solves the problem described in N4426, which is
>basically LWG
>issue 2456, which in turn is caused by CWG DR 1330. Some remarks:
>- the __is_swappable and __is_nothrow_swappable are at this time not
>meant to be general traits, they make the shortcut of automatically transforming
>their template parameter into a reference, which the truly general trait
>as described in N4426 wouldn't do.
>- swap is now constrained..
>- ..which means that everything from bits/move.h moves to type_traits since
>swap now needs type_traits. type_traits becomes ok to include in pre-c++14
>modes, but only enables the parts that work in pre-c++14 modes.
>- this patch does _not_ yet implement N4426.

I've committed this slightly altered patch, which doesn't move swap
into <type_traits> but just declares it there. The definition remains
in <bits/move.h> and including <type_traits> in c++98 mode is still
an error.

Tested powerpc64le-linux, committed to trunk.
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