[v3 PATCH, RFC] Implement LWG 2729 for tuple

[Ville Voutilainen <ville dot voutilainen at gmail dot com>]

First, pardons all around if I'm completely repeating what Marc
already tried to do.
I think I'm taking a different approach.

I'm not adding any defaulted or deleted functions, so I don't think I'm changing
triviality. Chances are I need to actually delete the copy operations I want to
get rid of, though; currently they are defined with a conditional
parameter type.
That leaves open the possibility of a move operation being implicitly defaulted
rather than suppressed, which can make them trivial where they weren't before.

I'm not sure whether this has abi impact. In the sense that this is making
previously-defined move operations suppressed, maybe. I guess I could
alternatively try making them conditional, and making the inverse of the
conditional private; that should keep the traits working and retain abi
compatibility, because I don't think our toolchains at least on non-Windows
platforms include access levels in mangling.

Tested for tuple on Linux-x64. Thoughts?

2016-08-30  Ville Voutilainen  <ville.voutilainen@gmail.com>

    Implement LWG 2729 for tuple.
    * include/std/tuple (_Tuple_impl(_Tuple_impl&&)):
    Suppress conditionally.
    (_Tuple_impl(_Tuple_impl<_Idx, _UHead, _UTails...>&&)): Likewise.
    (__is_tuple_impl_trait_impl, __is_tuple_impl_trait): New.
    (_Tuple_impl(const _Head&)): Constrain.
    (_Tuple_impl(_UHead&&)): Likewise.
    (_Tuple_impl(_Tuple_impl&&)): Suppress conditionally.
    (_Tuple_impl(const _Tuple_impl<_Idx, _UHead>&)): Constrain.
    (_Tuple_impl(_Tuple_impl<_Idx, _UHead>&&)): Likewise.
    (operator=(const tuple&)): Enable conditionally.
    (operator=(tuple&&)): Suppress conditionally.
    (operator=(const tuple<_UElements...>&)): Constrain.
    (operator=(tuple<_UElements...>&&)): Likewise.
    (operator=(const tuple&)): Enable conditionally (2-param tuple).
    (operator=(tuple&&)): Suppress conditionally (2-param tuple).
    (operator=(const tuple<_U1, _U2>&)): Constrain.
    (operator=(tuple<_U1, _U2>&&)): Likewise.
    (operator=(const pair<_U1, _U2>&)): Likewise.
    (operator=(pair<_U1, _U2>&&)): Likewise.
    * testsuite/20_util/tuple/element_access/get_neg.cc: Adjust.
    * testsuite/20_util/tuple/tuple_traits.cc: New.

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