[v3 PATCH] PR 67844

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

Tested on Linux-PPC64. The problem is a tad tricky, since
the bug triggered by the testcase is in a constructor that
will not be used, but will cause endless meta-recursion
via checking convertibility of an incomplete type that will
cause further recursion. While there might be ways to fix
that in the traits themselves, this is a straightforward fix,
for certain tuple-specific values of "straightforward". ;)

And hey, the problem and its solution were, once the bug
report came in, obvious to the resident tuple-hacker between
my chair and my keyboard. :)

2015-10-05  Ville Voutilainen  <ville.voutilainen@gmail.com>

    PR 67844.
    * include/std/tuple (_TC::_NonNestedTuple): Eagerly reject
    conversions from tuple types same as the target tuple.
    * testsuite/20_util/tuple/67844.cc: New.

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