[committed] libstdc++: Use correct argument type for __use_alloc [PR 96803]

[Jonathan Wakely]

On 26/08/20 19:34 +0100, Jonathan Wakely wrote:
>The _Tuple_impl constructor for allocator-extended construction from a
>different tuple type uses the _Tuple_impl's own _Head type in the
>__use_alloc test. That is incorrect, because the argument tuple could
>have a different type. Using the wrong type might select the
>leading-allocator convention when it should use the trailing-allocator
>convention, or vice versa.
>
>libstdc++-v3/ChangeLog:
>
>	PR libstdc++/96803
>	* include/std/tuple
>	(_Tuple_impl(allocator_arg_t, Alloc, const _Tuple_impl<U...>&)):
>	Replace parameter pack with a type parameter and a pack and pass
>	the first type to __use_alloc.
>	* testsuite/20_util/tuple/cons/96803.cc: New test.

While backporting 5494edae83ad33c769bd1ebc98f0c492453a6417 I noticed
that it's still not correct. I made the allocator-extended constructor
use the right type for the uses-allocator construction detection, but I
used an rvalue when it should be a const lvalue.

This should fix it properly this time.

libstdc++-v3/ChangeLog:

         PR libstdc++/96803
         * include/std/tuple
         (_Tuple_impl(allocator_arg_t, Alloc, const _Tuple_impl<U...>&)):
         Use correct value category in __use_alloc call.
         * testsuite/20_util/tuple/cons/96803.cc: Check with constructors
         that require correct value category to be used.


Tested powerpc64le-linux. Committed to trunk.



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