Relax std::move_if_noexcept for std::pair

[François Dumont]

Hi

 Â Â Â  I eventually find out what was the problem with the 
std::move_if_noexcept within associative containers.

 Â Â Â  The std::pair move default constructor might not move both first 
and second member. If any is not moveable it will just copy it. And then 
the noexcept qualification of the copy constructor will participate in 
the noexcept qualification of the std::pair move constructor. So 
std::move_if_noexcept can eventually decide to not use move because a 
_copy_ constructor not noexcept qualified.

 Â Â Â  This is why I am partially specializing __move_if_noexcept_cond. As 
there doesn't seem to exist any Standard meta function to find out if 
move will take place I resort using std::is_const as in this case for 
sure the compiler won't call the move constructor.

 Â Â Â  Note that I find __move_if_noexcept_cond very counter-intuitive 
cause it says if the move semantic should _not_ be used.

 Â Â Â  I am submitting this now cause it might be consider as a bug even 
if the end result is just that it pessimizes the occasion to use move 
semantic rather than copy.

 Â Â Â  * include/bits/stl_pair.h (__move_if_noexcept_cond<pair<>>): New
 Â Â Â  partial specialization.
 Â Â Â  * testsuite/20_util/move_if_noexcept/1.cc (test02): New.
 Â Â Â  * testsuite/23_containers/unordered_map/allocator/move_assign.cc
 Â Â Â  (test03): New.

 Â Â Â  Tested under Linux x86_64 normal mode.

François

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