Here is the patch to correctly deal with the new __dp_sign_max_size.

I prefer to introduce new __can_advance overloads for this to correctly 
deal with the _Distance_precision in it. _M_valid_range was also 
ignoring __dp_sign_max_size.

     libstdc++: [_GLIBCXX_DEBUG] Fix management of __dp_sign_max_size 
[PR 99402]

     libstdc++-v3/ChangeLog:

             PR libstdc++/99402
             * include/debug/helper_functions.h 
(__can_advance(_InputIterator,
             const std::pair<_Diff, _Distance_precision>&, int)): New.
             (__can_advance(const _Safe_iterator<>&,
             const std::pair<_Diff, _Distance_precision>&, int)): New.
             * include/debug/macros.h 
(__glibcxx_check_can_increment_dist): New,
             use latter.
             (__glibcxx_check_can_increment_range): Adapt to use latter.
             (__glibcxx_check_can_decrement_range): Likewise.
             * include/debug/safe_iterator.h
             (_Safe_iterator<>::_M_can_advance(const std::pair<_Diff, 
_Distance_precision>&,
             int)): New.
             (__can_advance(const _Safe_iterator<>&,
             const std::pair<_Diff, _Distance_precision>&, int)): New.
             * include/debug/safe_iterator.tcc
             (_Safe_iterator<>::_M_can_advance(const std::pair<_Diff, 
_Distance_precision>&,
             int)): New.
             (_Safe_iterator<>::_M_valid_range(const _Safe_iterator<>&,
             std::pair<difference_type, _Distance_precision>&, bool)): 
Adapt for
             __dp_sign_max_size.
             (__copy_move_a): Adapt to use 
__glibcxx_check_can_increment_dist.
             (__copy_move_backward_a): Likewise.
             (__equal_aux): Likewise.
             * include/debug/stl_iterator.h (__can_advance(const 
std::reverse_iterator<>&,
             const std::pair<_Diff, _Distance_precision>&, int)): New.
             (__can_advance(const std::move_iterator<>&,
             const std::pair<_Diff, _Distance_precision>&, int)): New.
             * testsuite/25_algorithms/copy/debug/99402.cc: New test.

Tested under Linux x86_64.

Ok to commit ?

François