In all these cases we know the value with signed type is not negative so
the cast is safe.
libstdc++-v3/ChangeLog:
* include/bits/deque.tcc (deque::_M_shrink_to_fit): Cast
difference_type to size_type to avoid -Wsign-compare warning.
* include/std/spanstream (basic_spanbuf::seekoff): Cast
streamoff to size_t to avoid -Wsign-compare warning.