* include/bits/shared_ptr.h (__enable_shared_from_this_helper): Use
nullptr.
* include/bits/shared_ptr_base.h (weak_ptr::_M_assign): Don't assign
if ownership is already shared with a shared_ptr object.
(__enable_shared_from_this_helper): Use nullptr.
* testsuite/20_util/enable_shared_from_this/members/const.cc: New.
* testsuite/20_util/enable_shared_from_this/members/reinit.cc: New.
* testsuite/20_util/enable_shared_from_this/requirements/
explicit_instantiation.cc: Instantiate with const and incomplete types.
From-SVN: r227232
6bc41b268e