[Bug libstdc++/100823] Special member functions of common_iterator should be conditionally trivial

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100823

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:5d7de4c8fd9af68011e1fa7bc1d022db81d89594

commit r10-11317-g5d7de4c8fd9af68011e1fa7bc1d022db81d89594
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jul 20 16:51:44 2022 +0100

    libstdc++: Fix std::common_iterator assignment [PR100823]

    This fixes the following conformance problems reported in the PR:

    - Move constructor and move assignment should be defined.
    - Copy assignment from a valueless object should be allowed.

    Assignment is completely rewritten by this patch, as the previous
    version had a number of problems. The converting assignment failed to
    handle the case of assigning a new value to a valueless object, which
    should work. It only accepted lvalue arguments, so wasn't usable to
    implement the move assignment operator. Finally, it enforced the
    precondition that the argument is not valueless, which is correct for
    the converting assignment but not for the copy assignment.

    A new _M_assign member is added to handle all cases of assignment
    (copying from an lvalue, moving from an rvalue, and converting from a
    different type). The not valueless precondition is checked in the
    converting assignment before calling _M_assign, so isn't enforced for
    copy and move assignment. The new function no longer uses a switch, so
    handles valueless objects as the LHS or RHS of the assignment.

    libstdc++-v3/ChangeLog:

            PR libstdc++/100823
            * include/bits/stl_iterator.h (common_iterator): Define move
            constructor and move assignment operator.
            (common_iterator::_M_assign): New function implementing
            assignment.
            (common_iterator::operator=): Use _M_assign.
            (common_iterator::_S_valueless): New constant.
            * testsuite/24_iterators/common_iterator/100823.cc: New test.

    (cherry picked from commit 56c999860bbbb2fd5091ba0985e2e5eaa90c6478)