[Bug libstdc++/69478] New: [4.9/5/6 Regression] std::copy/std::move broken with trivial move-only types

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69478

            Bug ID: 69478
           Summary: [4.9/5/6 Regression] std::copy/std::move broken with
                    trivial move-only types
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

Reduced from http://stackoverflow.com/q/35002402/2756719, repro:

#include <iterator>
#include <algorithm>

class A {
public:
    A() = default;
    A(A const & a) = delete;
    A& operator =(A const & a) = delete;
    A(A && a) = default;
    A& operator =(A && a) = default;
};

int main() {
    A a[1], b[1];
    std::copy(std::make_move_iterator(a), std::make_move_iterator(a + 1), b);
    std::move(a, a+1, b);
}

Compiles on 4.8.2 on Wandbox; fails with 4.9+ with a bogus static assert:

In file included from
/usr/local/gcc-head/include/c++/6.0.0/bits/char_traits.h:39:0,
                 from /usr/local/gcc-head/include/c++/6.0.0/ios:40,
                 from /usr/local/gcc-head/include/c++/6.0.0/ostream:38,
                 from /usr/local/gcc-head/include/c++/6.0.0/iterator:64,
                 from prog.cc:1:
/usr/local/gcc-head/include/c++/6.0.0/bits/stl_algobase.h: In instantiation of
'static _Tp* std::__copy_move<_IsMove, true,
std::random_access_iterator_tag>::__copy_m(const _Tp*, const _Tp*, _Tp*) [with
_Tp = A; bool _IsMove = true]':
/usr/local/gcc-head/include/c++/6.0.0/bits/stl_algobase.h:384:44:   required
from '_OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove = true; _II =
A*; _OI = A*]'
/usr/local/gcc-head/include/c++/6.0.0/bits/stl_algobase.h:420:45:   required
from '_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = true; _II =
A*; _OI = A*]'
/usr/local/gcc-head/include/c++/6.0.0/bits/stl_algobase.h:453:8:   required
from '_OI std::copy(_II, _II, _OI) [with _II = std::move_iterator<A*>; _OI =
A*]'
prog.cc:15:76:   required from here
/usr/local/gcc-head/include/c++/6.0.0/bits/stl_algobase.h:361:4: error: static
assertion failed: type is not assignable
    static_assert( is_copy_assignable<_Tp>::value,
    ^~~~~~~~~~~~~