[Bug libstdc++/78490] New: [c++17] libstdc++ has undefined behavior in operator= of propagate_const

[felix.morgner at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78490

            Bug ID: 78490
           Summary: [c++17] libstdc++ has undefined behavior in operator=
                    of propagate_const
           Product: gcc
           Version: 6.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: felix.morgner at gmail dot com
  Target Milestone: ---

The implementation of propagate_const libstdc++ 6.0.22 has undefined behavior
in operator=

The implementation reads as follows:

      template <typename _Up, typename =
                typename enable_if<is_convertible<_Up&&, _Tp>::value>::type>
      constexpr propagate_const& operator=(propagate_const<_Up>&& __pu)
      {
        _M_t = std::move(get_underlying(__pu));
      }

      template <typename _Up, typename =
                typename enable_if<__and_<is_convertible<_Up&&, _Tp>,
                                          __not_<__is_propagate_const<
                                                   typename decay<_Up>::type>>
                                          >::value>::type>
      constexpr propagate_const& operator=(_Up&& __u)
      {
        _M_t = std::forward<_Up>(__u);
      }

Both overloads of operator= are missing an appropriate return statement,
causing undefined behavior.