[Bug libstdc++/78490] [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

--- Comment #1 from Felix Morgner <felix.morgner at gmail dot com> ---
Created attachment 40124
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40124&action=edit
Proposed patch