[Bug libstdc++/85828] New: std::shuffle tries to swap element with itself

[soap at gentoo dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85828

            Bug ID: 85828
           Summary: std::shuffle tries to swap element with itself
           Product: gcc
           Version: 7.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: soap at gentoo dot org
  Target Milestone: ---

This bug is related to a previous one in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59603. For a full description, see
https://stackoverflow.com/questions/22915325/avoiding-self-assignment-in-stdshuffle.
The problem boils down to the fact that in std::shuffle, self-assignment in
some std::swap(x, x) will happen eventually. This will cause an "Error: attempt
to self move assign." failure to occur and kill the program when compiled in
debug mode. A chat on IRC with Jonathan Wakely confirmed that this is an
implementation bug, and that the likely correct fix for this is to avoid
self-swap in std::shuffle.