[Bug c++/78841] New: [6 regression] optimizer bug (silent bad codegen)

[Casey at Carter dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78841

            Bug ID: 78841
           Summary: [6 regression] optimizer bug (silent bad codegen)
           Product: gcc
           Version: 6.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Casey at Carter dot net
  Target Milestone: ---

g++ 6.2 (not 5.4, or 4.9, or 4.8, or trunk) generates bad code for the attached
repro at -O2 and higher. "silent bad codegen" just barely trumps "already fixed
on trunk" to my mind, so here's a repro; my feelings won't be hurt if you
WONTFIX this.

Apologies for the huge repro, I've had a hard time reducing it from here
without making the bug disappear.