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

[Casey at Carter dot net]

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

--- Comment #5 from Casey Carter <Casey at Carter dot net> ---
I have verified that gcc-6-branch compiles the repro correctly, so yes, this is
a dup of PR78047.