[Bug tree-optimization/100115] Bogus -Wmaybe-uninitialized warning with -O3

["boris at kolpackov dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100115

--- Comment #2 from Boris Kolpackov <boris at kolpackov dot net> ---
> I'm trying to reduce the test case to something manageable but that can take many hours, even days.

Right. On our side we have spent hours, even days trying to suppress this
warning (both by rearranging the code and with #pragma) but it keep popping up
in different places and on different targets (MinGW, Mac OS, etc). In fact,
we've wasted so much time on chasing this (and similar issues in the past),
that I am seriously considering just globally disabling this warning since it
seems more trouble than it's worth. Sorry, just venting a bit here...

> It would be really helpful if you could trim it down a bit, e.g., by removing the biggest non-essentials like <iostream>.

I will try. The thing is, such simplifications often make the warning
disappear. We have tens of similar code fragments in our codebase, but only
this one triggers a warning on x86_64-linux (other places trigger it on MinGW
and Mac OS).

> That being said, the context of the warning mentions std::optional which has been known to be challenging for analysis [...]

Yes, I can confirm that most of the issues with this warning we've encountered
in the past involved std::optional.