[Bug c++/126868] -ftrivial-auto-var-init=uninitialized should be incompatible with -std=c++26 (or at least warn about it)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126868

--- Comment #5 from Giuseppe D'Angelo <peppe at gcc dot gnu.org> ---
My code didn't have [[indeterminate]]? It was externally injected by the flag.
And that's why I think it's dangerous -- it gives my code a meaning I have no
way to prevent.

> Having a user a way to opt out is really necessary, while in a lot of code the extra cost isn't that significant, e.g. in huge switches with a lot of vacuously initialized variables in the scope of the switch body, the cost can be significant.  And the users should have a way to opt out, not just to use -std=c++23 forever because they aren't willing to accept the slowdowns.

But why doesn't the flag just stop the wiping, *without* re-engaging the UB, so
the missiles still don't get fired? This way the blast radius would be much
much smaller? (Yes, technically speaking it'd still be non-conforming because
it doesn't wipe.)