Pip Cet <pipcet@protonmail.com> writes:

> My understanding is it is safe to access an entire union at once, as the
> old code did, and that GCC concluded based on the undefined behavior
> elsewhere that the only possible union members were those with the holes
> in the right place.

I have reached the bottom of this unpleasant business.  The following
reproducer does not require enabling LTO or type punning (or casts at
all, come to that), just "gcc-15 -std=c99 -O3".