GCC 12.1, -isystem, -Wsystem-headers, and -Wmaybe-uninitialized

[Tom Kacvinsky via Gcc-help <gcc-help@xxxxxxxxxxx>]

We were happily using -Wall -Wextra -Werror with GCC 8.3.0 and -isystem
for Boost header files to squelch any warnings that would come out of
Boost.

I recently switched to GCC 12.1 and now we are getting
-Wmaybe-uninitialized errors out of Boost.  So I am not sure if
-Wsystem-headers was not part of -Wall -Wextra in GCC 8.3.0 but is now
part of GCC 12.1.0.

Optimization level plays a role in this.  With -O0 (GCC 12.1), the
warning does not happen, but with -O3, it does.

So I am not sure of the interaction between optimization, -Wall -Wextra,
-Wsystem-headers, and GCC 8.3.0 and GCC 12.1.0.

Any ideas?

Thanks, Tom