Do distros have tooling to deal with this type of change?
I imagine it would be very useful to be able to search through all the C/C++ source files for all the packages in the distro in a semantic manner, so that it understands typedefs and preprocessor macros etc. The search query for this change would be something like "find all union types whose first member is not its largest member, then find all lines of code where that type is initialized with `{0}`".
In theory it's just an extension of IDE tooling. A CLI with a little query language wrapping libclang. In practice I'm sure it's a nightmare just to get 20,000 packages' build systems wrangled such that the right source files get indexed by libclang, and all the endless plumbing for downloading packages and reporting results, and on and on.
Distribution build systems typically operate outside of an IDE. I suspect that it would be a nightmare to get 20,000 packages to compile in an IDE.
It is possible in theory to write a compiler plugin to generate an error when code that does this is found and it would make it easy to find all of the instances in all packages by building with `make -k`, provided that the code is not hidden behind an unused package flag.
As a retired Gentoo developer, I can say not really as far as I know. There could be static analysis tools that can find this, but I am not aware of anyone who runs them on the entire distribution.