GCC 15.1

> {0} initializer in C or C++ for unions no longer guarantees clearing of the whole union (except for static storage duration initialization), it just initializes the first union member to zero. If initialization of the whole union including padding bits is desirable, use {} (valid in C23 or C++) or use -fzero-init-padding-bits=unions option to restore old GCC behavior.

This is going to silently break so much existing code, especially union based type punning in C code. {0} used to guarantee full zeroing and {} did not, and step by step we've flipped the situation to the reverse. The only sensible thing, in terms of not breaking old code, would be to have both {0} and {} zero initialize the whole union.

I'm sure this change was discussed in depth on the mailing list, but it's absolutely mind boggling to me

using UNION was always considered sketchy IMHO. This is trivia for security exploiters?

No. This is how sum types are implemented.

And from a runtime perspective it’s going to be a struct with perhaps more padding. You’ll need more details about your specific threat model to explain why that’s bad.

a quick search says that std::variant is the modern replacement to implement your niche feature "sum types"