> -fno-strict-aliasing doesn't suddenly make pointer aliasing defined behavior.

No, it just protects you from a valid but unexpected optimization to your incorrect code. It's even spelled out clearly in the docs: https://www.gnu.org/software/c-intro-and-ref/manual/html_nod...

"Code that misbehaves when optimized following these rules is, by definition, incorrect C code."

> We have compiler behavior for incorrect code, and it's refusing to compile the code in the first place

This isn't and will never be true in C because whether code is correct can be a runtime property. That add function defined above isn't incorrect on its own, but when combined with code that at runtime calls it with values that overflows, is incorrect.