[still_grokking]

Stupid question, but how can a compiler flag "mitigate" some UB that is (to my knowledge) defined by the standard as UB?

This would be possible only if the standard would define the behavior in question as implementation defined. But that's not the case afaik.

Signed overflow is undefined behavior as it's afaik defined to be that. But maybe I'm just wrong here?

But at least SO is confining my knowledge:

https://stackoverflow.com/questions/16188263/is-signed-integ...

[pratk]

-fwrapv forces the compiler to handle signed overflow with 2's complement. UB essentially means the compiler is free to do anything, and with the option it chooses the wrapping behavior. See https://stackoverflow.com/questions/47232954/what-does-fwrap...