If a warning is issued in that case I'm tempted to say it's fair game. If you are sure it'd safe the compiler has extremely well established ways of telling it (i.e. assume or unreachable)
I wouldn't call it fair game, not least because nobody reads warnings. My central objection is that it's not what C is meant to be. C was made to write Unix, it was specifically created to be halfway between a portable macro assembler and a high-level language. That is a useful language that fills a particular niche. That is what C was for many years and a lot of code was written with that behaviour in mind. It can be argued that compilers which change semantics from what people intended are downright irresponsible given the foundational role that C has.
All this wouldn't be an issue if C were just some application language, but it's what all of computing is built on. It really should be simple by default and without adding more footguns than what directly programming in assembly gives you.
I wouldn't mind if all the assumptions-based optimisations were made opt-in. They are obviously useful in some way, but they're impossible to allow globally in a large legacy codebase. Which is pretty much every C codebase.
There is probably more than a million times as much C code that is not Unix as code that is. We could chuck in all the other OSes, and all the RTOSes besides, without changing the statement.
All this wouldn't be an issue if C were just some application language, but it's what all of computing is built on. It really should be simple by default and without adding more footguns than what directly programming in assembly gives you.
I wouldn't mind if all the assumptions-based optimisations were made opt-in. They are obviously useful in some way, but they're impossible to allow globally in a large legacy codebase. Which is pretty much every C codebase.