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And careful people keep in mind that only what's defined in the standard can be expected to be true across compiler updates.

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Good clarification, you really only know what you're going to get on (language, compiler, compiler version, platform).

You also need to know the exact implementation of your compiler's optimizer, otherwise you don't know whether it's safe to change your program. Any change might give the optimizer enough information to unleash the nasal demons in previously cromulent code.

Sure, for UB.

But extensions like typeof are different from UB. They have documented behavior, it is just documented by GNU instead of the C standards committee.

And it's stuff like this which is making me increasingly uncomfortable with using C++ as my main language. Currently it looks like Rust is the most suitable replacement but I'm not sure if it's there yet.

Not even that. Compilers had and have bugs.

It's not a bug if it's UB. If the compiler behaves differently than the standard says it should you can file a bug.

The compiler vendor can treat UB however they like. One option would be for them to specify that it does something in particular...

(The C standard explicitly calls this out as a valid way of handling UB.)

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