Like what, for example? To the contrary, I think that, other than constness, C++ has rather few facilities to communicate semantic invariants to the compiler.
If the thread you are on doesn't modify the variable (e.g. by const_cast), and that variable isn't atomic or volatile, the compiler should be allowed to treat it as invariant. Whether it does in practice probably depends on a lot of things though.
Yeah, but the compiler need to see through all the functions to find out that there is no const_cast and therefore makes const useless because it could as well see it is not modified.
Also it needs to be good at alias analysis.