One thing to realize in this discussion is that it is not possible in general to warn when a programmer is depending on undefined behavior, since by definition you cannot in general guess what misunderstandings the programmer has about C, and hence what behavior is expected.
There are some cases where you can guess well enough that you can provide warnings that won't have an excessive number of annoying false positivesm, and warnings in such cases are appropriate. But some contributions to this thread have seemed to come near to a position of requiring warnings whenever the compiler does something that might violate expectations of a programmer writing undefined C code, and as a general principle this is fundamentally impossible.
