Slightly tangential to the article but still important, it says "Some compilers infer that they can eliminate checks based on erroneous code elsewhere in the p

Slightly tangential to the article but still important, it says "Some compilers infer that they can eliminate checks based on erroneous code elsewhere in the program. For example, [obviously dodgy code snipped]some compilers will decide that ptr == NULL must always be false, since otherwise it would be incorrect to dereference it in call_fn()" So why does the compiler not report this error, or better still reject the code outright? It seems to turn a compile-time-detected fatal error into a critically dangerous runtime error.