The first 4 are implementation defined rather than undefined.
That said, warnings do not necessarily mean that the code is invoking undefined behavior. For example, with if (a = b) GCC will generate a warning, unless you do if ((a = b)). The reason for the warning is that often people mean to do equality and instead write assignment by mistake, so the compilers warn unless a second set of braces is used to signal that you really meant to do that.
That said, warnings do not necessarily mean that the code is invoking undefined behavior. For example, with if (a = b) GCC will generate a warning, unless you do if ((a = b)). The reason for the warning is that often people mean to do equality and instead write assignment by mistake, so the compilers warn unless a second set of braces is used to signal that you really meant to do that.