[ryao]

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.

[o11c]

In the cases involving overflow, it's implementation-defined whether there's undefined behavior.

[turminal]

> The first 4 are implementation defined rather than undefined.

Third and fourth are only defined in some implementations.

[ryao]

That is fair for 4, although would explain why it is the case for 3?

[turminal]

If char is signed and ' ' * 13 is bigger than CHAR_MAX, you get UB by signed overflow.