[vilhelm_s]

The "shall" in the standard means it's undefined behavior. This is explained in the "Conformance" section,

> 2. If a ‘‘shall’’ or ‘‘shall not’’requirement that appears outside of a constraint is violated, the behavior is undefined. Undefined behavior is otherwise indicated in this International Standard by the words ‘‘undefined behavior’’ or by the omission of any explicit definition of behavior. There is no difference in emphasis among these three; they all describe ‘‘behavior that is undefined’’.

Compilers will not refuse to compile the code, indeed the blog post we are all commenting on reports the results from a bunch of different compilers. Historically the reason the C standard specified a lot of undefined behavior is that the actually existing C compilers at the time compiled the code but disagreed about the output.

[thaumasiotes]

> Compilers will not refuse to compile the code, indeed the blog post we are all commenting on reports the results from a bunch of different compilers.

Yes, I see that. I just said they should refuse.

[masklinn]

Because this specific UB is static (not usually the case) both gcc and clang will flag it if Wsequence-point is enabled (and it is part of Wall) (technically the clang warning is Wunsequenced but aliased to the GCC version).

edit: apparently Wunsequenced is enabled by default so clang should warn you out of the box.