[asveikau]

> Many standard library functions return a pointer which they got as a parameter

Obviously. But this does not extend the lifetime of the buffer they are passed. Namely you can't use this as a technique to extend the life of automatic storage falling out of scope.

> Lifetimes of compound literals are specified in section 6.5.2.5 of both C99 and C11.

This is what I was missing. So it is valid by the standard. Which is good if you have looked it up. It remains not obvious when reading source without a copy of the standard on hand, or prior knowledge of that section. Passing an expression of that sort and keeping a pointer to it visually looks like the intent is to retain a pointer of more limited scope. Intuitively it would make just as much sense if the lifetime were shorter. If you are seeking clarity of intent this is not a great thing to rely upon.