Re: [hub] Andrzej's review

> Andrzej Krzemienski wrote:
> > 5. Compiling with -Wall gives me a warning in GCC when sorting an
> > empty
> > hub: a zero-sized heap array is allocated:
> > https://godbolt.org/z/dWvxv9n8s
> > This is technically fine to allocate such arrays, but maybe it is
> > worth to make -Wall clean.
> 
> The warning isn't caused by the zero sized allocation (which is legal.)
> 
> It's because the compiler can see that the array has zero elements, but can't
> see that the loop inside this call to visit
> 
> https://github.com/joaquintides/hub/blob/063cf1d3589d4893b53964084ee
> 56f4476eed471/include/boost/container/hub.hpp#L1628
> 
> is never executed. So it says "if this loop is executed, it would be UB because
> there aren't any elements there" but this is a false positive because the loop
> isn't executed.
> 
> (The subsequent loops suffer from the same issue and also cause warnings for
> the same reason.)
> 
> A common problem with GCC, and unfortunately nothing can be done about it
> short of disabling the warning.

On second thought, given that the warning only fires on empty containers, it
might be possible to avoid if `sort` checks for `empty()` (or `size() <= 1`) and returns.

Adding a branch to avoid a warning is a bit dubious in general, but in this case
it avoids a completely unnecessary allocation as well, so it's sound even in principle. :-)


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