Difference between void and zero types?

[Stephen R. van den Berg]

I see the following changes in pgsql_util.pmod:

957c958
<         Thread.MutexKey lock = closemux->lock();
---
>         void|Thread.MutexKey lock = closemux->lock();

<   private Thread.Queue qportals;
---
>   private zero|Thread.Queue qportals;

What is the difference between declaring a variable as void and as zero?
-- 
Stephen.