Frank Mittelbach wrote:
> Will Robertson writes:
>  > This is weird:
>  > 
>  >    \err_warn:nn{\temp~stuff~stuff}{}
>  > 
>  > Only the second tilde causes a space.
> 
> why would that be weird?
> 
> in plain TeX
> 
>   \errwarn{\temp stuff stuff} 
> 
> and only
> 
>   \errwarn{\temp\space stuff stuff}
> 
> in expl3 ~ is a space token and not an active char expanding to \space.

It took me a little while to realise this, but of course you are right.
 So that was not what confused me.

> It is a bitof a pity that space tokens are so deep down within the TeX engine
> behavior and that sometimes they get gobbled if explicit.
> 
> Interesting question if it would be possible to do ~ -> \space instead but my
> guess is that this would break a lot of the current code

I was rather expecting \c_space_token to save the day, but it didn't.
Hence my confusion: \space isn't a LaTeX3 name.
-- 
Joseph Wright