empty export warning

[Isaac Dupree]

Serge D. Mechveliani wrote:
> People, 
> 
> here is a more precise report about empty _export_ in 
> ghc-6.8.1-candidate:
> 
> I use
>   --------------------------------
>   module M1 (f, module M2)
>   where
>   import M2 ()
>   f = ...
>   --------------------------------
> 
> in the situation when  M1  imports some instances from  M2  and 
> reexports these instances.
> 
> And  ghc-6.8.1-candidate  reports 
> 
>   "Warning: The export item `module M2' exports nothing"
> 
> 1) Can I fix anything here?
> 2) Should this warning be different?

The warning is true.  The instances will be exported even if `module M2' 
is omitted from the export list, so that export item exports nothing.

Isaac