[GHC] #3714: Improve error message if an associated family declaration has excess parameters

[Stefan Holdermans]

Simon,

Regarding distinguishing between type indices and parameters, you  
suggested:

> type family T3 f !e :: *         -- The ! indicates a type parameter  
> (not
> an index)

I'd rather have indices, rather than parameters, explicated by mean of  
syntax. This seems more consistent with ordinary type declarations.

On paper, I often find myself writing

   type family T3 {|f|} e :: * .

(A remnant of overexposure to generic programming, I guess.)

Cheers,

   Stefan