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

[Stefan Holdermans]

Simon,

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

> Indeed.  But do you want to use that syntax for class parameters  
> too?  That would be a big change
>
> 	class C {|a|} where ....

Well... That would be the most consistent then. But... it looks weird.  
And it breaks code, of course. One could argue of course that visual  
markers are not needed for classes because all arguments introduce  
indices rather than parameters, but I see that you can argue the same  
for algebraic data-type declarations then.

Cheers,

   Stefan