Suggest -XGeneralizedNewtypeDeriving for newtypes

[guest]

newtype A = A Int deriving Enum

t.hs:1:27:

Can't make a derived instance of `Enum A'

(`A' has non-nullary constructors)

In the newtype declaration for `A'

But perhaps -XGeneralizedNewtypeDeriving should be suggested (ideally only if the wrapped type actually had such an instance)?

[simonpj]

Good idea. Fixed by

Mon Mar  1 11:15:55 GMT 2010  simonpj@microsoft.com
  * Suggest -XGeneralizedNewtypeDeriving (fix Trac #3888)
  
  If we can't derive a type, but it's a reasonable possibility that
  newtype deriving would do the job, suggest it. 
  
  A little refactoring too, moving non_iso_class to top level,
  and putting it with std_class_via_iso.

    M ./compiler/typecheck/TcDeriv.lhs -12 +23

Simon