'impossible' happened when deriving Functor on a type with a type family application

[patrick_premont]

The following should fail with an appropriate error message.

I discovered this by accident when I changed the definition of type 'U' and forgot to remove 'deriving Functor'.

> {-# LANGUAGE TypeFamilies, DeriveFunctor #-}

> data U a = U (G a) deriving Functor
>
> class A a where
>   type G a

[simonpj@…]

commit 88cd0d1fd68ec88dfe843fa14f3895a21e1ca07f

Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Mon Dec 12 08:12:18 2011 +0000

    Make derived functor stuff watch out for type functions (Trac #5686)
    
    The problem was simply that the side-condition check didn't test for a
    type function, and then splitAppTy fell over.

 compiler/typecheck/TcDeriv.lhs    |    2 +-
 compiler/typecheck/TcGenDeriv.lhs |   57 ++++++++++++++++++++----------------
 2 files changed, 33 insertions(+), 26 deletions(-)

[simonpj]

Thanks for the report!

Simon