id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
2851	Improve error message for failed deriving	guest	igloo	"The following does not work:

{{{
type family F a :: *

data D a = D (F a)
    deriving (Show)
}}}

The natural way of adding an instance does work (with enough extensions turned on):

{{{
instance (Show (F a)) => Show (D a) where
    show (D x) = ""D "" ++ show x
}}}

It would be nice if the deriving mechanism could derive this code.
"	merge	closed	normal		Compiler (Type checker)	6.10.1	fixed		lennart@…	Unknown/Multiple	Unknown/Multiple		Unknown	deriving/should_fail/T2851