id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
7346	Allow the use of `deriving` for GHC generics	kosmikus	dreixel	"Currently, a class that makes use of generic default methods (via the `DefaultSignatures`) extension can be instantiated by providing an empty instance declaration.

I propose to allow the use of `deriving` as well:

 1. Standalone deriving should be usable for a class not only for the specific set of classes supported by GHC now, but in addition for any class, if (
    * there's at least one generic default given for a method of the class, and
    * there are generic or normal default implementations for *all* methods of the class.

 There are a number of advantages of this solution over the empty instance declaration: we make it explicit that something generic is going on here; we ensure at compile time that we're not missing an implementation of a method; and we come syntactically closer to built-in derivable classes.

 In cases where a conflict arises between current GHC semantics and the proposed semantics (for example, when newtype-deriving is involved, I guess), I propose to stick with current GHC semantics, but I'm open for other suggestions.

 2. I'd also like for normal `deriving` to be useful under the same conditions as above. For normal `deriving` GHC has to figure out the class context automatically. I propose that if normal `deriving` is used, GHC uses the same heuristic for figuring out the class context that it uses for `Eq` in the case of `*`-kinded classes, and for `Functor` in the case of `* -> *`-kinded classes. That may not be optimal or even wrong. But in such cases, standalone deriving can still be used."	feature request	new	normal	7.8.1	Compiler	7.7			leather@… hvr@… v.dijk.bas@… cgaebel@… pho@… carter.schonwald@…	Unknown/Multiple	Unknown/Multiple	None/Unknown	Unknown