Can't generate data decls deriving multiparam type classes with TH

This applies to both data declarations and (more commonly) newtypes. Example:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

newtype T a = X { unT :: State Int a } deriving (MonadState Int)

The newtype declaration is impossible to express in Template Haskell's AST, as the field for the derived classes is [Name] when it should really be Cxt.

A workaround is to generate a StandaloneDerivD for each derived multiparam class, but this requires that the user enable the StandaloneDeriving extension which can be unintuitive, especially as there's no way (as far as I can tell) to detect whether this extension is enabled and report a proper error message if it isn't.

Trac metadata

Trac field	Value
Version	7.10.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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