Wrong suggestion when deriving Generic on an instantiated type

The following code

{-# LANGUAGE DeriveGeneric      #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE FlexibleInstances  #-}

import GHC.Generics

data A a

data B

deriving instance Generic (A B)

gives rise to the error

    Can't make a derived instance of `Generic (A B)':
      A must not be instantiated; try deriving `A B' instead
    In the stand-alone deriving instance for `Generic (A B)'

It should indeed fail, but the suggestion should be to try deriving A a instead.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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