[GHC] #11509: Incorrect error message in StandaloneDeriving: "The data constructors of <typeclass> are not all in scope"

[GHC]

#11509: Incorrect error message in StandaloneDeriving: "The data constructors of
<typeclass> are not all in scope"
-------------------------------------+-------------------------------------
        Reporter:  edsko             |                Owner:  RyanGlScott
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1-rc1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2558
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * status:  new => patch
 * differential:   => Phab:D2558


Comment:

 At least, I thought deriving strategies would come into play, but it turns
 out a different check already present in GHC gives a pretty reasonable
 error message for this scenario already. So now the code above gives the
 error message:

 {{{
     • Can't make a derived instance of ‘SC (Serializable (MyList a))’:
         ‘SC’ is not a stock derivable class (Eq, Show, etc.)
         Try enabling DeriveAnyClass
     • In the stand-alone deriving instance for
         ‘(Typeable a, SC (Serializable a)) => SC (Serializable (MyList
 a))’
 }}}

 `DeriveAnyClass` technically can't be used derive instances of kind `a ->
 Constraint` right now, but they will be able to once #12144 is fixed.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11509#comment:8>
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