DeriveFunctor should not require a functor instance for functorial data types used invariantly

This program:

{-# LANGUAGE DeriveFunctor #-}
data Silly a = Sillly a

data Baz o = Baz {
    foo :: o,
    bar :: Silly ()
  } deriving (Functor)

Produces this error:

/Users/mbolingbroke/Junk/Repro2.hs:7:14:
    No instance for (Functor Silly)
      arising from the 'deriving' clause of a data type declaration
                   at /Users/mbolingbroke/Junk/Repro2.hs:7:14-20
    Possible fix:
      add an instance declaration for (Functor Silly)
      or use a standalone 'deriving instance' declaration instead,
         so you can specify the instance context yourself
    When deriving the instance for (Functor Baz)

But there is absolutely no need for such an instance, because I'm not using "Silly a", just "Silly ()".

Trac metadata

Trac field	Value
Version	6.12.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture