Misleading error message when deriving Functor

Compiling this trivial demo program:

data Container
  = Container [Wibble Int]
  deriving (Eq, Show)

data Wibble a
  = Wibble a
  | Wobble
  deriving (Eq, Functor, Show)

results in the error message:

a.hs:6:13: error:
    • No instance for (Eq (Wibble Int))
        arising from the first field of ‘Container’ (type ‘[Wibble Int]’)
      Possible fix:
        use a standalone 'deriving instance' declaration,
          so you can specify the instance context yourself
    • When deriving the instance for (Eq Container)

a.hs:6:17: error:
    • No instance for (Show (Wibble Int))
        arising from the first field of ‘Container’ (type ‘[Wibble Int]’)
      Possible fix:
        use a standalone 'deriving instance' declaration,
          so you can specify the instance context yourself
    • When deriving the instance for (Show Container)

a.hs:12:17: error:
    • Can't make a derived instance of ‘Functor Wibble’:
        You need DeriveFunctor to derive an instance for this class
    • In the data declaration for ‘Wibble’

The "No instance for (Eq|Show)" are rather misleading, because the *actual* error is that the {-# LANGUAGE DeriveFunctor #-} pragma is missing.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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