Surprising error message when deriving Functor for a GADT

{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE GADTs #-}

data T a b where
    Mk :: Int -> b -> T Int b
    deriving (Functor)

GHC says:

    Can't make a derived instance of ‘Functor (T a)’:
      Constructor ‘Mk’ has existentials or constraints in its type
      Possible fix: use a standalone deriving declaration instead
    In the data declaration for ‘T’

This is not true - Mk does not existentially quantify anything, nor does it have a constraint in its type. The suggested fix of using a standalone deriving declaration works though. I'm guessing GHC is turning my declaration into Mk :: (a ~ Int) => a -> b -> T a b?

Tested on 7.10.3 running in OSX Mavericks.

changed weight to 5

added Tbug Trac import labels

changed title from Incorrect error message when deriving Functor to Incorrect error message when deriving Functor for a GADT

changed the description

changed title from Incorrect error message when deriving Functor for a GADT to Surprising error message when deriving Functor for a GADT

mentioned in commit cc92a446

closed

changed milestone to %8.2.1

Good idea thanks.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed
Test case	- → deriving/should_fail/T12163

mentioned in commit 210a2e12

added error messages label

added Pnormal label