Crash when using (->) instead of (=>) in a typeclass instance

[Helkafen]

import qualified Data.Vector as V import Test.QuickCheck? import Control.Monad (liftM)

instance (Arbitrary a) -> Arbitrary (V.Vector a) where

arbitrary = fmap V.fromList arbitrary

main = do

print "bla"

The arrow is obviously incorrect.

runghc <thisfile> tells me: *** Exception: compiler/rename/RnSource.lhs:429:14-81: Irrefutable pattern failed for pattern Data.Maybe.Just (inst_tyvars,

_, SrcLoc?.L _ cls, _)

[monoidal]

Thanks for the report, the bug is already fixed in GHC 7.6.1, see #5951. Now you get

T.hs:5:10:
    Malformed instance: (Arbitrary a) -> Arbitrary (V.Vector a)