GHC accepts overly general instance sigs

{-# LANGUAGE InstanceSigs #-}

data I a = MkI a

instance Functor I where
  fmap :: (a -> b) -> f a -> f b
  fmap = undefined

is accepted when the type is too general, even things like:

  fmap :: (a -> b) -> f a -> g b
-- &
  fmap :: whoami

are accepted

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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