New GeneralizedNewtypeDeriving check still isn't permissive enough

Roman Cheplyaka writes:

I just tried compiling smallcheck with GHC HEAD, and it didn't work out:

 Test/SmallCheck/SeriesMonad.hs:41:7:
     Can't make a derived instance of ‛MonadLogic (Series m)’
       (even with cunning newtype deriving):
       it is not type-safe to use GeneralizedNewtypeDeriving on this class;
       ‛msplit’, at type ‛forall a.
                          m a -> m (Maybe (a, m a))’, cannot be converted safely
     In the newtype declaration for ‛Series’

So GHC now looks at the methods, but the problem is still there.

I would agree that msplit's type is problematic (due to the nested m's), but Simon and Richard previously said that it should work, so I'm confused.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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