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I've reproduced this issue on both 7.6.1 and 7.6.2. It does not exist on 7.4.2.

Given the following code (simplified from an actual case in Yesod):

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}

data Foo b = Foo deriving Show

class ToFoo a b where
    toFoo :: a -> Foo b

instance ToFoo (c -> ()) b where
    toFoo _ = Foo

bar :: Foo ()
bar =
    baz ()
  where
    baz () = toFoo $ \_ -> ()

main :: IO ()
main = print bar

This compiles and runs correctly with 7.4.2, producing the output "Foo". However, with 7.6.1 and 7.6.2 I get the following error message (identical between the two versions):

test.hs:16:5:
    Overlapping instances for ToFoo (t0 -> ()) b
      arising from the ambiguity check for `baz'
    Matching givens (or their superclasses):
      (ToFoo (t -> ()) b)
        bound by the inferred type for `baz':
                   ToFoo (t -> ()) b => () -> Foo b
        at test.hs:16:5-29
    Matching instances:
      instance ToFoo (c -> ()) b -- Defined at test.hs:9:10
    (The choice depends on the instantiation of `b, t0')
    When checking that `baz'
      has the inferred type `forall b t.
                             ToFoo (t -> ()) b =>
                             () -> Foo b'
    Probable cause: the inferred type is ambiguous
    In an equation for `bar':
        bar
          = baz ()
          where
              baz () = toFoo $ \ _ -> ()

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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